Exploiting sunshine in house design

•

~ Prepared by Eclipse Research Consultants on behalf of the Bunding Research----.: Establishment for the Department of Energy's Energy ~echnolog:f Support Unit

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Contents

PrefaceIntroduction

SECTION 1 Solar gain and energy efficient design

Why energy efficIent design?Twenty steps to energy efficiency in house designWhy glaZing?Why solar gain?Solar gain and glazing

3

468

1020

Site planning and solar radiationRoom arrangement

SECTION 2 Site layout and house planning 27

2834

35

364450

WindowsSECTION 3

Window systems. Window orientation

Window area

SECTION 4 Glass shading, insulation and thermal mass

Overheating and the effects of direct sunshineOccupants and shading/insulation devicesExternal shading devicesInternal shading and insulation devicesThermal mass and window design

57

5860637080

Conservatories in energy efficlent designConservatories as attached greenhousesConservatories as living spaceConservatories and energy performanceConservatories as buffer zonesConservatories as high temperature solar collectorsConservatories for pre-heating ventilation airConservatories and energy savingsConservatory shading and ventilationConservatories and the Building Regulations

SECTION 5 Conservatories 85

8688909899

100103105108110

SECTION 6 Heating systems and solar energy

Space heating and solar gain

117

118

123

124125127

References and indexSECTION 7

Useful organisationsSources of figuresIndex

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Preface

Much has been written over the past decade andmore on the importance of using energy efficientlyin the buildings we live in. In addition toassessments of energy conservation measures,there has been a good deal of research into thepotential for exploiting solar gains in housing. Atthe Building Research Establishment (as part ofour work for the Department of Energy) we havebeen fortunate enough to be able to survey thisinformation, digest it and then produce what weexpect to be a useful handbook for UK housedesigners.

I can say that we expect it to be useful (rather thanjust hope) as we have also been able to tryout adraft of this handbook on potential users. Theirreactions were both helpful and encouraging.Comments we received on the contents and layouthave helped to make it a better, more user friendlydocument. Other responses have confirmed ouroriginal views that such a book of designguidelines does not exist at present and that UKdesigners will be keen to use it.

The contents aim to help you, the designer, toproduce better, more efficient houses. Initially youmay not find the book makes your job any easier­it will probably raise problems that you haven'tconsidered before - and it may make you thinkmore deeply about the decisions you take. But ouraim is to help you have a better understanding ofthe conflicting options that confront designers oflow-energy housing. Then you can make up yourown mind on how to produce better housing.

One final point - this Is not a research document.The main authors of this handbook are botharchitects with practical experience of housebUilding and of teaching architectural design.They've tried to provide you with sound informationand concrete advice - whenever currentunderstanding allows.

Building Research Establishment

May 1988

Introduction

This handbook summarises the best current information on designing windows andconservatories to be energy efficient. It draws particular attention to the value ofuSing solar gains to offset heat losses. Design guidance is offered across a rangeof topics from site planning to the use of coated glass.

Our aimIn this handbook we attempt to draw together andto condense the best information and guidancecurrently available about the effect of glazing onthe environmental performance and energyconsumption of housing. In particular, we focus onWhat is known about using solar gain as a meansof reducing fuel consumed for space heating.Special attention Is given to making houses energyefficient by means of their siting, orientation,Internal planning, and the iocatlon, size andspecification of their windows. The potential ofconservatories for energy saving Is also discussed.

How to use this handbookThe arrangement of the handbook Is based iooselyaround the RI SA Plan of Work, from site analysisand site layout to construction details andcomponent specification, so presentinginformation in the order required to assist decisionmaking.

Each section of the handbook is organised broadiyon the following lines:• general requirements are stated

• general principles are given

• facts and figures about cost and performanceare described

• wherever possible each section or subsectionis concluded by design guidance.

The design guidance of the handbook takes theform of summaries of the advantages anddisadvantages of particUlar measures orcomponents, together with simplifiedrecommendations for designers to follow. Whereappropriate, warnings are given about thelimitations of this guidance and other publicationsgiving more detailed Information are cited.

The scope of the handbookOver the past ten years, many design features forcollecting, storing and distributing the sun's energyhave been Illustrated In the International literatureon passive solar design. They include trombewalls, roof collectors and c1erestoreys, as well asdirect gain systems, conservatories as solarcollectors and a variety of hybrid systems.

Not all of these so called "advanced" passive solardesign options are applicable to UK circumstancesbecause of climate, occupant behaviour, orpeople's expectations. Some remain experimentaldesigns that have never been built and had theirperformance monitored In this country. Others,have been found to have practical disadvantagesor to be so expensive to build that the additionalcapitai cost of the solar features would not berecovered by the saving in conventional fuel.

Advanced passive solar designs also demand alevel of simulation modelling to ascertain theirInternal comfort conditions and energyperformance. This puts them beyond the scope ofthe simplified guidance of this handbook andconsequently they are not dealt with here.

In this handbook attention Is focused on optimisingdirect solar gain through windows, a featurecommon to all houses, and on the potentialthermal benefits of conservatories.

The main topics of the handbook are:• site layout, orientation, and solar access

• room arrangement

• window and conservatory location, sizing andconstruction

• selection and specification of buildingcomponents

• responsive building services.

1

Our sourcesThe handbook draws on many sources• statutory documents such as the Building

Regulations, British Standards and codes ofpractice

• published advice on low energy design

• specialised research reports aboutexperimental designs and computersimulations

• articles in the technical press

• trade literature.

Much of the advice comes from sources not easilyaccessible to practitioners:• the simulated performance of house designs,

commissioned from architects by the EnergyTechnology Support Unit of the Departmentof Energy, which were analysed using athermal simulation model called SERI-RES

• simulated performance of a typicalsemi-detached house whose designparameters were systematically varied by theBRE and analysed using SERI-RES.

• monitoring studies of the performance anduse of conventional and low energy designs,particularly

- test cells- the Linford and Pennyland housing- an attached greenhouse at New Bradwell,

Milton Keynes.

2

SECTION 1Solar gain and energy efficient design

3

Why energy efficient design?

Energy is only one of a number of issues designers have to consider. Often it is byno means the most important. But in any design many decisions have to be takenabout glazing. Although the reasons behind these decisions may have little ornothing to do with energYl there are important consequences for occupants'comfort and for energy consumption.

Increasingly, designers are being expected to beaware of energy issues. Both through the BuildingRegulations and accompanying British Standards,such as BS 8211,1 they are being asked to designbuildings that are energy efficient. However, whilethese documents set standards, they not provIdedetailed design guidance on, for example, how todesign windows and conservatories that conserveenergy. The handbook tries to fill this gap.

The design consideratIons described In thishandbook are not new, As Barry Parker remarkedaround the turn of the century,

If we are to derIved health and pleasure fromthe time we spend in our homes we canhardly attach too great an Importance to thebringIng of light and sunshIne into the veryhearts of them.

And so in his influential designs, such as Figure 1,2

The sun, if it does shine, shInes Into the livingroom from morning to night.

As long ago as the 19308, the Royal Institute ofBritish Architects felt obliged to offer guidelines3 onthe correct orientation of houses and aVoidance ofobstruction, see Figure 2, and observed,

Until the habit of thInkIng in terms of sunshinehas been acquIred, buildIngs will continue todeprIve their occupants of values whichmight have been enjoyed without cost everyday over the whole life of the buildIng.

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1 British Standards Institution, BS 8211, British standard code of practice for energy efficlenoy In housing, 8SI, London.2 HaWkes, D, 1986, Modern country homes in England: the Arts and Craft arohitecture of Barry Parker, Cambridge University

Press, CambrIdge.3 Royallnstltute of British Architects, 1933, The orientation of buildings, RIBA, London.

4

Recently, the importance of sunshine andorientation has been restated by the RIBA in itsHousing Design Brief1,

Livihg rooms and rear gardens should have asoutherly aspect as far as possible andgenerally dweillngs should be sited tomaximise solar gain.

What has changed over the past 50 years is bothour understanding of the scientific principlesinvolved and the glazing technology at ourdisposal. This makes exploiting sunshine topractical effect a more realistic objective for usthan it was for our predecessors, however goodtheir intentions.

Institute-of Housing & Royal Institute of British Architects, (undated), Housing desigh brief, RIBA PublicatIons, London,

5

Twenty steps to energy efficiency in house design

The Building Regulations are concerned only with setting minimum standardsbelow which designers may not fall. For those who decide to build to higherstandards, there are many measures which may be taken to reduce theconsumption of conventional fuels and to provide added comfort, amenity andenjoyment for occupants.

We list below the twenty most important measures which designers should consider to make their designsmore energy efficient. These measures are listed in the order in which they need to be considered. Only thosein bold type are covered In this handbook.

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7

Why glazing?

The amount, type and position of glazed elements included in the external envelopeof a house have a significant effect both on how comfortable it is to live in and onits space heating consumption.

Windows are a prime focus In energy efficientdesign. Traditionally they have been seen as theweak spot in the thermal envelope of a house, asthe holes In its insulation. And it Is true that there ishardly a quicker way to lose heat, and to reducecomfort conditions, than through a windowcomposed of a single layer of glass. Glassconducts heat almost nine times faster thansoftwood and thirty-five times faster than expandedpolystyrene of the same thickness, 1 Yet, despiterepeated reductions to the rate at which externalelements - walls and roofs - are allowed to loseheat under the Building Regulations, the onlyrestriction on glazed elements Is a limitationin their total area, Single glazing is stili permittedas the minimum standard.

Windows do not have to be a weak spot in ahouse's thermal envelope. If they are orientated,sized and constructed correctly they can bethermally neutral, that is their heat losses may bebalanced by their solar gains, Under somecircumstances they may even become a netsource of heat gain during part of the heatingseason.

The effect of windows on thermal performancebecomes more important as insulation standards inhouses are increased. In existing houses, up to25% of heat escapes through wlndows.2 ln recenthouses, constructed since the 1982 revisions to theInsulation standards required under the BuildingRegulations, and with the maximum amount ofsingle glazing permitted, 30- 35% of the heat canbe lost this way. And another 15% will be lostthrough ventilation, mainly as heat flowIng outthrough inadequately sealed cracks and gapsaround window and door openings.

So, as the insulation values of other elements in thebuilding envelope are improved, the importance ofheat lost through windows and ventilatIonincreases, Figure 3

1 Everett, A., 1978, Materials, Mitchell's Building Construction, Batsford, London.2 Energy Efficiency Office, 1986, Monergy News, Department of Energy, London.

8

This handbook describes how to improve thedesign of windows by focussing on the fiveenergy-related functions they serve:• collecting wanted solar gain during the

heating season

• providing insulation and airtightness duringthe heating season

• offering natural ventilation, especially duringtemperate weather

• rejecting unwanted solar gain during thesummer

• admitting daylight all the year round.

For the past century or more, windows haveoccupied a surprisingly stable proportion ofexposed external wall area of British dwellings. AsFigure 4 shows, this proportion has remainedbetween 15% and 20% across housing types andover tlmea

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Modern glaZing technology now offers much morecomplex and sophisticated window systems thanhave previousiy been avallabie. These hold out thepromise, for example, of fully glazed south-facingfacades that deliver a net energy gain all the yearround.

Before they can decide whether to exploit thistechnology, either in the form of windows orconservatories, designers need to understand howglaZing affects the performance of a house, Interms of both comfort and costs.

In this handbook, we try to help designers torespond positively to increasing pressures on themto provide glazed elements in housing that areattractive and serviceable without being an energyburden for occupants.

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3 Steadman, P, & Brown 1 F., 1987, Estimating the exposed surface area of the domestic stook, In Hawkes, D. et ai, Energy andurban built form, Butterworths, London.

9

Why solar gain?

All houses have windows for daylightl ventilation and view. So all houses benefit tosome extent from solar radiation. In energy efficient design, the aim is to maximisesolar gain when it is wanted and to exclude it when it is not.

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However, while solar energy is fairly predictablefrom year to year, It can vary considerably fromday to day. It is also at its maximum during thesummer, whereas demand for energy Is greatest inthe winter. Nevertheless, recent research anddemonstration work undertaken for theDepartment of Energy suggests that, if carefulconsidered and integrated into design, solar gainscan, on average, contribute a third of total heatIngneeds.3 In small well-insulated houses, thepotential for using solar energy is less becausegains from occupancy can be enough to meetmost of the heating requirement.

The potential of solar radiation.Solar energy is plentiful. 0.01 % of the total amountwhich lands on the earth's surface would meet theworld's present needs.1 The solar energy falling onthe UK in a year Is 80 times our annual energyneeds. And, in any year, a typical house in Britainreceives more solar energy than the total energyconsumption of Its household.2

Factors affecting incident solarradiationSolar radiation Is the radiant energy emitted by theouter layer of the sun called the photosphere. Thecharacter of the radiation is determined largely bythe temperature of the photosphere, about 5,500°C.

Solar radiation can be divided Into threecomponents on the basis of wavelength. Thevisible component spans a wavelength range fromabout 0.4 to 0.7 microns. (A micron Is one millionthof a meter.) The wavelengths of visible light aredIstinguished by their colours from red at thelongest wavelength (0.7 microns) to violet at theshortest (0.4 microns). Ultraviolet (beyond Violet) isradiatIon of a wavelength that begins at violet andextends to shorter wavelengths, down to about0.01 microns. Infrared (below red) Is radiation withwavelengths longer than that of red. Solar radiationis often called short wavelength radiation todistinguish It from the radiation emitted fromobjects that are much cooler than the sun, which isreferred to as long wavelength radiation.

Figure 6

1 Department of Energy, 1975, Solar energy: its potential contribution within the United Kingdom. HMSO, London.2 Department of Energy & Centra! Office of lnformalion, 1980, Energy. a key resource, Department of Energy, London,3 US & UK Departments of Energy, 1985, US/UK Energy R&D: P<'lssive solar design.

10

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Some of the transmitted radiation is diffused by theparticles of the atmosphere and reaches theground as diffuse radiation, and some istransmitted direct. Global radiation Is the sum ofthe direct and diffuse radiation reaching theground, measured on a horizontal surface, Theamount of global radiation received on the earth'ssurface is not constant but varies according to:• the place,• the season• the time of day,• meteorological conditions.

Figure 9 shows the average daily solar radiation ona horizontal surface for various latitudes in theabsence of atmosphere,

Once solar radiation reaches the earth'satmosphere It is• partly absorbed (15%)

• partly reflected (6%)• partly transmitted to the earth's surface (79%),

The 23° 27' tilt in the earth's axis, Figure 8,accounts for the change in radiation intensity,length of day and seasonal climate. If the axis wereat right angles to the plane, there would be noseasonal changes In the weather.

Extraterrestrial radiation is the amount of solarenergy that reaches the limit of the earth'satmosphere. measured perpendicular to thedirection of the radiation. This is fairly constant, atabout 1.4 kW/m2

, see Figure 7, varying onlybecause of the elliptical orbit of the earth whichresults in the sun being closer to the earth inDecember and further away in June.

For any location, cloudless days provide the mostsolar radiation. But even totally cloudy days canprovide Useful radiation, up to about a third of thatavailable on a cloudless day at the same time ofyear.

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11

In addition to the direct and diffuse radiation, thevertical surfaces of buildings receive somereflected radiation, depending on the groundreflectance value, see Figure 10,

The apparent path of the sunSeen from a position on the earth's surface the sunappears to travel in an arc across the sky. Thisapparent path determines:• the hours of direct sunlight which a window or

conservatory receives

• the angle of incidence of the sun's rays

• the overshading characteristics of a site andIts surroundings.

The path of the sun varies with the seasons andwith the latitude, At any gIven time the position ofthe sun may be defined by Its angles of azimuthand altitude. The azimuth is the angle between truesouth and a point on the horizon directly below thesun; the altitude is the vertical angle between thehorizon and the sun, as shown In Figure 11.

As the sun paths In Figure 12 show, in the summerthe sun rises north of east and sets north of west.In the winter, it rises south of east later in themorning, travels In a lower arc, and sets south ofwest earlier In the evening.

The height of the sun at midday at the equinoxes(21 March and 23 September when the length ofday and night are approximately equal) can becalculated by the formula:

h = 90° - <p

where <p Is the latitude of the place. Thus for placeswith the latitude 51 ° North, the height of the sun atmidday is 39°, At the summer solstice (22 June, thelongest day) the height of the sun Is given by theformula:

At the winter solstice (21 December, the shortestday) the formula is:

12

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Figure 11

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Figures 13 and 14 (overleaf) show examples of sunpath diagrams in the form of stereographicprojections for London and Glasgow. These aretaken from BSI DD67:1980 Basic data for thedesign of buildings - sunlight. Such diagramsenable the sun's altitude and azimuth angles to beread off, for a given location, at any time of day andany time of year. The numbers around the outsideedge show the azimuth angle relative to anobserver facing north, and the concentric linesshow the altitude.

For example consider June 22. The sun risesbefore 4.00 am at 50' east of north, and by 5.00 amit Is 10' above the horizon. It is 60' above thehorizon between 11.00 and 1.00 pm. It sets after8.00 pm.

It can be seen from the stereographic projectionthat the altitude of the sun does not change at aconstant rate, but that the year is divided Into fourdistinct periods:• two, around the summer and winter solstices,

where the maximum altitude of the sunchanges only slowly from day to day

• two, around the spring and autumnequinoxes, where the maximum altitude of thesun changes rapidiy from day to day.

Sun path diagrams, such as the stereographicprojection, may be used:• to examine overshading in site layouts

• to check periods of direct sunlight for facadeswith particular orientations

• to investigate the effect of shading devices.

13

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Solar energy in the British IslesThe total amount of solar energy (diffuse, directand reflected) and the angle of incidence (theangle between the direction of the sun's radiationand a line perpendicular to the surface of thebuilding) of the direct radiation that reaches abuilding are determined by:• its latitude

• the season

• meteorological conditions.

Figure 15 shows that the south of BritaIn receivesabout one and a half times as much solar radiationon a horizontal surface as the north.

For any given facade of a building, the totalradiation will depend on the facade's orientationand the degree to which the radiation is obstructedby surrounding topography, buildings or plants.

Where solar radiation penetrates via windows toheat a space directly, Insolation on verticalsurfaces is the important consideratlon. Figure 16shows the total solar radIation in kWh/m2 for Kewon vertical surfaces with different orientations on aclear day.

There are three main conclusIons to be drawn fromthis graph:• the high altitude of the sun in summer results

In a large angle of incidence of the directradiation, and this in turn prevents a peak Inincident solar energy on south facing facadesfrom occurring between May and August.ThIs helps to reduce the risk of summeroverheating in houses with large south facingwindows

• onclear days in the heating season ahorizontal surface receives less solar radiationthan a south facing vertical one

• the summer peak in solar radiation on ahorizontal surface helps to explain grosssummer overheating in south facingconservatories with unshaded sloping glazedroofs.

16

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In contrast with Figure 16 which assumes clearskies, Figure 17 shows the total radiation onvertical surfaces at each of the four cardinalorientations for ail months of the year except June,July and August, for three locations in the BritishIsles. These amounts have been calculated frommeasurements of solar radiation on horizontalsurfaces made during the period 1966·1975.

Another way of gaining a broad understanding ofthe potential contribution which solar radiation canmake to reducing fuel consumption in a particularlocality Is to compare solar radiation on ahorizontal surface with the number of degreedays. Degree days record the daily differencebetween a base temperature, often taken as 15.5'Cin the UK, and the 24 hour mean ambienttemperature when it fails below the basetemperature. Daily totals may be added to give themonthiy total, and monthly totals added to give theannual total.

Figure 18 shows the "crude solar viability" indicescalcuiated by Oppenheim1. These appear to bebased on the foilowing calculation:

For each month caiculate

(daily solar radiation on a horizontal surface x number of days)(number of degree days divided by 30)

2 Add the monthly totals for the year excludingJune, July and August to find the crude solarviability indices for each location.

It is disputable just what these indices show. This isbecause the index becomes meaningless whenextreme conditions are used. For example, if thenumber of degree days were to approach zero, thesolar viability index would get very large. Yet in alocation with zero degree days there would be nopotential at ail for solar gains since there would beno heating requirement. Conversely, if the numberof degree days were to approach a very largenumber, implYing a long cold winter, then solargains would be increasingly viable and the indexshould enlarge to show this trend, yet in fact itdecreases.

This index represents the British climate as:• becoming more severe from west to east

• becoming sunnier from north to south.

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Figure 18

Oppenheim, D., 1981, Small solar buildings in cool northern climates, Architectural Press, London.

17

As latitude increases the length of the days islonger during the winter (giving rise to the midnightsun at very high latitudes) so that solar radiationoccurs for longer periods. However ambienttemperatures are lower In the north, and althoughthe heating season therefore extends into thesunnier spring and autumn weather, the amount ofheat lost through windows is increased. Thus theenergy balance between heat losses and solargains is poorer in the north than the south, seeFigure 19. 1

Another factor which may militate against the useof solar radiation In the north is that the effect ofobstructions is greater there because the sun is ata lower altitude during the heating season than Inthe south.

On the basis of computer simulations, it has beensuggested2

, 3 that in Scotland glazing areas mayneed to be smalier than in the south of England.So, for example, it has been proposed that in theShetland Isles the area of south-facing glazingshould be 20% smaller than for a comparablehouse at Kew.

At present, it is probably safest to conclude that itIs too early to judge precisely how designrecommendations for direct solar gains should bevaried to exploit solar radiation in differentiocations within the British Isies.

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Bartholomew, 0,,·1984, Possibilities for passive solar designs in Scotland, Occasional Paper ETSU L14, Energy TeohnologySupport Unit, Harwell.

2 Porteous, C" 1985, Partial solar heating for Hebridean housing using sunspaces and air collectors, The sLin at work In Britain,No. 20, UK-ISES, London.

3 Millbank, N" 1986, The potentia! for passive solar energy in UK housing, PO 102/86, Building Research Establishment, Garston,Watford.

18

19

Solar gain and glazing

If the sun's energy is to be used for heating, a building has to have a means ofcollecting it. Glass has particular properties which allow it to do this· by trappingheat from the sun.

Solar transmission through glassWhen solar radiation meets a transparent elementsuch as glass, it is• partiy reflected

• partly absorbed

• partly transmitted.

The fraction absorbed Is re-emitted as longwaveradiation from each side of the glass. The sum ofthe fractions directly transmitted and of thosere-emitted inward constitutes the total transmissionthrough the glass.

.r,~gl~

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~1

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,

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t~lJ'Iift~ at vaniJur l#t1!ff 1-lfrtti)b1,{rFigure 20

Figure 20 shows that the proportion of energytransmitted changes little within angles ofincidence from zero to 45', and then begins todecrease rapidly as the angle of incidenceincreases towards 90'.

Figures 21 to 24 illustrate the relative proportions ofreflected, absorbed and transmitted radiation forvarious giazing arrangements. Body tinting and/orcoatings applied to the glass modify thetransmission properties.

The proportion of solar radiation falling on a glasssurface which is transmitted is dependent on twofactors:• the altitude of the sun and the orientation and

inclination of the surface; these three result Inthe angle of incidence of the direct solarradiation falling on the glass

• the physical properties of the transparentmaterial.

20

,

Figure 24

6m'l1 i?trllf tin1'J- j/all (abffJrPIh.f)(.!l'Un)-- -_. -_ .. _"""_._-_._,,.

Figure 22

80

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Figure 23

Figure 21

21

The angle at which sunlight intercepts a windowalso determines the solar aperture or projectedarea of the window, This Is the area of a windowprojected onto a plane perpendicular to the sun'srays. The projected area becomes smaller as theangle of Incidence increases. The amount ofenergy transmitted is therefore reduced owing tothe decreased area exposed, as shown in Figures25 and 26. The projected area is also affected bythe position of the glass in relation to the face ofthe external wall, I.e. by the depth of reveal andframe thickness.

22

plaY}

!J0j,cftd wiJPh tfM'1IIlt'N w,1I1!leTMa! wj-Pt"tt

Figure 25

W,t t(cp,l WJitlOl,1/~ hay!: a I!WJIY"'I'D) f-diJ /I Y"t4, ('1 WMffr thlt 11 {fun",,.,.-

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Figure 28

~~

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Figure 27

Capturing solar radiation: thegreenhouse effect in buildingsThe process by which solar energy appears asuseful heat in a house is a complex one. When thesuns rays strike a window, part of the visibleradiation and Infrared is transmitted through tileglass. This is then absorbed by the opaquesurfaces within the room, such as the furniture,walls and floor, depending on their absorptioncharacteristics. These become warm, and in turnemit Infrared radiation at long wavelengths in alldirections.

When the long wavelength radiation strikes awindow from the inside, it is partially reflected andpartially absorbed, because glass is almost opaqueto radiation at wavelengths longer than about 2.5microns. The absorbed fraction is re- emitted oneither side of the glass. Some radiation is thereforetrapped inside, resulting in an increase in theinternal temperature. This phenomenon is knownas the greenhouse effect.

Figure 27 shows the spectral curve of the energytransmission of 6mm glass, Figure 28 shows thespectral CUNe of the energy emission of plaster,and Figure 29 shows the superimposition of thesetwo CUNes, illustrating the greenhouse effect, withthe hatched part representing the trapped solarenergy.

Solar gain as useful heatDuring the heating season the greenhouse effectoffers the possibility of reducing the amount of heatrequired from the house heating system. It hasbeen calculated that, on average, about 16% of theenergy requirements in existing houses are metdirectly by the sun, see Figure 30. 1 But thisproportion can vary greatly, depending onorientation, insulation levels, and infiltration rates.

Milbank, N., 1986, The potential for passive solar energyIn UK housing, PD102/86, Building ResearchEstablishment, Garston, Watford.

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Figure 29

23

Simple aryd complex passivesolar designsA great deal has been researched and writtenabout the potential for raising the proportion ofspace heating provided by solar energy above thelevels usual in conventional housing, and manydifferent types of system have been invented toexploit solar energy, see Figure 31.

In the UK climate, the potential for many of thesesystems is limited by the need for sophisticatedcontrol devices such as insulating shutters toreduce heat loss at times of low or zero solar gains(for example at night) and/or shading devices orblinds to prevent glare and overheating in summer,In advanced passive solar designs such devicesand their means of operation are an Integral part ofexploiting solar energy because they allow windowsizes to be increased to benefit from solar gainswithout incurring negative side effects,

Although advice is offered in this handbook on thedesign of such devices (because some designerswill want to employ them in their schemes), it is notnecessary to adopt complex solutions to design ahouse that benefits from solar gains. Simpleoptions are available. These are, in passive solarterms:• direct gain where solar radiation penetrates

through windows to heat the house directly

• isolated gain where solar radiation iscollected outside a room space, for examplein a conservatory, and transferred as required.

All houses can maximise the benefit from passivesolar heating by careful attention to certain designparameters:• site layout to minimise overshading

• orientation to maximise the potential for solargains

• room layout with main habitable rooms on thesouth

• window location, area, and glazing type

• heating system controls which react quicklyto solar and other incidental gains.

These are the main topics covered in the rest of thehandbook.

24

Ur~ I rafar 161-

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L j)jrfCt j/ANY'ft"t IJO/~ti)

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~

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25

26

SECTION 2Site layout and house planning

27

Site planning and solar radiation

The layout of buildings on a site, their relations to each other and to other featuresof their natural and built surroundings can all affect the extent to which solarradiation can be exploited.

General requirementsPlanning the layout of houses on a site isinfluenced by a wide range of constraints, most ofwhich lie outside designers' control, such as:• the shape, topography and existing

landscape of the site

• the position and type of the surroundingsinclUding adjoining buildings

• the iocation of the site access.

Within these constraints, the iayout must beplanned to satisfy a wide range of requirements,that include:• the density of the development

• the mix of house types

• vehicular circulation

• parking provision

• height restrictions

• building lines

• privacy distances

• tree preservation.

By their handling of these, designers can safeguardor prejudice the extent to which sunlight can beexploited on a site.

SunlightBSl's Draft for Development 0067: 1960 Basicdata for the design of buildings - sunlightrecommends that main living rooms receivesunlight for at least part of the day, see page 41. Tomeet this recommendation, iet alone make use ofdirect solar radiation for heating, considerationmust be given in the design of site layouts to:• the orientation of houses to benefit from solar

gain,

• the avoidance of overshading from, forexample, other buildings and trees.

28

OrientationThe effect of orientation on the auxiliary heatingdemands of a conventional house and a soiarhouse, as predicted by the computer simulationprogram SERI-RES, are shown in Figure 37. 1

These simulations are based on houses built atLinford, Milton Keynes. The solar house representsthe houses as they were designed with iargesouth-facing windows and small north facing ones.The conventional house Is the same but with theglazing equally distributed on the north and southfacades. Both houses are on unobstructed sites.

The results of the simulations show that:• orientation has a large effect on the

performance of a house which relies onsoutherly-facing giazlng to exploit solar gain.Changing the orientation of the solar housefrom due south to due west increases itsannual auxiliary space heating load by 17%

• the conventional house is relatively Insensitiveto orientation. Changing the orientation of theconventional house from due south to duewest increases the space heating load by only3%.

One of the main conclusions that can be drawnfrom these findings is that houses with largesoutherly-facing windows are more sensitive tochanges in orientation than conventional ones.

Orientation, as It relates specifically to windowlocation, is discussed in more detail on page 44.

NBA Tectonics, 1986, A study of passive solar estate layout, Report to the Energy Technology Support Unit, Report no.ETSU-S-1126,1986.

29

Oyershaqing, house spacing andprivacy distancesApart from existing features of the natural and builtsurroundings, the other factors which cancontribute to overshading are:• house spacing• site slope• roof shape.

House spacing Is likely to be determined by thedistances needed to maintain privacy betweenopposing house facades containing the mainhabitable rooms. There are no national standardsfor privacy distances, and advice should beobtained from the local planning authority.Acceptable privacy distances are likely to fall withinthe range 18m to 27m.

In order to minimise overshading of south-facingelevations, houses need to be set out so that theydo not cast excessive shadows over one another.In the NBA Tectonics study mentioned above,calculations were made to explore the relationshipbetween privacy distances, roof angles andresulting obstruction angles. Figure 32 shows theresults of overshading calculations in the case oftwo parallel rows of two-storey terrace houses ofmedium depth (7.2m) on a flat site. It gives, forvarious privacy distances (D) and various roofpitches, the resulting angle of obstruction.

The effect of overshadlng by detached houses wasalso studied by NBA Tectonics. They used thesimulation model SEAl-RES to look at the effect onauxiliary space heating load of overshading asouth-facing house by a row of detached housesto the south of it. The results showed that forhouses of a given plan area (104m2

), roof pitch(30°), and privacy distance (21 m), set out 7.2mapart, no difference could be discerned betweentwo layouts:• one with the houses directly opposite each

other

• one with them offset.

For a given roof pitch, narrow frontage housesblock less solar radiation than wide frontage ones.And, for a square plan, a pyramid roof blocks lessradiation than dual pitch. The direction of a dualpitch roof was found to make little difference. Inailthese comparisons, unless the houses beingshaded rely heaVily upon the contribution of solarradiation for their heating, the differences found areinsignificant. Even for the Linford passive solarhouses, the effect of overshading on increasedspace heating load was less than 0.5% for any ofthe cases described.

30

IIIo<1It !)~tJ"J #tl ""I l.flTcJ!I /'Y 4~ ~JJtr"'lIIrl""jfl' '1 15" ([~!""I'")

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/J <Ofl'

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lIe'f titcf/c (l'JrH-r)

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h.J-tJr 11"-£:;":.1 lUI.) OOltrJf.Gtw/J IWi:1!H

Figure 32

Techniques for assessingovershadingA graphical method using shadow prints, Figures33 and 34, was developed and applied in the layoutof the Pennyland low energy houses in MiltonKeynes, to help avoid overshadlng. 1 Its aim was toensure that olthe total incident solar radiatIonfalling on the south facing windows there was nomore than a 10% reduction caused by overshadlngfrom the other houses between November andMarch.

There is also a rule of thumb which can be used forthis purpose, This is considered to be equivalent tothe 10% shadow print, and is based on ageometrical method. The rule is that the midwintersun should just graze the roofline of one row ofhouses In order to penetrate the ground floorwindows of the next row,

The formula given on page 12 shows that themidwinter sun is at an angle of 90° - 23°27' - ¢where ¢ is the latitude. For Kew at a latitude of51°29' the maximum midwinter altitude of the sun i:'i15°04'.

If appropriate, Figure 32 may now be used, Thebox within the figure shows the relationshipbetween privacy distance and roof pitch for anangle of obstruction of 15°, In particular it illustratesthat decisions about roof pitch influenceovershading, particularly at close spacing.

In the case of dwellings which do not fit thedimensions in Figure 32 (for example bungalows,three storey houses, shallow depth houses) orwhich are on sloping sites, a simple sectionaldrawing may be prepared using the appropriatesun angle projected from the sill of the shadedhouse,

In the North of Britain where the maximummIdwinter altitude of the sun WIll be lower (1 0 lowerthan at Kew for each degree of latitude north ofKew) even greater attention WIll have to be paid toavoiding overshading. .

JhMtIW.fYJ~1r .pr IO~ W bY,. .rHuc:tlIJVl /J1 ItlCi)'4f [D!lty rMIA-tUl/l

Figure, 33

hnl1f/~Ii) ~rt'r.t&-, MtltMlI K"i1t1: Iltt- /~p...u­

11M rAuwl1j uft'- ~ rhfl.nw ,Ni~/i

Figure 34

Lowe, R" ChapmM, J, and Everett, R" 1985, The Pennyland project, Report by the Open University Research Group to theEnergy Technology Support Unit, reference ETSU-S-1 046, HarwelL

31

•

igure 36

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Figure 35

The interaction of orientation andovershadingIt has already been shown above that houseswhich rely on solar gains are more affected bychanges in orientation than conventional houses.Similarly they are more likely to be affected byovershading. And overshading and orientation caninteract in their effects on solar gains.

Other conclusions drawn from the study were:• for the conventional house once an

obstruction angle of 20° is reached it ceasesto be beneficial to orientate It towards south

• for the passive solar house it continues to bebeneficial to orientate it towards south up toand Including an angle of obstruction of 50°,

The InteractIon between orientation andovershading was explored in the NBA Tectonicsstudy by simulating the case where a conventionalhouse and a solar house are shaded by acontinuous terrace at specific angles ofobstruction, across a range of orientations fromdue west to due east. Figure 37, compiled from thestudy, shows some of the results, from which thefolloWing conclusions may be drawn:• for both the conventional and the passive

solar house the effect of obstructions isgreatest when the houses are orientated duesouth

• the effect of overshading is greater for thesolar house than for the conventional housefor all orientations.

Other techniques for assessing overshadlng Include• shadow masks with a stereographic sun

path diagram to plot obstructions (Figures 35and 36)

• a solar envelope used to define a volumethat can be occupied without causingovershading

• photographic methods

• a heliodon used with a physical block modelto cast shadows and allow rapid explorationof alternative arrangements

• computer-based graphical methods offeringthe same facilities as the heliodon.

Figure 37

32

Microclimatic planning and solaraccessThe microclimate of a site can be assessed, andthis is usuaily done with a view to planning thelayout to create a sheltered environment,particularly in terms of wind protection. If windspeeds around buiidings are decreased then:• infiltration into and heat loss from a bUilding

can be reduced• the external ambient temperature can be

raised

The relationship between the benefits of designingfor solar access and those of designing to enhancethe microclimate remains unquantified. It ispossible that there may be conflict between somesoiar recommendations and some microclimaticones. For example, the spacing requirements forsolar access may conflict with the microclimaticrequirements for wind protection. In such cases atrade-off will have to be made between theconflicting recommendations.

!,Ol~~:s ~~0~7d~;~~':;ed for their .' . terraces, which are the le8$t flexiblepotential to provide layouts with good hOlJae form, should be positioned firatsolar access and given the most favourable

* sitelJ should be plaoned to enhance the orientations; lJami-detached or detachedaccess of sunlight to habitable roomlJ houlJes are more flexible and may be

, as many houses as possible should be of ~~~:~:at~;.,~oads with less favourable

a southerly orientation '. * avoid 10cati~g terra"es to have gardens, as many houses as possible should be "

placed to avoid causing overshadlng and which are north· faCing and In permanent

*t:h~~:a~otvWhe:sf:d~~~~lw denstity h?lluslilng at . * ;;::~~~u::~~n~~I:~~frer a s.unnye sou ° eve opmen s WI a ow

solar access to the houses at the north 'any new trees should be carefully* tall buildings should be positioned where selected and positioned to prevent

they will cause least overshadlng, by overshading.being to the north, at corners ofdevelopments and at road intersections

33

Room arrangement

The orientations given to rooms within a house will affect both how comfortable andpleasant they are to live in and the energy consumption of the house.

There is no one set arrangement of rooms in ahouse that will best exploit solar energy. In eachcase, their arrangement will be influenced by ahost of factors beyond those concerned just withcomfort and energy efficlency, such as:• house type - detached, semi-detached,

terraced, bungalow, etc

• floor area, especlally as this relates tofrontage and depth of plan

• specific siting and planning considerations,particUlarly:

- density of development- access-view- privacy~ external noise sources.

Designers' handling of these other factors willaffect the extent to which they can take advantageof the solar energy available. The general rule tofollow is - arrange rooms to exploit the position ofthe sun as it changes throughout the day.

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34

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Figure 38

F/rft ftoor

A hour'" flA-l)/! H wtlt.:• fh~t1 IiI/inti rouy; J ftUf- JMti1• k/fch f{IJ/ ./Pllth ("'DO;'VJ I ]f4Jr r fi MrlA

• 7;t'tu11hf /1)1;'1,a,;",.,~ JU /?jfjfr/"

• cOlJr"s/tlf~ryFigure 39 .

SECTION 3Windows

35

Window systems

The specification of a window system· its type of frame and type of glass· affectsits performance and influences both comfort and energy consumption in the house.

General requirementsThe specification of a window system affects howwell It performs in providing:o sunlight

o daylight

o sound insulation

o thermal insulation

o ventilation

o weathertightness.

Of these requirements, for a given window size,o sunlight, daylight and sound Insulation are

influenced mostly by the choice of glazing

o ventilation and weathertlghtness areinfluenced by the choice of frameand

o thermal insulation Is influenced by bothglazing and frame specifications.

Sound insulationRequirements for sound insulationo between dwellings

o between a dwelling and an external sourcemay affect your choice of window construction.Advice on Keeping noise out of your house isavailable In the Glazing Manual 1 and in BS Code ofPractice 153 Part 3: 1972 Windows and rooflights ­sound insulation.

The Building RegulationsPart L of the Building Regulations, which deals withthe conservation of heat and power, makes nodirect stipulation about window construction.Instead there are restrictions on the total area ofwindow allowed, depending on the specification ofthe glazing type, see page 50.

The Glass and Glazing Federation, 1988, The Glazing Manual, G&GF, London.

36

Thermal performance of windowsConventionally windows are regarded as weaklinks in the thermal integrity of the externalenvelope of houses because they lose more heatthan the walls:• by conduction through the frames

• by conduction and radiation through theglazing

• by unwanted infiltration losses due to gapsaround opening lights and cracks betweenthe frame and wall through cold bridgingacross the structural opening.

The level of heat losses through a window dependson its:• exposure

• size

• frame type

• glazing specification.

Under some circumstances, solar gains throughwindows are also important. The energy balancebetween heat losses and solar gains is discussedon page 44.

Window frame performanceThe thermal performance of a window framedepends on:• Its degree of exposure

• the rate at which it conducts heat

• its weathertightness.

ExposureFigure 40 gives values showing the Influence ofexposure on the thermal transmittance (U-vaiue) ofwindows with different frame materials. The moreexposed a window is, the poorer its U-valuebecomes.

ConductivityThe rate at which heat is conducted through aframe depends on Its material and detailing. Woodand UPVC are poor conductors of heat whiieaiuminlum and steel are relatively good ones.Hollow plastic frames are also poor conductors butmetal reinforcements in them may provide extrapaths for conductive losses.

37

Figure 41

Figure 40

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Figure 41 shows classifications based on testingprocedures laid down in BS 5368 Methods fortesting windows.

Figure 40 gives the thermal transmittance ofdifferent frame milterials according to theproportion of window area that they represent. Itshows that the proportion of window openingtaken up by a frame has a considerable influenceon the overall U-value of a window.

WeathertightnessWeathertightness reqUirements for windows are setout In BS 6375 Part 1: 1983 The performance ofwindows which gives separate classifications for:• air permeability

• watertightness• wind resistance.

The purpose of these classifications is to assist inthe choice of windows for particular exposureconditions.

Selecting window framesA frame should be selected In accordance with BS6375 when stringent levels of performance arerequired. For example, where exposure Is severe, awindow achieving the specified levels ofperformance at the following pressures should bechosen:• at 600Pa in the air permeability test

• at 300Pa In the watertightness test

• at 2,300Pa In the wind resistance test.

Some frames include a thermal barrier or breakbetween the component in contact with warmroom air and those In contact with cold outside air.Where metal frames are selected, types with athermal break will not only reduce heat loss butalso reduce the risk of condensation on the frameitself.

Even in a moderate or sheltered situation, wherewind speeds are unlikely to cause permanentdamage that would reduce the performance of thewindow, there are still benefits in selecting awindow with a performance higher than iald downin BS 6375:• unwanted infiltration of cold air and loss of

warm air will be reduced

• draughts will be reduced, with the result thatcomfort conditions can be reached at lowerair temperatures.

38

If window frames are specified whose performancehas a high rating in wind resistance and airpermeability tests, it is not possible to rely onadventitious ventilation around opening lights toprovide sufficient air changes In the house. Insteadspecific arrangements have to be be made toenable ventilation to occur, Adequate provisionshould be made in the frames, in the form ofcontrolled or trickle ventilators, to provideventilation that can be regulated when windowsare closed, see Figure 42. Mechanical extractsshould be fitted in kitchens and bathrooms toremove steam before It condenses and providesthe conditions necessary for mould growth. 1 Ahumidistat can be Installed to measure the relativehumidity of the air and keep the extractor runninguntil moisture content is below the level set.

In fitting windows Into the structure, care should betaken to avoid cold bridges. Whenever possible, aform of construction should be chosen at lintels,sills and jambs whIch has the same U-value as theremainder of the wall. Local use of thermalinsulation can be used to reduce cold bridgingwhen, for example, steel lintels are used whichwould otherwise provide a direct path betweenInside and outside, see Figures 43 and 44.

fI,f-M/,O-#IJjj toCJ<./rr vp.ld;/a1Pr j/rrfbin !hr I1tM If >It W/r;pb/,tl fr4-m1-

Figure 42

IflJU/amn m4Ji7tlflJ1~III C:j;;-fI---r'--l+---

evPrt wh(t' /ilttfi/

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SE.CTIONigure 43

. .

. ' .

y

.....

' ''.'. <>." •.••../ ..... < ......}<

...•..•••«".>'.

Bullding Research Establishment, 1985, Surface condensation and mould growth in traditionally-built dwellings, BRE Digest 297,BRE, Garston, Watford.

39

Selecting glazingStandards for glazing are set down in BS 6262Code of Practice for Glazing for Buildings and inthe Glazing Manual.

Glass is a dense material which is a poor insulator ­it conducts indoor heat outwards in winter andoutdoor heat inwards in summer. However itssurface resistances are high so use of double ortriple glazing provides improved thermal insulation.The U-values for a variety of glass types are givenin Figure 45. The rate of heat loss through a singlepane of glass (U-value 5.6 W/m2K) is e;tbout tentimes that through a wall insulated to the 1985Building Regulations standard (U-value 0.6).

Heat losses through the glass are not the onlyconsideration in selecting glazing, The orientationof the window (see page 44), the severity of theseasons (see page 17), and the heat gained fromlighting, people and equipment also need to betaken into account.

For windows which receive direct sunlight, theshading coefficient can be important, see Figure46. This is a relative measure of the amount of solarenergy that is transmitted to the interior. Glasswhich transmits 87% of the total incident solarradiation is taken as the base for comparing otherglass types (this corresponds with clear glass ofbetween 3mm and 4mm thick). This is given ashading coefficient of 1,00. The shading coefficientof a glass provides a comparison between its solarradiation transmission and that of single glazing.So, just as its U.value is a measure of the amountof heat transmitted by a window to the outside, theshading coefficient of its glass Indicates theproportion of incident solar radiation admittedrelative to single glazing.

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Figure 44

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Figure 45

Figure 46

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~

Designers who want to take advantage of solargains for heating need to select glazing which• has low heat losses

and• maintains a high transmission of solar energy,

The relationship between these two factors formsthe basis for calculating what Is known as theenergy balance of a window, see page 44.

40

How to improve the thermalperformance of windowsThe balance between the heat losses and solargains of a single glazed window is poor throughoutthe heating season, whatever Its orientation, Butthe thermal performance of glazing can beimproved to levels above that of a single pane ofglass by the following means:• multiple glazing - adding one or more layers

of glass

• coated glasses

• gas-filled cavities,

Multiple glazingMultiple glazing improves the insulating propertiesof a window by adding a layer (or layers) oftrapped air and introducing additional glass withsurface resistances, But soiar transmission is aisoslightly reduced,

If the panes of glass are not hermetically sealed attheir edges, water vapour can condense on theinside of the outer pane as it does on singleglazing, Such units should have their cavityventilated to the outside to reduce the likelihood ofcondensation,

Factory sealed units contain dry air so thatcondensation cannot form inside the cavity anddust cannot enter. Variations in the external airpressure and temperature subject the units tostresses which, in time, tend to break down theiredge seals,

In sealed units, heat has to be transferred byradiation and convection to cross the air space(s),So the width of the air space in multiple glazingaffects thermal transmittance, The optimum widthof cavity for all types of glass is about 20mm, seeFigure 47, Beyond this size, a wider cavity allowsair to circuiate more freely, When this happens, airin contact with the warm sheet of glass rises andair in contact with the cold sheet settles, So acyclical movement of air is established, Thismoving air transports the heat from the warm tothe coid glass,

Figure 47 shows, among other things, that:• double glazing with a 15mm air space is

equivaient to triple glazing with air spaces of3mm

• low emissivity double glazing is better thantriple glazing for cavity widths above 8mm(and will be lighter in weight and of lessoverall thickness)

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41

• triple glazing composed of 6 +4 +6 +4 +6(total width 26mm) has a better U-value thandouble glazIng of the same overall widthcomposed of 6 + 14+6, although itwlll beheavier and more expensive.

Installing double or triple glazing improves theenergy balance and therefore the thermalperformance of a window. It also Improvesoccupants' comfort. Because multiple glazingresults In higher temperatures in the inner pane ofglass, the amount of heat radiated back Into aroom will be greater than with single glazing. So itis no longer uncomfortable to be be near a windowon cold days, see Figure 48. Comfort conditionscan also be achieved at lower room airtemperatures which can lead to energy savings.

Figure 48

Coated glassesThe heat absorbIng and radiating characteristics(termed emittance) of the two glass surfacesfacing towards the air cavity will affect the rate atwhich heat is radiated across it Special coatingsapplied to the surface of glass can reduce itsemissivity and so improve Its insulation value.

Low emissivity coatings, commonly called low-E,maintain a high transmission In the shortwavelength end of the spectrum where solarradiation is concentrated and so allow a highproportion of it to pass through. ThIs shortwavelength radiation is absorbed by the floor, wallsand furnishings of the room, and then re-radlatedat longer wavelengths. A low emissivity coatingcauses long wavelength radiation to be reflectedback into the room and so reduces outward heatlosses.

42

The U-value of a double glazing unit with lowemissivity coating is about 2.0 W/m2K comparedwith 3.0 for Its clear glass equivalent. Lowemissivity coatings result in a surface temperatureof the inner pane of glass only 1 or 2°C below roomtemperature, so improving comfort, see Figure 49,

The light transmission of a sealed coated unit is inthe range 60-75% as compared with 75-80% for anordinary sealed unit. This small reduction in Ifghttransmission has a neglfgible effect on the level ofdaylight.

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FIgure 49

Gas-filled cavitiesBy filling the cavities of multiple glazing units withcertain gases, such as argon, their insulation isimproved. The mix of gases in the air space affectsthe rate of heat transfer, For example, heat flowthrough a multiple glazed unit is reduced by 14%when the air space Is filled with Krypton. Areduction of as much as 20% may be possible byusing carbon dioxide.

43

Window orientation

Decisions about the orientation of windows affect the energy consumption of ahouse. But what specifications for windows should be used and where should theybe positioned to minimise consumption?

General requirementsThe 1985 Building Regulations make nostipulations about window orientation. But theBSI's 0067: 1980 Basic data for the design ofbuildings - sunlight recommends that livingrooms, and possibly bedrooms, should receivethree hours of sunlight on March 1st. This Isintended as a minimum. Where circumstancespermit or where solar heat is to be used In thewinter, then 0067 recommends that a moregenerous provision (preferably based on statisticaiexpectation of hours of sunlight) shouid beconsidered.

If fuller use Is to be made of solar energy by thedirect gain method, prolonged exposure toIntense sunshine in rooms used as collectionspaces will occur. This can have effects on bothoccupants and their possessions, see page 58.

In spaces that receive sunshine, windows shouidalso be located to give solar radiation access tosufficient thermal mass (see page 80) so thatenergy can be stored:• to even out excessive swings in dally

temperature at any time of yearand

• to avoid summer overheating.

The energy balance of windowsThe location of windows should also be consideredin terms of their energy balance. This depends onthe rate of heat loss through a window minus anyheat gains through it. Windows with differentorientations have very different energy balancesbecause of the varying quantities of direct anddiffuse solar radiation failing on them. The dallyenergy balances for unobstructed vertical singleglazing for different orientations are shown InFigures 50 to 53. These diagrams are drawn fromWilberforce's theoretical study based onmeteorologlcai data for Bracknell (51' 23'N, A'47'W) during the period 1967to 1973.1 An internaltemperature of 18'C, averaged over 24 hours, withcurtains not closed at night, has been assumed.

Wilberforce, R., 1976, The energy balance of windows, Building Services Engineer, Vol. 43, No, 12, pages 241~243.

44

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45

North-facing windowsThese have a negative energy balance, see Figure50, because over the heating season they allowmore heat to escape from inside the house thanthey admit from diffuse solar radiation. Computermodelllng conducted for ETSU shows a distinctrelationship between a house's area of north-faclngglazing and its requirement for auxiliary heating,see Figure 54. 1 If the area of north-faclng glazing isreduced, the requirement for auxiliary heatingdrops significantly.

The clear message from this piece of computermodelling is that, where possible, windows placedon the north side of a house should be kept to aminimum. In practice, the size of north-facingwindows Is likely to be affected by factors otherthan energy co'nsumption, such as daylighting andventilation requirements, the need for views, andexternal appearance,

South-facing windowsOver the heating season, south-facing windowswhich are double glazed or better can have apositive energy balance. Even if it is not possibleto fit glazing of this quality, it can still be desirableto concentrate any large areas of glazing on thesouth side of a house because windows In thisposition benefit most from solar gain, see Figure 17.

The extent to which a south-facing window makesa positive contribution to reducing requirements forauxllary heating in a house depends on:• its construction - single/double glazing, frame

type, weatherstripping, see page 37

• its area, see page 50

• how people use it (particularly for ventilation)and

• the temperature and duration of heatingmaintained within the room,

Wilberforce's theoretical study, like CAP Sclentific'scomputer modelling conducted for the EnergyTechnology Support Unit, suggests that doubleglazed south-facing windows in a house heated toa constant temperature can be a source of netenergy gain for ten months of the year, see FIgure51. Only December and January show net energylosses. But even these windows, which have a netpositive annual energy balance, lose energy in themiddle of the heating season,

7hf' fH~fitc~JhAp bttWffr/ nt},Tt1 fttC/~fJIlt Z.fYJ(j ~ 1fJ4ctr hf~;J1 Y"!-fu/Yp./J1H/'t

Figure 54

Energy Technology Support Unit, 1984, Passive solar study group: windows, ETSU, Harwell.

46

East and west-facing windowsWindows with these orientations receive 35% lesssolar radiation than south-facing ones in totalduring the heating season, see Figure 17. Becauseof this, they are net energy losers over the whole ofthe heating season, see Figures 52 and 53, unlesslow-E double or triple glazing is specified. Toreduce energy consumption, windows placed oneast and west elevations should be kept to aminimum - unless their area is thoughtfullyconsidered and their specification up-graded so asto improve their energy balance.

"Effective U-values"The glazing areas permitted by the 1985 BuildingRegulations, see page 50, are calculated on thebasis of conventional U- values. These only takeaccount of heat loss through a window due toconduction from Its warm to its cold side. Noconsideration Is given in the Regulations to otherfactors which affect a window's energy balance.

The concept of modified or effective U-values hasbeen developed, and popularised in the UK byPllklngtons, to provide a simple method ofrecognising the significance of two way energyflows across a window. An effective U-value is aconventional one modified to allow for usable solarradiation failing through a window during theheating season.

Figure 55 shows Pilkington's original estimates ofthe effective U-values of different window systemsIn relation to orientation. 1Figure 56 shows theresults of their more recent calculations to modelthe same effect.2 Here the equation employedcontained a utilisation factor to allow secondaryinfluences (such as thermal storage and mass,ventilation rates, internai gains and heatingschedules) to be taken Into account.

The two Figures differ as to the precise degree ofdifference between conventional and effectiveU-values. But they agree about the direction of theeffect. As before, these studies offer a clearmessage. They both suggest that, once theinfluence of solar radiation is included, south (orsoutherly) facing glazing has a much superiorenergy balance to other orientations.

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Figure 55

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Figure 56

Button j D" 1982, Energy options for housing design: glazing options, Journal of the Royal Institute of British Architects, Vol. 89,No. 10.

2 Owens, P., 1984, Low emissivity coatings on glass in windows, UK·1SES Conferenoe Proceedings (C38) I Coatings for energyefficiency and solar applications, pages 11 "21,

47

Orientatiop and energyconsumptionSoutherly orientations, free from obstructions andovershading by trees or other buildings, are to bepreferred if maximum use IS to be made of solarradiation during the winter. Of course, not allwindows can face south. Neither can they becompletely unobstructed. The effects of otherorientations and of various degrees of skylineobstruction are shown in Figure 57.

This figure is based on computer simulations of theLinford houses at Milton Keynes conducted byCAP Scientific. These suggest that windowsintended to provide useful solar gain shouldpreferably be orientated within 300 of south.1

Where this is not possible, orientations up to 45°either side of south realise half the possiblesolargain. Modest obstructions, of up to 10°, are notsignificant. But, once the angle of obstruction hasreached 25°, the potential solar benefits arereduced to less than half the unobstructed value.

However, other computer simulations of aconventional and a passive solar house conductedby BRE suggest that domestic energyconsumption is fairly insensitive to variations inorientation between +1- 45° of south.2 Suchvariations alter the total amount of solar radiationfalling through windows by only about 5 to 10%.Similar conclusions about the relativeunimportance of orientation for energyconsumption were drawn from a third set ofcomputer simulations of low energy housesconducted for BRE by CAP Scientific3 and fromstudies of the passive solar houses at Pennyland inMilton Keynes by the Energy Research Group ofthe Open Unlversity4, see Figure 58.

The orientation of a house has less effect on itsenergy consumption than the levels of insulationspecified for the opaque elements in its externalenvelope. In addition, computer simulations,conducted by CAP Scientific for the BRE study oflow energy houses cited earlier, suggest that theeffect of changes in orientation on consumption isreduced as the Insulation value of the opaqueelements is increased.

Figure 57

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~ ~t-......~- ---~~~ rt(CT~) . '5000

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Milbank, N" 1986, The potenllal for passive solar energy In UK housing, PO 102/86, Building Research Establishment, Garston,Watford.

2 Bloomfield, D., 1987t lEA Task VIII· Dooumentation of parametrlo study, Building Researoh Establishment, Garston, Watford.3 Nielsen, 0., 1984, Options on energy, Arohltects Journal, Vol. 179, No. 21, pages 47-62.4 Lowe, R., Chapman, J. and Everett, R., 1(:'85, The Pennyland project, Report by the Open University Energy Research Group to

the Energy Technology Support Unit, Contract No. E5NCON/1046/174/040.

48

The findings of all these computer simulations ofthe effect of orientation on energy consumption arenot unequivocal. This is not surprising, The studiesare not strictly comparable, They each modeldifferent houses. And each set of simulations isbased on slightly different sets of assumptions. Atpresent, it is probably safest to conclude thatorientation can be Important, depending on howwindow area is distributed between elevations, seepage 50, But it may be less important than theU-value of walls and roofs. And it seems to becomeless important as these values are Improved.

49

Window area

The glazing placed on each elevation will have different effects on energyconsumption, depending on its area and its orientation. How much glazing shouldbe put on each facade to optimise energy consumption?

The Building RegulationsNo stipulations are made in the BuildingRegulations 1985 about about permitted areas ofglazing for different orientations. Instead Part L ofthe Regulations simply states that, where singleglazing Is installed, the aggregated area ofwindows and roofllghts shall be no more than 12%of external wall area, Figure 59,

Table 1 In Section A 1.1 of Part L2/3 stipulates thatareas which are double glazed may have up totwice the single glazed area, see Figure 60.Likewise areas which are double glazed with a lowemissivity coatIng or triple glazed may have up tothree times the permitted single glazed area, seeFigure 61, The areas used in this calculation shouldbe those between the finished Internal faces of thebuilding. Section A 1.1 of Part L2/3 specifies theother conditions governing this calculation,

Procedure 3 of the Regulations describescalculated trade-offs which permit other ways ofincreasing window areas. This Involves:• first calculating the rate of heat loss through

elements of the building allowed if thestipulated U-values and percentage glazedareas are followed

• this establishes the allowable heat lossesthrough the fabric elements

• then larger areas of (for example, double)glazing can be proposed which do notexceed the allowable heat loss,

Trade-offs can also be made between walls androof U-values so long as the allowable heat loss isnot exceeded. A similar trade-off can be madebetween glazed and opaque areas. So, If aproposed wall construction has a better U-valuethan that required by the Regulations, thepercentage of glazing may be increased to thepoint at which the calculated rate of heat loss is nogreater than if the stIpulated maximum U-values ofopaque and glazed elements had been used. Inpractice, this means that where triple glazing isemployed (and depending on how window area isdistributed between elevations), the Regulationsmay allow completely glazed south-facing facadesto be used, see page 55.

50

J/n.J(r J/47..i~ lfrPJ'i,t Mt1) vhW w~Ufi~fil1Jclft,{ f jrdt~ wa«r!J /tr;vzk

Figure 59

lJot1lJlr3In..in'1, tolAr~ "if WtlJ),f

ipf#tic~t I jJ/btYwattr blav"kFigure 60

The lintel, jamb and sill of a window must either bedesigned to comply with the U-value of theremainder ofthe wall or be included as part of thewindow area, see Section A 1.1 (t).

Building costs and window areaSo the 1985 Building Regulations containprocedures which allow the glazed area, as apercentage of the external envelope of a house, tobe increased. But is it worthwhile to do so?

Davis, Belfield & Everest have produced anelemental breakdown of building costs forhousing1

, see Figure 62. This shows that opaqueelements in external walls typically account forappproximately 10 to 20% of total constructioncosts. Windows and external doors usually onlyaccount for half or a third as much, ie. betweenabout 5 to 15%. However, in the passive solardesigns produced for ETSU's House DesignStudies1, the total cost of windows was about thesame as the total cost for the opaque wall elements.

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Figure 61

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ftAfl~rl""r 1f-·5tof·f If ) .. r. 7·'PIreq-n:t4jrr e/1#1 hltJtb /?r"ak~oUh1 t7f pui/'J/~ fAtuI 'Y1J!lIffJ j,!J J/~ /JA1) e..

Figure 62

Traditionally, windows occupy about 15 to 20% ofthe exposed wall area of British dwellings, seeFigure 4. Any increase in this proportion is likely toIncrease building costs. Even single glazing in astandard softwood frame is 1.5 times moreexpensIve per square metre than the insulated wallit replaces.

Davis, Belfield & Everest Consultancy Group, 1986, Newbuild design studIes: summary of quantitative data from the costanalysis project, Report to the Energy Technology Support Unit, DB&E, London.

51

Improvi'1f.! the insulation value of a wall from 0.6 to0,3 W/m K increases its cost by about 15%, seeFigure 63, Improving the insulation value of glazingIs more expensive, as Davis, Belfield & Everest'scasted comparison of window systems shows, seeFIgure 64. Double glazing In a standard softwoodframe can cost up to half as much again. However,the cost of windows Installed in a cavIty walldecreases as the window area increases, seeFigure 65. As these figures reveal, there are nogrounds - at least in terms of initial costs - forincreasing window area as a proportion of theexternal envelope.

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Figure 63

52

The optimum area for south-facing glazing

Nevertheless, is it worth Increasing south-facingwindow area in an attempt to cut running costs byreducing expenditure on energy?

Single glazing on a south facade is a net heat loserover the heating season in houses with typicalinternal temperatures. So there are no grounds forincreasing south-facing window area if singleglazing is installed.

\\~'rgJ #/)d I ~"I{b.'/" j!4uo

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Double glazing (or single glazing where nightinsulating shutters are consistently operated) isthermally neutral over the whole of the heatingseason, Ie. heat loss and gains are approximatelyequal. So varying the proportion of double glazingin a well-Insulated south-facing facade is unlike tohave a dramatic effect on a house's annual heatingrequirement.

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FIgure 64

53

BRE has conducted computer simulations of theeffect of increasing the glazed percentage of asouth.faclng elevation for a conventional, privatedeveloper's middle market semi-detached house of80m2, see Figure 66. 1 Here the originalsouth-facing elevation contains 18% glazing andthe house is assumed to be heated between07,00-23.00 and sited at Kew, Figure 67 shows theresults of this modelling. Increasing thesouth-facing window area to 30% (of thesouth-facing wall) brought a 1% reduction inannual heating requirement. Increasing it to 50%brought about a 4% reduction,

From the slope of the graph in Figure 67, therewould appear to be no obvious optimum area fordouble glaZing on south- facing facades. With eachIncrease in area, a small Improvement in energyconsumption is obtained. Figure 66

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Both predict only modest energy savings. Energyconsumption is suggested to be fairly insensitive tochanges in the area of south-facing windows wheredouble glazing is used. As Figure 67 shows, withtriple glazing, the savings are more significant.

These two studies are not directly comparable,They employ different computer models tosimulate different situations, using differentassumptions, But they suggest similar conclusionsabout south-facing window area. They broadlyagree about the direction of the effect.

Similar predictions of the effect of increasingsouth-facing window area on energ~ consumptionhave been published by Pilkingtons for one roomof a house sited In Glasgow, see Figure 68.

Bloomfield, D., 1987, lEA Task VIII- Documentation ofparametric study, Building Research EstablIshmentGarston, Watford.

2 Button, D., 1982, Energy options for housing design:glaZing options, Journal of the Royal Institute of BritishArchitects, Vol. 89, No. 10.

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rrifJlt

54

Computer simulations conducted for the HaslamHomes house at Energy World, Milton Keynes, seeFigure 69, predict that, once designers haveaccess to window systems with:• U-values of about 0.5W/m2K (such as

quadruple glazing argon filled sealed units)and

• a solar transmission of 0,5,then any increase in the area of south-facingglazing up to 100% may reduce annual energyconsumptlon,1

In practice, factors other than energy consumptionare likely to be just, if not more, critical in decidingwindow area. For example, where south-facingwalls have more than 20% glazing, the BRE'ssimulations cited above predict that overheatingmay, on occasion, be severe for bUildings ofconventional British construction" unless specificmeasures are taken to prevent or control 1t, seepage 80.

The balance between north and south-facingglazing

The simulation studies cited so far suggest thatthere Is little value in increasing either:• a house's overall area of glazing

or• simply the glazed percentage of Its

south-facing elevation.

But, if the overall area of glazing is kept constant, Isit worth:• reducing the area facing north

and• redistributing the saved glazing from the north

to the south?This strategy was adopted by Greenberg &Hawkes in their design to reduce the energyconsumptIon of Wlmpey's Supawarm houses. 2

Their design had less glazing than the Wlmpeyoriginal, resulting in a cheaper building envelope.North- facing glazing was also reduced, leaving agreater proportion to be orientated to the south,see Figure 70. Computer simulation showed thatselective double glazing to the north and eastwindows was more cost effective than inclUdingthe south-facing ones as well,

Figure 69

a 0

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WtItJ'hJ ft1;tt'NY/Y) !JPlAfL, /'}'Jp'Jif/e#.Figure 70

Littler, J. & Ruyssevelt, P., 1986, The role of thermal mass in UK housing, in The efficient use of energy in buildings, 2ndUK-ISES Conference (C46), UK·ISES, London.

55

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Figure 71

------------

BRE has also conducted computer simulations totest the effect of this strategy. The standarddeveloper's house (described earlier, see page 54)originally had equal areas of glazing on its northand south elevations. BRE modelled the effect ofreducing the northern window area to 10%. Thisled to a predicted saving of 0.4% of the annualheating requirement. Transferring the window areataken from the north to the south-facing elevationproduced a further reduction of 2%.

So a small cut in consumption (which requires noadditional Initial cost) can be achieved by reducingnorth-facing glazing and redistributing the windowarea saved to an elevation facing due south. But,for other orientations, this strategy is morequestionable. For houses orientated up to +/-45°of south, reductions in consumption will still bemade but they wlll be smaller, falling to 1.5% at 45°in the south of England. For houses orientatedmore than +/ -45° of south, redistributing glaZingarea In this way will worsen energy performance,see Figure 71.

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Greenberg, S. and Hawkes, 0 .. 1984, Report on Wlmpey passive design study, ETSU Passive Solar Programme, Greenberg &HaWkes, London.

56

SECTION 4Glass shading, insulation and thermal mass

57

Overheating and the effects of direct sunshine

Sunshine is not always wanted inside houses, especially in the summer. And, evenduring the heating season, sitting in direct sunshine is not always a pleasantexperience. Consideration should be given to steps to avoid both these conditions.

General requirementOne of the main challenges of expioiting solarenergy is controlling daily and seasonalfluctuations in air temperature in rooms that aresunlit. If this is not done successfully, importantconsequences can follow:• occupants become uncomfortable on ciear,

sunny days - especially during the summerbut even during the heating season

• they may pull curtains or blinds - if they dothis during the heating season, potentiallyusefui solar gain may be lost

• they may open windows to induce extraventilation - if they do this during the heatingseason, purchased heat and potentially usefulsolar gain Is lost.

This unwanted chain of events can be avoid by acombination of design strategies:• properly sized windows

• shading, preferably external

• a balanced use of thermal mass

• a responsive heating system and controls.

Summer overheatingOverheating due to solar gains has not beenreported as a major problem In Britain in houseswith conventional areas of glazing, see page 55.And there is little documented evidence about thisphenomenon in houses with enlargedsoutherly-facing windows. There Is, however,evidence of very high temperatures inconservatories, see page 91.

in the Pennyland scheme (currently the onlyreported monitoring in this country of houses withincreased southerly glazing), it was concluded thatsummertime overheating is not a significantproblem where:• glazing is 40% or less of the south facade

and• buildings are of medium weight constructioll. 1

Lowe, R., Chapman, J and Everett, R., 1985, The Pennyland projeot, Open University Energy Researoh Group, Report for theEnergy Technology Support Unit, Contract No. E5A/CON/1946/174/040.

58

However, computer simulations conducted by BREsuggest that overheating is likely to be severe(defined as above 25 DC for 10% of the time) insouth·facing living rooms where• windows are more than 20% of the wall area

and• the building is of conventional masonry

constructlon. 1 Large windows on the east andwest facades are also predicted to causeoverheating problems at specific times of theday.

The effects of direct sunshineSoutherly-facing glazing, even if it does notproduce overheating (in terms of excessive airtemperatures), may cause other adverse effects:• discomfort to occupants (glare or heat stress)• fading or even fire risk to furniture and fittingsbecause of the intensity of direct sunshine.

The former can be dealt with, like overheating, bythe use of shading devices, To avoid the latter,textured window glass (such as hand-spun bullionand Flemish glass panes) should not be specifiedfor doors or windows subject to direct sunshine,2

1 Bloomfield, D" 1987, lEA Task 8 • Documentation of parametric study, Building Research Establishment, Garston, Watford,2 Goldstone, B" 1982, Hazards from the ooncentration of solar radiation by textured window glass, BUilding Research

Establishment Report, Department of Environment, HMSO, London.

59

Occupants and shading/insulating devices

Few households in Britain currently use external blinds or shutters. Nearly all usesome form of internal covering at their windows. Where glazing is designed toexploit solar radiation, careful thought must be given to how occupants' use ofwindow coverings will affect its energy balance.

The effectiveness of devicesThere is little documented evidence available abouthow window shading and InSUlating devices areused in practice. A study of measurements madeat 70 passive solar houses across the USAconcluded that the usefulness of such devices Ishighly dependent on how frequently and how wellthey are used. 1 Movabie window insulation, forexample, has to be operated properly for at least70% of the time if any net energy benefit is to begained. It is suggested that this makes the viabilityof manually operated devices questionable. Unlessconsistent and appropriate use can be assured,up-grading the energy performance of the glazingitself Is presented as a more efficient andcost-effective strategy.

Window coverings and privacyTypical window coverings in Britain are translucentnet curtains and/or opaque fabric ones. Both areusually employed to improve privacy. Net curtainsprevent outsiders from looking in during thedaytime. Opaque curtains offer the sameprotection at night. The latter may also beemployed, on occasion, to intercept unwantedsunshine. But curtains are only thought ofsecondarily, if at all, as a means of reducing heatloss.

What is known about the use of shading andInsulation devices in Britain comes from themonitoring of passive solar houses In MiltonKeynes.2 There the energy savings realised fromsouth-facing glazing were, In practice, smaller thanpredicted because of occupants' use of windowcoverings. The solar absorption of windows wasreduced by the presence of net curtains,half-drawn blinds and insulating shutters. A parallelInvestigation revealed that, given the particularlayout of the Pennyland housing (where largewindows to habitable rooms face on to public

Swisher, J" Bishop, R. and Frey, D" 1983, Effectiveness of movable window insulation in passive solar homeSj Report by theNational Association of Home Builders Research Foundation to the US Department of Energy, Rockville, Maryland,

2 Everett, R" Horton, A., Daggart, J. and Willoughby, J., 1985, Linford low energy houses, Report by the Open University EnergyResearch Group 10 the Energy Technology Support Unit, Report No, ETSU-S-l025, Harwell, axon.

60

--------------------------

circulation areas), residents chose the increasedprivacy offered by nets and curtains rather than thereduced fuel bills offered by uncluttered windows. 1

A Dutch study of occupants' reactions to rollershutters found an unwillingness to accept them asa valid substitute for traditional window coveringssuch as nets or curtalns.2 Roller shutters weredisliked for a range of functional, aesthetic andsocial reasons beyond simple expense. And theseare findings which would probably be repeated in aUK study.

Designers who specify shading or Insulationdevices that require the intervention of occupantsneed to think about their design and use. Properattention needs to be paid to:• the layout of sites and the Internal

arrangement of houses to avoid problems ofprivacy in rooms or conservatories whereglazing Is intended to trap solar gain

• occupants' own needs and priorities (whichmay place less emphasis on reduced energyconsumption than other aspects of theirdomestic lives such as avoidance ofoverlooking)

• occupants' understanding of how to operatedevices, especially since (even if they haveencountered them before) they are unlikely tohave thought of them specifically asmechanisms for improving the energybaiance of windows.

Mlekle, S, (ed,), 1984, SocIal survey; Pennyland residents, Milton Keynes Development Corporation, Saxon House, MiltonKeynes,

.2 Dubbled, M. (ed.), 1984, Energy saving by using roller shutters, Commission of the European Communities, Brussels.

61

An occupants' manualTo provide occupants with information on:• how to use shading and insulation devices

and• how to operate their heating systems without

incurring energy penaities,designers can provide them with an instructionmanual.'

This would:• describe the design of their home

• explain how it is meant to perform in terms ofenvironmentai conditions delivered andenergy consumed

• offer advice on servicing and maintenanceand

• give guidance on what to do in the case ofmalfunctions and repairs,

Points to remember• movable Insulation may be less efficient

and cost effective than up-gradlnlltheenergy performance of glazing Itselfautomatic or motori,sed shading orinsulation devices are likely to be moreeffectiVe than manual ones but onlyspecify them If the system is very simple,reliable and the cost Is reasonable

, before installing or recommendingmanual dellices, give carefulconsideration to whether occupants arelikely to operate them in ways compatiblewith reduced energy consumption

• 'if manual or motorlsed devices areInstalled, make sure that they are as,convenient to use as possible

• provide simple and clear Instructions tooccupants about the operation of anymovable shading ot insulation devices(automatic, motorised or manual);stressing the need for appropriate dailyoperation If insulation Is to provide anenergy benefit.

Building Research Establishment, 1987, Pre~design and post-construction issues, Design Information Booklet 8, Passive andHybrid Solar Low Energy Buildings, International Energy Agenoy, BRE, Gerston, Watford

62

63

External shading devices

Shading devices can be used to exclude unwanted solar radiation. They are moreeffective if they are installed outside rather than inside glazing.

General requirementsThe diurnal energy balance of glazing in thesummer, especially daytime heat gains (asopposed to night-time losses), can be Important tooccupants' comfort. Excessive heat gains can beavoid by installing a shading device. Theeffectiveness of these depends principally on their:• position

and• shading coefficientalthough other factors such as orientation,exposure and the temperature difference acrossthe window are also Important.

Shading devices are most effective when they arelocated outside glazing since unwanted solarradiation is best intercepted and excluded before itreaches the glass.

The shading coefficient of a device Is a relativemeasure of the amount of solar energy falling on itwhich is transferred to a building's interior, seepage 40. So a device with a shading coefficient of0.2, for example, means that it permits 20% of thesolar radiation incident on it to pass through.

External shading devices can be:• part of the roof

• part of the wall in which a window stands

• part of the window system itself.

For example, shading from solar radiation can beachieved using:• roof overhangs

• sun screens or louvres

• awnings or blinds

• shutters.

The further south a building is located (in thenorthern hemisphere), the more important shadingeast and west windows becomes while the needfor south-facing shading grows less so. This isbecause the high altitude of the summer sun Insouthern latitudes which results in less direct solarradiation failing on south-facing windows, seeFigure 16.

64

Roof overhangsSouth-facing windows are most effectively shadedfrom summer solar gains by horizontal projections,such as roof overhangs, which shade the surfaceof a window from direct radiation. During thewinter, they allow winter sun to reach windowswhen solar gain is needed.

The effect of a projection depends upon itsgeometry. Figure 72 illustrates this for a simpleoverhang above a south-facing window for thewinter and summer solistlces. Because the sun'smovement is symmetrical around June 21,south-facing horizontal projections which shadeglazing in the summer wHl also partly shade it incooler months (March and April) when the sunmight be welcome. So, unless carefully planned tosuit their specific orientation and latitude, fixedhorizontal projections cut off potentially usefulsolar radiation in the autumn and spring.

A graphical method for establishing the depth ofroof overhang required for specific roof pitches atparticular latitudes can be found in the Solar anglereference manual. 1

External louvresThe effectiveness of horizontal louvres in shading awindow also depends upon their geometry. But thereflectivity of their material is important as well .Their geometry determines how high the sun mustbe above the horizon before the louvres block allthe direct sunlight. Their reflectivity determines howmuch light penetrates Indirectly by being reflectedoff the surface of the louvres, see Figure 73.

East and west-facing windows are more effectivelyshaded from solar gains when the sun is low In thesky by vertical projections, such as fixed ormovable louvres.

If air can circulate between a shading device andthe glazing, and if the latter is completely shadedfrom direct sunlight, then solar heat gain can bereduced by as much as 80%.2

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1 Sibson, R., 1983, Solar angle reference manua), John Wiley & Sons, New York.2 American Society of Heating, Refrigeration and Air Conditioning Engineers. 1967. ASHRAE Handbook of Fundamentals.

ASHRAE, New York, p. 485.

65

External awningsThe capacrty of a external awning to shade awindow from the sun is dependent on how opaqueits material is to both direct and diffuse radiation,The surface of an awning should be a light colourto minimise the amount of sunlight absorbed. Solarenergy absorbed by the shading device raises itstemperature. This heat can be transferred to thewindow in two ways• by radiation

and• by raising the air temperature between the

device and the glazing.

Light coloured materials are more effectivebecause they stay cooler and transfer less heat tothe window, see Figure 74,1

Ont¢t?t!PI4 .

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Figure 74

Hastings, R S & Crenshaw, R" '1977, Window designs to conserve energy, NBS Building Science Series 104, National Bureau ofStandards, Washington DC,

66

To be effective, an awning must be designed toprovide adequate coverage of the window area forits specific orientation. A south-facing window onlyneeds a minimal horizontal projection to becompletely shaded throughout the day in thesummer, But the sides of the awning should beclosed to prevent the sun from shining in fromeither end in the morning and afternoon, seeFigure 75. East or west- facing windows need anawning which extends further down in order toprovide protection from low angle sun, againespecially early in the morning or late In theafternoon.

External blindsThe performance of a translucent material used ina shading device can be described In terms of Itssolar gain factor which is made up of:• the fraction of the solar radiation falling on a

material that passes through itpius

• the energy absorbed by the material and thenre-radiated into the building,

Solar gain factors are latitude dependent. Figure 76gives values for the UK of various types ofmaterial. 1

External rOller blindsThese can provide shading from summer sun andreduce winter heat loss through windows.Horizontal slats on a roller can be housed at thehead of a window or at the ridge of a conservatory.These can be lowered to form an opaque barrier tosummer sun, blocking both direct and diffuse solarradiation. Blinds can be lowered with slats open tocut off sunlight but allow circulation of air todissipate heat absorbed by the blind,

In the winter, they can be lowered with slats closedto trap air between the blind and the glazing to actas an additional insulating layer. At night, which isthe period of greatest heat loss, closed blindsproVide added insulation, decrease exposure tothe cold night sky, and further reduce heat loss.

Figure 75

77fe rTf jltll./~

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Figure 76

Achard, , P, and Glcquei, R, (eds), 1986, European Passive Solar Handbook. Commission of the European Communities,Brussels,

67

External shuttersLike external blinds, external shutters have theadvantage that, if occupants operate them asintended (see page 60), they can provide shade inthe summer and reduce heat loss and infiltration inthe winter. Like blinds, a shutter's shadingperformance depends upon how well heatabsorbed by it is dissipated to outside air, Louvreswhich can be adjusted to block the sun but allowair to circulate improve shading performance. Theyare reported to be 30% more effective than similarInternal shading devices.1

Winter heat loss through a window with closedshutters Is reduced because the air space betweenshutter and glass proVides additional resistance tooutward heat flow. A shutter's Insulatingperformance depends on the air tightness of thespace between it and the glazing it covers. Thisconflicts with shading requirements, Pivotinglouvres which can be opened and closed meetboth needs, However even fixed open louvresreduce winter heat loss by• sheltering the insulating film of air at the outer

surface of the glass from the scouring actionof the windand

• reducing infiltration through window cracks.

Achard, P, & Gicquel, R. (eds), 1986, European Passive Solar Handbook, Commission of the European Communities, Brussels,

68

69

Internal shading and insulation devices

Traditional window coverings, such as curtains and blinds, are not an effective wayof excluding unwanted solar gain. But they can help reduce night time heat loss.

Internal shading devicesExternal shading devices are more effective thaninternal ones. When the sun shines on a shadingdevice inside glazing, a proportion of its energy Istransmitted or absorbed and then re-radlated toother room surfaces. While the uncomfortableexperience of direct sunshine may be avoided,unwanted heat gain within the room still occurs.

The effectiveness of an Internal device is mainlydetermined by the solar energy which is:• reflected

• absorbedand

• transmitted.

A increased proportion of unwanted solar radiationcan be rejected by an internal shading device if itssurface facing the glazing has a low emissivity. Thiswill then reflect near Infrared radiation which theglazing will re-transmit to the outside.

During the heating season, shading or insulationdevices should be opened when glazing is sunlit toallow solar radiation into the interior. Wheredevices are left closed, they will absorb the heat,much of which is then re-radiated straight back tothe glass.

During daylight hours in the heating season,shading or insulating devices should be storedclear of the window area so that penetration ofsolar radiation is not impeded. Even curtains(Whose primary purpose is not to alter a window'senergy balance but to provide privacy) are bestfitted to a track which allows them to be openedclear of the window in order to allow all availablesunlight into the room If and when it is wanted.

70

Internal insulation devicesAlthough inefficient at preventing undesired solargain, movable window insulation can be a cheapand effective method of reducing heat lost throughglazing by:• conduction

and• air infiltration.

Movable insulation reduces conductive losses bytrapping a volume of air which is a poor conductorof heat, see Figure 77. However, unless cracksaround glazing are tightly sealed, heat is alsocarried to the outside by air escaping around theframe,

Wv'll/teuPIJ ifIro/4.tH fl»hJ ;rptJrJ1

Cof!(/~17 Cilryf11"trov;r,w'li; WM'fJ'/ rpott/ air-

Infiltration losses can be considerable. Forexample, it has been estimated that a poorly fittingsingle glazed window can lose twice as much heatby infiltration as through Its glazed area byconduction. 1

The amount of heat lost by such air leakagedepends on• the size of the cracks around the frame

and• external wind speed and direction,

Computer simulation suggests that, In a typicalhouse in the UK, 40% of all Infiltration losses aredue to leakage around closed windows. 1

Weatherstripping is an effective method ofreducing infiltration, see page 38, MovableInsulation which has well sealed edges can alsoreduce infiltration losses If it fits tightly betweenwindow reveals. The effect of a trapped volume ofair is increased If the edges of a device are sealedsince this prevents convection currents inside theroom from circulating across the window. Suchinsulation should be put in place in the eveningsand throughout the night since these are significantperiods of heat loss. If left open, insulating devicesmay cause part of the wall they cover to remaincool and result in condensation,

Nets, lightweight and insulatingcurtainsLittle is known about the effect of the use ofcurtains in occupied houses on comfort conditionsor energy consumption. But their performance in atest-cell has been investigated in Britain. 1 This

Figure 77

Ruyssevelt, P, & Littler, J" 1985, Movable window Insulation, School of Building, Polytechnic of Central London, Final Report tothe CommIssion of European Communities, EUR 9996 EN, Brussels,

71

study found that, when closed, lightweight curtainsare ineffective for shading purposes - almost halfthe solar radiation failing on glazing is transmittedinto the room since the curtain operates as a solarcollector, see Figure 78.

Nor are net curtains an effective means of reducingheat loss. The material In them typically fills lessthan 30% of the window area which they cover. Asa result, they reduce heat loss through a windowby radiative transfer by less than 10%, see Figure79.

These test cell measurements suggest that drawingcurtains at night can have a very significant effecton window heat loss. They show that lightweightcurtains can reduce it by nearly 20%, whileheavyweight ones (with a thermal lining) can do soby 40%. However, North American studies suggestthat, when closed, both insulated and un-Insulatedcurtains have similar effects on a window's rate ofheat loss if their edges are not seaied. Figure 80shows measured results from a North Americantest-cell study of the 'winter effectiveness' - ie. thepercentage reduction In heating costs - of threetypes of curtains, including one foam-backedvariety sold as 'energy saving,. 2

Results for each type of curtain were similariy poor,ie. only marginally better than a bare window. Withdouble glazing their 'winter effectiveness' was evenless. It was concluded that, because curtains hangso loosely in front of a window, cold and heated aircan easily move around them. A heat- shrunkplastic vinyl film (0.15 mm thick) was much moreeffective since it sealed more efficiently. Similarresults have since been found in another NorthAmerican study where measurements were madein an unoccupied room of a house.3

Energy Monitoring Company Ltd, 1987, Studies inpassive solar test cells, Test oell studies 1: windowcoverings, report to Energy Technology Support Unit.reference ETSU-S-1162.

2 Tomany, R., 1982, The measurement of windowtreatment effectiveness in reducing residential heatingand cooling costs. ASHRAE Trans, 88 (2): 235-248.

3 Nicol, K., 1986, The thermal effectiveness of varioustypes of wIndow covering, Energy and Buildings,9:231-237.

72

Tipe "I U(Yilt!" fhl.()IVtJ C'~f/t'I&1t

lIet Ui,ta,,,,0.76({,Vle wet:>ve)

Net w. rTlUlJ o. r?(will. we~r1e)

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~ntWl-'J1IIt U/"ta,,, 0.63(0 e", 1000 - / rOf) )

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Figure 78

Tlpe. of turTIlI'v7 PevcHltlA.J' rV/oldiOI/

Net curtail! 1-(fi;1e fIItave)

Net &lNrt'aill 2(Wi~e weave)

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Nijnt UNrra/~ 11-1(/1,etllv,'f weiq/lbt)

pevcMfaJl r~uctIP" (~ flW loffthYPUj/!J wi",,'Dw WIth PJ/fY'UftTjptr 4f &/'vY'ta,/J I (f-KprU'!i); 1</[

;evCWlfagt of/lteld IO!! thY'D1o/1UY/cpl/f#; rll'/jle flll2.eJ WI';? ow}

Figure 79

"fpe --f CMdtll'.1 PWClf,tt~t r"l"lct/04in htt:tril1lJ CPrts

/flO'/o ~0<1 /1o?[it/Ull' rt"aiYl ,

677- YP-Ij'''' / I to5B 1, acet-ate U/,rn.i",

6/f.'(ocot{..-. /36 iI ;0tyI to?(foa... back.e~) alr"tai",

fetU~ O.fIjMI>/ v'-"ii film n /0 4-3

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Figure 80

Figure 81

~

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"

T!U fhM'::J 1~I'mMC~ ofMftv}14-/ 'b/IJIIJJ

..

Internal blindsSunlight absorbed by a blind raises its temperatureand this heat is then dissipated into the room by:• radiation to room surfaces

and• convection of room air in contact with the

warm surface of the blind.

The effectiveness of roller blinds, measured Interms of their shading coefficients when used Inconjuction with different types of glazing is shownin Figure 82. 1

The colour of a blind and and Its degree ofopaqueness affect its performance, see Figure 81. 1

Hastings, S R & Crenshaw, R., 1977. Window designstrategies to oonserve energy, NBS Building ScienceSeries 104, National Bureau of Standards, WashIngtonDC,

Ill/Vi () chV4CfiYirhci

Cj/A-rr type.7iMfUJceurt, (J!~tr-t(1 O!41p,t(" ,

wjV1t cP/~'W white ~.itHC

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Figure 82

73

Venetian blindsVenetian blinds can be used for three purposes• to reflect summer sun back out through a

window

• to reflect daylight on to a ceiling to providedeeper light penetration into a room

• to prevent overlooking.

These uses are not necessariiy compatible. Foreach, the slats of the blind may need to be set at adifferent angle.

The extent to which venetian blinds act assun-shades also depends on their colour. Figure83 shows the performance of two differentcoloured blinds with their slats set at 45' when thesun Is at right angles to the slats. 1

North American figures for the effectiveness ofvenetian blinds, measured in terms of their shadingcoefficients when used in conjunction with differenttypes of glazing, are shown in Figure 84. 1

These differ from figures for the performance ofvenetian blinds as measured in a British test-cellstudy, see Figure 85.2

Despite their differences, both the North Americanand British figures suggest that venetian blinds arenot partlculariy effective as sun-shades, especiallywhen closed. In this position, they act as a solarcollector. Air trapped between the blind and thewindow is heated, rises and then flows into theroom. Nor are venetian blinds more effective atreducing heat loss. Their performance here is on apar with net curtains, reducing heat loss by about10% even when ciosed.2

Hastings, S R & Crenshaw, R., 1977, Window designstrategies to conserve energy, NBS Building ScienceSeries 104, National Bureau of Standards, WashingtonDC.

2 Energy Monitoring Company Ltd, 1987, Studies Inpassive solar test cells, Test cell studies 1: Solardistribution, solar lost and wIndow coverings, report toEnergy Technology Support Unit, referenceETSU-S-1162.

74

!

/JeYC~t~t- of YM/~rIOY/

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Figure 83

BltnJ citVIKtiVI ftrc j

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AmWlcan ShrUfn.J C()e/flclentr .ofvu/etftt'1 b/f~QJ .

Figure 84

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.

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" \

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Figure 85

Insulating blindsMany versions of this type of blind are marketed Inthe USA but few products are currently available inthe UK. The Roman blind, see Figure 86, hasmagnetic strips to seal the edge of the blind.Where this is done, a U-value of 0.8 has beenclaimed when used in conjunction with singleglazing,1 MUlti-layer blinds are also available, oftenwith low emissivity surfaces, which require greaterstorage space when not in use, see Figure 87.Where such systems have sealed edges, a U-valueof 0.4 with single glazing has been claimed. 1

Do-it-yourself quilted roller blinds, with analuminised layer In the centre of the material,require a large storage area at the window head.These blinds use a crude sealing system of timberbattens or sprung hinges. 1

Computer simulations suggest that a doublelayered blind, incorporating two low emissivitysurfaces and effective edge sealing, is mosteffective In reducing energy consumption in theUK1These simulations also showed that it isdifficult to achieve energy savings with windowinsulation devices that do not offer a means ofedge sealing.

Figure 86

---

<.-Iatjlv'f ~XPPw&O 1i-{DvlJ1 ~tY' r,~ Cer

f--JpMefl

~---rtaftD)t-tMPI ANl)he-a,)

Figure 87

Ruyssevelt, P. & Littler, J., 1985, Movable window insulation, School of Building, Polytechnic of Central London, Final Report tothe Commission of European Communities, EUR 9996 EN, Brussels, p. 14.

75

Internal shuttersAn Internal insulating shutter can have a significanteffect on the energy balance of a window, Figure88 shows a computer prediction of the effect of a25mm polystrene shutter, closed during the hoursof darkness, With such a shutter fitted, asouth-facing double glazed window becomes asource of net energy gain throughout every monthof the year, Similarly shuttered, a single glazedwindow has an energy balance which equals thatof unshuttered double glazing,1

However, while the principle behind shutters issimple and their operation is technically sound,their success in use depends upon the willing andappropriate efforts of occupants, see page 60,

There is little monitored evidence of the effect ofinternal shutters in occupied houses, Figures areavallable from tests conducted on an unoccupiedhouse In Canada where a set of shutters cut from25mm polystrene and without any special edgesealing were placed in position at 18,00 each nightand removed at 08,30 each morning for 18 days,2

When the outside temperature was DoC, theshutters reduced the energy consumed to heat thehouse by 12%,

,(

-.t~ t 1r-----.--,--------,,------,--,--.----.----,r-,~

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Figure 88

Everett, R" 1980, Passive solar In Milton Keynes, Energy Research Group, Report ERG 031, Open University, Milton Keynes,page 25,

2 Nicol, K., 1986, The thermal effectiveness of varIous types of window covering, Energy and BUildings, 9:231-237,

76

Although internal shutters hold out the promise ofimproved comfort and reduced fuel consumption,In practice, their use Is problematic.

Internal shutters come In three forms:• push-in

• bi-fold

• roller.

Push-In shutters are the simplest, cheapest, haveto be tailor-made, and are inconvenient to storewhen not in use, see Figure 89. A polylsocynurateinsulating board with a compressable edge sealhas been developed in Denmark1 where thebuilding regulations require a high degree ofthermal Integrity for the building envelope.

BI-fold shutters fold away, concertina fashion, seeFigure 90. While easier to store, It is more difficultto achieve an effective seal for each panel of theblinds when placed against the window.

Figure 89

Figure 90

Byberg, M. et ai, 1985, Insulated shutters, Commission of the European Communities, Brussels.

77

rolle-r

holfDW PVCJryrnentJ

f-PVC r;~r. r.,'acJu

Roller shutters are quite common in Europeanhouses though they are usually installed to provideshading or security, not insulation, The shutterillustrated, see Figure 91 , comes from the USA. Asis clear from the illustration, it is both expensIveand requires a large storage space at the head ofthe window,

Figure 91

78

79

Thermal mass and window design

In the past, thermal mass has often been included as a means of storing surplusenergy in buildings designed to exploit solar radiation. Now it is thought moreimportant as a means of moderating daily temperature sWings.

What is thermal mass?Thermal mass is a way of referring to the heatstoring capacity of materials used in buildingconstruction. Its identifying characteristic is theability to absorb and later release heat. Anyphysical material - solid, liquid or gas - has thisability to some degree. The storage capacity of awide range of materials used In construction,expressed in terms of their specific heat (J/kg'K)and density (kg/m\ is given in Figure 92.

The ability of a material to store the solar energywhich falls on it also depends on its surfaceabsorptance and its emissivity. The absorptance ofsolar radiation depends not on colour but on themolecular structure of a material's surface.Polished metals have low absorptances and iowemlsslvities (typically between 0.1 to 0.15) in thefar-infrared region of the spectrum. Most otherbuilding materials, including glass, have highemissivities and absorptances (typically 0.9 to0.95) in the far-Infrared region, regardiess ofcolour. The solar absorptance and emissivity of arange of materials and surface finishes used Inbuilding construction are given in Figure 93.

The thickness of a material used also affects itsability to store heat. The layers of the materialclosest to an Irradiated surface participate most inthe process of heat accumulation.

The thermal response rate ofmaterialslightweight materials do not store energy verywell. When exposed to even a small quantity ofsolar energy, their temperature increases rapidlybecause they have a low voiumetric thermalcapacity. Lightweight materials then transfer heatgained from solar radiation to a room byconvection from their (relatively) hot surfaces. Somaterials with a low thermal capacity should not belocated where solar radiation will fall directly uponthem or over-heating may result.

Heavier materlais, with higher thermal capacities,can absorb greater amounts of solar radiation. Sothis type of material can safeiy be placed in directsunlight without risk of over-heating occurring.

80

M4tfri~1 IN'1'lJi; JjJfiJjtihff.t

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Figure 92

/11 Po te-vi~l AlP!.,", ftil~7

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Figure 93

Buildings of heavyweight construction (heavymasonry or concrete with external Insulation) warmup and cool down slowly. They are said to have aslow thermal response.

Lightweight buildIngs (timber framed constructionwith insulation located within the frame orinternally) have a short response time, warming upand cooling down quickly,

Neither heavyweight nor lightweight construction isnecessarily the best strategy for reducing energyconsumption, In most UK housing, a balancebetween the two would be advantageous.Lightweight materials can be used for a fastthermal response in areas which receive no solarenergy or in rooms which are only intermittentlyused. Heavier materials are more appropriatelyused In positions where they can be used toabsorb Incoming solar radiation, see Figure 94.

The purpose of thermal massThermal mass can be useful in houses for tworeasons• to moderate temperature swIngs which result

from sudden peaks In solar or Incidental gains

• to store surplUS heat for periods when solar orincidental gains are reduced and auxiliaryheating is required.

In the UK, where intermittent heating Is common,computer simulations conducted by BRE suggestthat the primary function of thermal mass is tomoderate temperature swings rather than to storesolar energy.1

Until recently, it was believed that the more thermalmass put Into a house in which solar gains were tobe exploited, the better its performance would be,So much of the advice offered in literature onpassive solar house design over the past decadehas focussed on where and how to include morethermal mass than Is found in conventionalhousing, In Britain, this no longer accepted asnecessary or even desirable, Now, on the contrary,It is thought that inclusion of excess thermal masscan lead to Increased heating bills and reducedcomfort.

, I ,

"'0""_. '-"'j i \'"

ThPrJ1llIt( /I11ft/! /P(;A'ffJ to rtttitlrI,j'nt JI/A-r ra)/~tJ();-;

Figure 94

Bloomfield, D" 1987, lEA Task Vlll • Documentation of parametric study, Building Research Establishment, Garston, Watford,

81

It appears that only the surface layer of thermalmass plays a role In diurnal heat control In the UK.Monitoring of occupied houses at Abertrydwrrevealed that only about the top 10 mm ofplasterboard underwent temperature changes in aspace exposed to solar radiation. Similar resultswere found in the monitoring conducted atWoodbridge Cottage. 1

These findings suggest that in the UK, Ifconventional masonry construction and windowareas are employed (see page 55), then the massInherent In large areas of plastered internal walls issufficient to control diurnal temperature swingsduring the heating season.

Where particularly lightweight forms ofconstruction (such as timber frame) are employed,especially in conjunction with large areas ofsoutherly-facing glazing, additional thermal masswill need to be provided.

Whatever the form of construction employed,however, summertime overheating is more fittinglydealt with by shading and ventilation strategies.Solar radiation is best deait with before itpenetrates the external envelope, not afterwards.

The dangers of too little and toomuch thermal massIt has been estimated that, on a clear sunny dayduring the heating season, solar energy can entera well-Insulated building with large south-facingareas of glazing four times faster than the rate atwhich it is needed.2 Some of this energy has to bestored If air temperatures are to be kept atcomfortable levels. With too little thermal mass,over- heating may occur.

Littler, J. & Ruyssevelt, P" 1986, Role of thermal mass In UK housing, in The effloient use of energy in buildings, 2nd UK~lSES

Gonference (046), UK·IS.S, London.2 Lebens, R. & Myer, A. (eds), 1983, Draft CEe Passive Solar Handbook, Commission of the European Communities, Brussels, p.

102. The text was changed in the 1986 Preliminary Edition, edited by Achard and Glcquel, to read Ilthe solar energy gained maylargely exceed the heat demand" (page 4,15),

82

Computer simulations also suggest that, In a well­insulated house, there are likely to be few days Inthe heating season when solar gains exceed thedemand for heating throughout the dwelling. 1

Adding too much thermal mass may Increase fuelconsumption. Heat from the heating system will beabsorbed by the thermal mass In the morningsbefore the sun (if it comes out) can have any effect.So too much thermal mass will also have importantconsequences for comfort and costs:• the low temperature of the thermal mass

during the warm-up period when the heatingfirst comes on will give rise to low radianttemperatures in a space

• occupants will need compensatory higherthan normal air temperatures to feelcomfortable

• for both these reasons, runnIng costs in theform of the annual heating bill will increase

• capital costs will be increased as wellbecause during the warm-up period, the peakdemand placed on the heating system will begreater - so a larger heating plant thannecessary will have to be Installed

• outside the warm-up period, the heating plantwill operate at low efficiency and this too willincrease running costs.

Where to locate thermal massThe best way to get solar heat into mass is to allowthe sun to shine directly on it. It gets much hotterthis way than if it Is merely in contact with warm air.Walls and floors are the most convenient place tolocate thermal mass for storage purposes.

Walls used as thermal mass do not necessarilyhave to be directly sunlit since they will be heatedby scattered sunlight and by warmed air in theroom by convection. But they do have to belocated in the space that receives r~diatlon toperform correctly.

Where the mass in a floor is to be used for storagepurposes direct sunlight Is required for propercharglng.~ This Imposes limitations on the kinds offloor finishes and coverings that can be used onfloors intended to act as thermal mass. Carpets, forinstance, are Inappropriate because they insulatethe floor from solar radiation. Appropriateconstructions are illustrated in Figure 95.

..r-q}fArY'~ niH OIl Ilc p~'i i

.................................... :. k'fcn# _

~~J7ZZ4IFf-ctlntYftt PltltkrN .:.~ IJVVVVV\ '4l.i<" 1)\ fl'e-lfl'!'IJH COlIC.

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Figure 95

Littler, J. and Ruyssevelt, P., 1986, Role of thermal mass in UK housing, in The efficIent use of energy In buildings, 2nd UK·ISESConference (C46), UK-ISES, London.

2 Architectural Energy Corporation, 1986, Design guidelines for energy efficient passive solar homes, AEC, Boulder, Colorado.

83

The thermal mass provided by walls and floors canbe divided Into three types:• primary, those parts of walls and floors

directly sunlit

• secondary, those areas not directly sunlit butto which heat can be transferred by radiationfrom those that areand

• remote, areas hidden from view of primaryand secondary ones to which heat transferoccurs by convection only,

The last of these is the least effective way ofproviding thermal mass,

Where it is possible to spread primary thermalmass over most of the surfaces in a room, thentheir temperature will rise less and so fluctuationsin room air temperature will be reduced,

Points to remember• the main purpOlle of thermal mass in uK

houses Is to moderate diurnaltemperature swings, not to store surplussolar energy until It is needed. The massInherent In plastered lightweightblockwork should be sufficient for thispurpose

plasterboard on stud work (Inner skin ofexternal walls or internal partitions) maynot be sufficient (in sunlit rooms withlarge areas of unshadedgla;zing) tomoderate temperature sWings or avoidoverheating

84

. ';::::,;,:<:-;:~:;:,:,:;::,:,:::):::i:- ::i;::,:'::-i:-:"'::'::::\(:':,':':::::";;:::..::,::· ::::.:':'/: '':;::;ii;::;::!:':,;,:::,:::,::,:?::;':D:,:(:::i: :;:,::;?",:;,:;,:;,::!(:!:,:;::;;,=:,\:,::;;,:.;,':'

'. use Ollllioor aS,thermal m8$s placeslimits on the forms of construction andfinishes Which clln be used withoutundermining its performance

• proper provision lor shading andventilation Is a better way of avoidingsummer-time overheating thandependence on large amounts of thermalmass

; excess thermal mass can lead to ,Increased capital costs, poor comfortconditions, and higher than nllcessaryenergy consumption

SECTION 5Conservatories

85

Conservatories in energy efficient design

Conservatories offer an opportunity to combine economical seasonalaccommodation with energy savings. Achieving this combination in practice willdemand careful balancing of environmental conditions with an acceptance oflimitations on use.

What is a conservatory?A conservatory is a predominantly glazedenclosure accessible directly from a dwelling butphysically separated from it by some combinationof door, wall and/or window.

To be classified as a conservatory for the purposesof the Building Regulations, a conservatory musthave a transparent or translucent roof. If it does, itmay be exempt from some of the Regulations, seepage 110. If instead the roof is opaque, theseexemptions will no longer apply.

There are three broad ways in whichconservatories can be used:• as an attached greenhouse

• as habitable accommodation- for living purposes- for access or circulation- for storage

• as an energy saving feature.

These uses are not fully compatible. An unheatedconservatory, which is used as habitableaccommodation only when ambient environmentalconditions allow, can reduce energy consumption.But it is not suitable as a greenhouse for thosekinds of plants which need to be protected againstlow winter temperatures. If a conservatory isheated to protect plants, or so that it can be usedas habitable accommodation throughout the year,then this will lead to Increased fuel consumption.

So conservatories occupy an ambiguous place inenergy efficient design. They are highly visiblearchitectural features which have beenIncorporated in many houses that are intended tobe energy efficient by exploiting solar gain toreduce auxiliary heating demand. But they are alsocommonly depicted as a bright and luxuriousspaces, fully furnished complete with exotic plants,which can accommodate the full range of livingfunctions. If heated, conservatories can easilybecome a symbol of conspicuous consumption.

86

Figure,96

Figure, 97

The ambiguity which surrounds the contribution ofconservatories to energy saving arises because oftheir glazed fabrIc. This offers a degree ofenvironmental control over precipItation, airmovement, humidity, and temperature. But thiscontrol is less than is provided by the main fabrIcof the house. Glass is a poor Insulant and it allowslarge conductIve heat losses. It also leads,because of the greenhouse effect, to large solargains. These can raise the temperaturedramatically within a conservatory at tImes of directsunshine. It Is the thermal performance of glazingwhIch results In both advangages anddisadvantages associated with the conservatory.

87

Conservatories as attached greenhouses

Many people will use their conservatories for growing plants. If they are to do sosuccessfully, this will affect both the environmental conditions they need tomaintain in the conservatory and their home's fuel consumption.

Environmental conditions ingreenhousesHistorically, conservatories were devised asspaces for nurturing plants, not for housingpeople. 1 So the environmental conditions in themare like those which occur In greenhouses. Theseare typically used:• to enable controlled propagation of seeds

• to extend the growing season

• to preserve valuable plants through the wintermonths

• to provide a protected environment fornon-native or exotic plants and flowers.

Conservatories have an energy and temperatureadvantage over a detached greenhouse. Theybenefit from heat losses from the house whichkeep them warm. This marginally increases theirpotentiai for plant conservation. But for moreextensive use as a greenhouse, a conservatory hasto be heated.

The environmental conditions that can bemaintained in greenhouses fall into four types:• the alpine house· unheated• the cool greenhouse· minimum

temperature about 6'C• the intermediate greenhouse· minimum

about 10'C• the warm greenhouse· minimum about

15'C.

Huxley, A., 19S3, An Illustrated hIstory of gardening, Macmillan, London.

88

Energy consumption in attachedgreenflousesFigure 98 shows the temperature requirements ofeach type and the kinds of plants that can begrown In them. The energy consumptionassociated with these requirements has beenpredicted by computer simulation for a 13,5m2

conservatory attached to standard semi-detachedhouse.2 The results show that the cost of heatingthe conservatory to 15°C is half as much as heatingthe main house. It is clear that USIng heatingappliances In a conservatory, or allowing warm airfrom the house to leak into it through an open dooror window, carries a very heavy energy penalty,

1jjJt tf ! rUY1hPtA!(. A/!/~ t hpt1fl, (pol) hilt! flllrJy l"!tvlJ1P:)ltt tt W4r1?7

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1. In Itl' 'pe-/ r1'f jl'"tVln,t.tu if if errtAtit4 Ii ~YJtrol MaX/mum J'u1'J'J//7V ttJnlf.Figure 98

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.' ··Withopt!(rilSlhgsolargains coolingmeblJanisrn inthe sl.iful1'let'.*' :-. . )(f~{~f:.tQ:r:jJI:~:n't~;::~_t1~_<~S~O,¢i_(1t:~:~t,;lltg-~ .'-",:.' :.:::':.'.:',-.::,..':" '.:,', -::, :"- :.:,.' ,:'.: ,',:',' .: .'.',-.': .

2 Penz, F., 1986, The energy Implica~lons of keepIng plants In conservatories, Eclipse Research Consultants, Cambridge,

89

Conservatories as living space

Many people will want to use their conservatories as extensions to their livingspace. Depending on the times of year at which they do so, this use will affect boththe environmental conditions they need to maintain in the conservatory and theirhome's fuel consumption.

Environmental conditions inconservatoriesCertain factors will influence the extent to whichconservatories can be used as habitableaccommodation:• large dlurnai temperature swings

• large seasonal temperature range

• direct sunlight and risk of glare

• condensation risks

• high infiitration rates - air leakage

• watertightness - especially In driving rain.

Other factors also need to be taken Into account:• security

• privacy• safety of glass, glass fixings, and frame

• maintenance and cleaning.

Awareness of these factors at the design stage isvital. If they are understood then either• users can be advised of the environmental

conditions that are likely to occur at varioustimes of the day and yearor

• designers can tailor the conservatory todeliver those environmental conditionsparticularly suited to intended uses.

Conservatories as storage orcirculation areas .Items stored in conservatories must be able towithstand the range of expected environmentalconditions described below.

For some occupants, temperatures outside thenormal comfort range may be acceptable inaccess or circulation areas, such as entrancelobbies. For others, especially the elderly or Infirm,they will not. Designers need to give carefulconsideration to the kinds of people who are likelyto live in their schemes before deciding to useconservatories for storage or cirCUlation. In allcases, privacy and security need to be maintained.

90

FIgure 99

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S 0 N b J F M A

Comfort conditions inconservatoriesIn a conservatory which is used primarily as Hvingspace, habItable for as much of the year aspossible, the emphasis should be on achievingacceptable internal comfort conditions. These willdetermine the practical uses to which aconservatory can be put and its amenity value asliving space.

Internal conditions: measuredresultsVery little data has been collected on comfortconditions in conservatories. In a monitoringexercise conducted by the Royal College of Art, asingle glazed conservatory attached to arefurbished terraced house in Milton Keynes wasinvestigated. 1 Temperatures in the conservatorywere recorded over the period from October 1980to December 1981. Figure 99 shows the house andconservatory. Figure 100 records the monthlyaverage air temperatures. The conservatorytemperature remained above ambient but belowliving room temperature. Figure 101 recordstemperatures over a period of four days In Aprll1981. It illustrates the extremely high temperaturesthat can be reached in conservatories duringperiods with high levels of insolatIon. It also revealsthe wide diurnal swing in temperature from 40°Cmaximum to 10°C minimum.

Figure 100

....... !iY'17 Y(Jo;J/j

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Ford, B, 1983, Thermal performance monitoring of aterra.ce house with conservatory, New Bradwell, MiltonKeynes, Report by the Department of Design Research,Royal College of Art, to the Energy Technology Support

Figure 101

91

Internal conditions: computersimulationsSimilar trends have been predicted by simulation,such as those performed by CAP Scientific forETSU using the thermal simulation modelSERI-RES, to simulate conditions in the house withconservatory shown In Figure 102. The plan of thehouse is shown in Figure 124. Figure 103 showsthe results for a 48 hour period In February.

For the whole of this period the conservatorytemperature was at least 4°K above ambienttemperature. During the first day, which wasovercast, the conservatory temperature evenduring the daytime remained at about 4°K aboveambient. During the second, sunny, day theconservatory temperature rose to reach 20°C for ashort period in the early afternoon.

J.eh7i-?tfllt~i; hDV1~t with (,p/jJtrvllfli')",JI-) 111 Sll11ulqliO;'JJ: ;1Ilr; rff I'J,PJ-f/l­

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Figure 102

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Figure 103

James, R and Dalrymple, G., 1985, Modelling of conservatory performance, in Norton, B (ed), Greenhouses and conservatories,UK·1SES Conference Proceedings, UK-ISES, London.

92

Simulations petiormed by Penz at the University ofCambridge produced the predicted petiormanceshown In Figure 105,1 Maximum temperatures forDecember, January and February were in theregion of 19-22°C with minima about 2-4°C, givinga range of 20°K. Maximum temperatures from Aprllto October exceed 30°C and they exceed 25°Cfrom March to November.

Figure 104

N c F M A

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Figure 105

Penz, F., 1983, Passive solar heating In existing dwellings, unpublished PhD thesis, University of CambrIdge,

93

A series of computer simulations were conductedat the University of Louvain of an unheatedattached conservatory in the Belgian climate at alatitude similar to the south coast of England. 1 Theeffects on internal temperatures of theconservatory resulting from changes to its glazingand to the elements dividing it from the house wereInvestigated, see Figure 106, This study suggeststhat double glazing a conservatory with clear glassor with coated glass raised its minimum Internaltemperature in the winter by about 3 to 4°K. Butthis measure also raises its maximum temperatureIn the summer, thereby increasing the risk ofoverheating and the need for shading andventilation.

Figure 107 shows the effects of changing theorientation of this conservatory between East andWest. One conclusion that may be drawn fromthese simulations is that minimum temperaturesvary only slightly (by about 2°K) with the changesIn orientation, while maximum temperatures varymuch more. The maximum for an east facingconservatory is 21°C, compared with 23°C for awest facing one, and 30°C for one facing south,

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Figure 107

Ie Paige, M" Gratia, E, and de Herde, A, 1986, Architecture et climate: guide d'alde a la conception bioclimatlque, Services deProgrammatlon de Is Politi que Sclenlfflque, Bruxelles,

94

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Figure 108 shows some of the maximum andminImum temperatures achIeved in thisconservatory in Belgium during the day in variousmonths of the year:• maximum temperatures from May to

September reached 36 to 40°C

• minimum temperatures were about 2°C inJanuary

• for about three months of the year, theconservatory is unlikely to reach a habitabletemperature of 20°C except for a possibleshort period between midday and earlyafternoon on sunny days

• at the beginning and end of the heatingseason the conservatory will be above 20°Cfor up to 8 hours on sunny days.

The monitored results presented in Figure 100confirm the same trend for the UK climate as thesesimulations. Overall, conservatories appear to becapable of achieving a phenomenon known as theseasonal shift: the Internal temperature In aconservatory in March is likely to be equivalent tothe ambient air temperature about six weeks later.

o'-t--+---f:--+--+-+-4o I/- /2. I' 2.0 2Jt

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Figure 108

Conservatory form andperformanceInternal temperatures In conservatories arise fromdecisions about their form and fabric.Comprehensive information about theinter-relationship between all the design variablesinvolved Is not available. It is possible, however, togive preliminary advice about most of them.

OrientationSouth facing conservatories will achieve• higher maximum temperatures

and• higher minimum temperatures In both winter

and summer than those orientated In otherdirections.

FormAttached conservatories achieve• higher maximum temperatures from solar

gain because of their large areas of glazingand

• lower minimum temperatures because of thelarge conduction losses from theconservatory and because of the smalleramount of heat conducted through the morelimited area of external elements of the housethat are covered.

95

Recessed conservatories achieve• lower maximum temperatures because of

their smaller glazed areaand

• higher minimum temperatures because oftheir reduced conduction iosses from theconservatory, and the iarger amount of heatconducted through the increased area ofexternal elements of the houses they cover.

Ratio of glazing to solid elementsLarge areas of glass cause• higher maximum temperatures because of

solar gain

• iower minimum temperatures because ofincreased conduction iosses.

Glazed roofs in particuiar cause the greatestoverheating problems in the summer.

Insulation values of wall covered byconservatoryIf a conservatory covers mainiy windows or doors,more heat will escape into the conservatory fromthe house so raising its minimum temperature.However, these same windows and doors willprovide less thermal mass in the conservatory soIts maximum temperature will also be raised duringsunny conditions.

Glazing and frame typesSpecification of double glazing andlor coated glasswill achieve• higher minimum temperatures

and• higher maximum temperatures.

Timber frames or metal frames with thermal breakswill also achieve• higher minimum temperatures

and• higher maximum temperatures.

All conservatories with direct access from thehouse, whether intended to be used asaccommodation or not, should be glazed inaccordance with BS 6262: 1982 Glazing forbuildings. Also many cheap prefabricatedconservatories have very crude fixings forattaching glazing to their frames. These may be asafety hazard if the conservatory is used as livingspace.

96

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Figure 109

97

Conservatories and energy performance

Conservatories act in a number of related ways to modify the energy balance ofhouses to which they are attached. These need to be understood if designers are toexploit the potential conservatories possess for reducing energy consumption. Thissection only deals with how unheated conservatories do this.

The energy I?alance ofconservatonesConservatories can modify the energy balance ofhouses to which they are attached in three differentways:

conservatory as buffer zone• providing an extra iayer of buiiding fabric and

of trapped air, so reducing the rate of heatloss from the house

conservatory as high temperature solar collector• because of the greenhouse effect,

conservatories can be used to capture andstore solar radiation

conservatory for pre.heating ventilation air• by raising the temperature of in-coming air,

drawn through the conservatory beforedistribution about the house for ventilationpurposes.

Anyone of these three types of conservatory cancontribute to reducing total energy consumption if:• there are no heating appliances in it

and/or• Its internal temperature is not raised by

occupants' allowing heat from the rest of thehouse to warm it.

A conservatory which is heated, by either of thesemethods, will increase total consumption, seepage 88.

98

Conservatories as buffer zones

Buffer zones are parts of a building which, during the heating season, are attemperatures between the warmed interior and the cold outside air. They canreduce the rate of heat loss by decreasing the temperature difference across thatportion of the fabric of the heated accommodation which they cover.

The buffer effectsIn a conservatory, these result from:• the additional layer of building fabric

Whatever materials the conservatory is madeof, whether transparent or opaque, it:- provides an extra layer of building fabric- keeps the enclosed surface of the main

fabric dry- reduces wind velocity at the surface of the

main fabricand

- traps a volume of air.In effect, it increases the insulation value ofthe external wall elements that it covers togive an improved resistance to outward heatflow,

• solar gain in the buffer spaceWhere a conservatory is mainly transparent,the fabric effect is enhanced by collectingsolar radiation within it (described In moredetail later). This gain raises the temperaturein the conservatory and so further reducesthe temperature differential across theexternal fabric of the building,

• the draught lobby effectBy reducing the wind speed and windpressure on the windows and other gaps inthe external wall which it covers, aconservatory reduces infiltration lossesthrough these elements,

Although the direction of the prevailing wind mustbe taken Into account, the additional fabric and thedraught lobby effects operate under mostconditions and are both independent of orientation.But both effects are reduced in influence• as the insulation values of the walls and

windows are improvedand

• as the building becomes more airtight.

In a double glazed, well-Insulated and well-sealedbuilding, heat loss through the fabric iscomparatively low, In these circumstances, theaddition of a conservatory makes only a smallimprovement to reducing fabric and Infiltrationlosses,

Figure 110

Pointstotemember..If it is desired to rnaXimlseacdfiservatory'$bUffer~ffects,itshot.Hd:· ••••.• \\./>\»<•.••..

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·<h~Yl!a~QlIt~~rIYQtll:1r1t~tibl"l.· ..•.......•..........

99

Conservatories as high temperature solar collectors

Solar radiation which enters a conservatory raises the temperature of the airtrapped inside by means of the greenhouse effect. In the UK, direct use of this solargain for space heating purposes is limited.

The greenhouse effectThe principles behind this form of solar gain aresimilar to those described for windows. Shortwave solar radiation enters a conservatorythrough the glass, see Figure 111. It is thenabsorbed by the surfaces of the wall and floor andconverted into heat. This is then transferred bythree means:• by convection to the air in the conservatory

• by radiation to colder surfaces in theconservatory (for example the glass)

• by conduction into the materials of the wallsand floor.

In this way, solar gains raise the temperature ofboth the air and the surfaces In the conservatory.The heat is stored partly in the fabric of theenclosed wall of the house and partly in the floor ofthe conservatory. These solar gains and theirstorage result in conservatory temperatures whichare almost always above external air temperatures.

0p.timum climate for solarcollectionIn climates where there is a combination of:• low air temperatures

and• high solar radiatIon,air temperatures in a conservatory are likely to beabove the demand temperature of the house forsubstantial periods of time. Under theseconditions, which do not occur In the UK veryfrequently, solar heated air can be transferred tothe rest of the house, Figure 112. Opening doorsand windows between the house and theconservatory will cause air to circulate between thetwo spaces, resulting in a net heat gain by naturalconvection into the house. The openings must beclosed when the air temperature in theconservatory falls below the house temperature orelse there will be a net heat loss from the house asreverse flow takes place.

100

FIgure 111

Figure 112

UK climate and solar collectionIn the UK climate, both monitoring and simulationstudies suggest that, while air temperatures inconservatories will normaily remain above ambienttemperature, there are probiems in designing aconservatory that will operate as a warm airexporter:• the air temperature In the conservatory will

oniy rarely be above the demand temperatureof the house during the heating season, seeFigure 100, and very rareiy sufficiently aboveIt for moving the warmed air Into the house tobe worthwhile

• warm air Itself is a very poor medium fortransferring significant amounts of heat.

Transferring solar-heated air tothe houseIf climatic conditions do allow a conservatory to bedesigned as a high temperature solar collector,solar-heated air can be transferred from aconservatory by two means:• manually by opening linking doors and

windows whenever the conservatorytemperature Is above the demandtemperature of the main house, This willachieve convective exchange of air betweenthe house and conservatory, These openingsmust be closed again when the conservatorytemperature drops beiow demandtemperature

• mechanically by the use of a thermostaticallycontrolled fan and ductwork linking theconservatory with the house, The fan needsto be controlled by thermostats which detecttemperatures in both the main house and theconservatory, When the temperature in theconservatory Is above the demandtemperature required for the house (andprovided the temperature of the house is notalready above demand temperature), then thefan is switched on and pumps warm air fromthe top of the conservatory Into the house,The heating system for the house must bedesigned to be able to respond to thisImported warm air,

101

Limitations on the direct use ofsolar gainBesides the questionable ability of the UK climateto support high temperature solar collection, thereare other factors associated with trying to optimiseconservatory as a solar collector:• comfort conditions in such a conservatory will

be affected detrimentally, Because of solargain, temperatures will vary greatiy both dailyand across the seasons, This may make aconservatory less suitable as living space orfor growing plants and reduces its amenityvalue,

• solar control measures in the form of shadingand ventilation will be essentiai to preventoverheating in summer

• if the conservatory is also used as forcultivating plants, its air may be at a highrelative humidity, When this is transferred tothe house interior, It may cause condensationon cooi surfaces,

Where there is a limited budget for measures toreduce energy consumption, it is more worthwhileto Insulate and draughtproof the rest of the housebefore trying to improve the performance of aconservatory as a solar collector, 1

Points to rememberIf a consetvatory Is to be used as a hightemperature solar collector, emphasis should beplaced on raising Its air temperature as much aspossible, This can be achieved by:• an attached conservatory with as mUch

glazed area faCing south as possible

, a high (a1io of glazed conservatory areato house wall area covered

lightweight surfaces within theconservatory to reduce absorption of heat

double glazing (or better) to r$duce heatloss

movable Insulation to the roof and walls.

If benefit Is to be gained from exporting solarheated from the consetvatory to the house,, the conservatory must to be warmer than

the heated parl of the houseand

, an efficient manual or mechanical rOlitefor transferrirlg the warmed air has 10 beprovided.

James, R and Dalrymple, G" 1985, Modelling of conservatory performance, In Norton, B. (ed), Greenhousesand conservatories,UK·ISES Conference Proceedings C39, UK-ISES, London,

102

Conservatories for pre-heating ventilation air

Solar gain and storage, and conductive losses from a house to its conservatory,raise the conservatory's air temperature above ambient temperatures, even in theUK climate. With controlled ventilation, this can be used to pre~heat incoming airbefore it is brought up to internal comfort levels by the heating system.

Ventilation pre~heatIf ventilation in a house can be controlled so that ahigh proportion of incoming air enters via theconservatory, then pre-heating this air can reduceauxiliary heating demand. There wlll be nothreshold above which the conservatorytemperature has to rise before solar gain can beused, Any increase in temperature represents auseful heat gain,

Consider as an example a conservatory attachedto a well Insulated house, The day-time airtemperature in the conservatory on a cloUdy winterday might be say goC, while the externaltemperature could be 4°C. Any ventilation airdrawn from the conservatory will thus have to beheated through only 1oaK to reach a demandtemperature of 1goC, rather than through 15°K. If allthe house ventilation air could be pre-heated in thisway, the heating load due to ventilation could bereduced by a third,

In the case of very well insulated houses, wherefabric losses have been reduced to low levels,there is stili a need for ventilation of at least 0.5 airchanges per hour. And, in such a house, theventilation load may be similar In magnitude to Itsconductive loss, Pre-heating of the ventilation aircan then begin to make a major contribution toreducing the auxiliary heating load.

Naturaj ventilation and solarpre-heatIn practice, controlling the flow of air from theconservatory to the rest of the house andextracting contaminated air to the outside bynatural means demands• investigation of the site micro-climate

• knowledge of the principles of wind pressureand stack effect on air movement

• control over the permeability of the buildingfabric

These issues are beyond the scope of thishandbook. A detailed discussion of them can befound elsewhere, 1

Figure 1131I

Baker, N" 1985, The use of passIve solar gains for the pre-heating of ventilation air in houses, Repor! by ECO Partnership to theEnergy Technology Support Unit, Report No. ETSU·S·1142.

103

Mechanical ventilation and solarpre-heatAn alternative to a reliance on natural means of airintake, distribution and extract is to introduce amechanical ventilation system. This can be linkedto a conservatory to achieve both ventilationpre-heat and control over airflow within thebuilding. Although there are likely to be costimplications, there will be fewer uncertainties aboutthe final performance than may be experienced ifattempts are made to use natural means.

Several of these mechanical ventHatlon systemslinked to conservatories were constructed inhouses at the Milton Keynes Energy Worldexhibition In 1986. Generally they operate by thewarm air being ducted from the conservatory up tothe roofspace, which Is used as a warm fresh airstore. Air handling plant In the roof space heats theair further. It is then ducted around the house fordelivery as required. Air extracts in particularrooms may lead either directly to outside or to heatexchangers in the roofspace. For summer use,there are large ridge ventilators on the main houseto let the warm air straight out by stack effect.

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Figure 114

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104

Conservatories and energy savings

Some occupants will want to use their conservatories to help reduce their fuel bills.At present, there is very little evidence available to show how effectiveconservatories are as a means of doing this.

Evidence on energy savingsUnder certain circumstances, conservatories canreduce fuel bills. But there is little documentedevidence about what actually happens In practice.Work that has been undertaken falls into two types:measured and simulated results.

Measured resultsThe thermal performance of an attachedconservatory at Milton Ke¥nes, has beenmonitored, see Figure 99, From measurementstaken, calculations were made to estimate thesavings due the conservatory as follows:• the buffer effect

(due solely to the additional insulationprovided by the conservatory) was calculatedto be worth 120kWh/year, This couid beestimated for any design by a simple U-valuecalculation.

• solar gainsthese, transferred to the house from theconservatory principally by conduction, wereestimated at 270kWh/year

• ventilation pre.heatingbecause the house was in a terrace, theconservatory covered about one third of itsexternal wall areas. It was assumed that halfthe ventilation air entered the house via theconservatory. As the air temperature in theconservatory was higher than the externalambient temperature, this pre-warmed airgave an estimated energy saving of about 270kWh/year. The house also had athermostatically controlled fan between theconservatory and house which drew warm airfrom the conservatory into the house whenappropriate. This was estimated to be worth150kWh/year.

Ford, B-, 1983, Thermal performance monitoring of a terrace house with conservatory, New Bradwell, Milton Keynes, Report bythe Department of Design Research, Royal College of Art, to the Energy Teohnology Support Unit, Report No, ETSU-S-l056B,

105

Simulated resultsComputer simulations have been performed toexplore the effect of attaching conservatories toeXisting dwelllngs,1 The results for a single glazedconservatory attached to a semi- detached house,see Figure 104, suggest the energy savings shownin Figure 115.

For this poorly insulated house (the U-value of thewalls was 1.36W/m2K),the conservatory waspredicted to save energy at the rate of 100kwh/yearper square metre of house fabric covered. Thisreduced annual consumption by 20-25%. Savingsas large as this are unlikely to be achieved,however, with a new house built to the 1985Building Regulations standards.

Computer simulations have also been conductedfor ETSU by CAP Scientific using SERI-RES for thehouse design study illustrated In Figure 102,2 Themodelled house was a new semi-detached with afloor area of 100 m2

, constructed to the 1982Building Regulations standards (that is walls 0.6W/m2K, roof 0.35 W/m2K) and with single glazedpatio doors abutting the conservatory, If anoccupancy schedule with intermittent partialheating to 21"C is assumed, the simulationssuggest the figures for space heating demandshown in Figure 1i 6.

Figure 115

])t-Jijll w,#/pur elM rtIVlJ"tOry 711-DO

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Figure 116

1 Penz, F., 1983, Passive solar heating in existing dwellings, unpublished PhD thesis, University Of Cambridge.2 James, R and Dalrymple, G., 1985, Modelling of conservatory performance, In Norton, B. (ed), Greenhouses and conservatories,

UK-ISES Conference Proceedings C39, UK-ISES, London,

106

J5000

/'1 300

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Figure 118 shows a further breakdown of thesources of the savings.

A further set of simulations on the same housewere performed but with the house kept ataconstant 21 cC. This gave the figures for spaceheating demand presented in Figure 117.

These simulations leave two important questionsunanswered:• what would the energy demand figures be for

a design of equivalent floor area but with asimple square plan shape?and

• what are the cost implications of theadditional perimeter wall that results from theL-shape plan?

I

Figure 117

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107

Conservatory shading and ventilation

If people and/or plants are to be comfortable in a conservatory, adequate shadingdevices and means of ventilation need to be provided, particularly to avoidoverheating during the summer. There are also other steps which can be taken inthe design of conservatories to minimise this risk.

Figure 119

Roft up b/t/Jl!

Summer overheatingThis is a common problem with conservatories,confirmed both by monitoring and computersimulation studies. A range of methods areavailable to combat overheating:• opaque elements, especially in the roof or

side walls

• shading devices

• ventilation

• thermal mass.

These methods are best used in combination.

Shading devicesAs with windows, the most effective position forthese devices is outside the glazing. In thisposition, they will prevent solar gain from enteringthe conservatory. Although external devices arepreferable, they have higher costs andmaintenance requirements. Internal devices arecheaper but will be less effective in preventingoverheating since they absorb solar radiationthemselves before re-radiating their heat to theInterior of the conservatory.

The main types of shading devices are:• roller or blinds, fabric or sheet material

• folding blinds, fabric or card

• slats that may be rolled up, typically wooden

• venetian blinds, wood or aluminium

• matting, jute or hessian.

For information on the performance characteristicsof these devices, see pages 63-78.

h

VentilationVentilation occurs naturally in conservatoriesbecause of the temperature difference betweeninside and outside. The rate of ventilation isaffected by the vertical distance between the airinlet and outlet, see Figure 120. It is important tokeep the distance marked as 'h' in this figure aslarge as possible.

On hot sunny dflYS, 30 air changes per hour ormore are necessary to bring the temperature in aconservatory close to ambient.

108

Figure 120

The size of inlet and outlet required to achieve thisventilation rate Is about 20% of a conservatory'sfloor area, 1 For a 4,Om x 2.0m conservatory, thiswould mean an inlet and outlet of 1.6 m2 each.These are comparatively large areas toaccommodate in what is only a medium sizeconservatllry,

The most common type of ventHators are:• manually operated louvre blades at low levels

and• automatic vents for high outlets.

These are made for the commercial greenhousemarket, are reliable and comparatively inexpensive,They may be preset to the required ventilatingtemperature, usually between 18°C and 28°C.Automatic controls are preferable, if they can beafforded, because manual ones place heavydemands on occupants if overheating is to beavoided.

Thermal massThe inclusion of materials within a conservatorywhich absorb and store solar radiation can also beused to combat overheating. This solar storage ismost likely to be sited in the floor and the wallbetween the house and the conservatory, Althoughthis thermal mass is helpful in reducingtemperature swings, its rate of heat absorption istoo slow for it to be relied upon to controlmaximum temperatures.2 Additional mass, such asthat provided by containers full of water, Is alsolikely to be too slow in absorbing heat to preventquick temperature rises from solar gain.Simulations by Penz suggest that if the theequivalent of four 200 litre barrels of water werestored In the 13.5m2conservatory he modelled, theair temperature could be lower by 4 to 5°K whenoverheating was a risk,3 The total volume occupiedthis amount of water Is almost 1 m3.

.

':d;!lf~illli'lllllf:'~~I;!'!III;iijwail$~r~ri;oreeffective·. '...' •.•......

*extel'nalshadingdevicesare moreeffe¢Uvethan ihternalones

!:.':..::'" :.'~-.' :..::::.':-;"':'.-":.::. <,... ::" ","::-'.,,': ,> ::;- ,>::.:: :'<:<:. _: :.:";:" :::'. :-:::.: :::':-:-';./:,-'-:'.:".:' .;::';~::',:,.:::>:~ ..<-:-:- :<:.:::,;::':'.:::\.:::- :

·1~t~e.air.ihl~t~and •• ()utlet9·.·~~e~e6e9;~·rV· .. t()prdyi~~.t~(!irate6f",el1til~!i()hf~CfUjredlo6blhbaloVEH'heatih ..:· .•··•· •••••: .•·..... ·.·.·.H··· ..·.·.,,····••.·: ··:·il'lletsshOl.!ldbep"o"idE!d~floWlevel, .outlefsafhigh level' . .

the rate of ventllatfonimproyesthefurtheJ'apartverticallytl1~inh~tand.ol1tl~tareloqatecJ;> . ..

Penz, F,,1986, The energy Implications of keeping plants in conservatories, Eclipse Research Consultants, Cambridge,(summarised on pages 88 to 89).

2 Littler, J, and Ruyssevelt, P" 1986, The role of thermal mass in UK housing, in The efficient use of energy in buildings, 2ndUK-ISES Conference C46, UK-ISES, London.

3 Penz, F" 1983, Passive solar heating In existing dwellings, unpublished PhD thesis, University of Cambridge,

109

Conservatories and the Building Regulations

Whether a glazed extension to a house falls within the Building Regulations'definition of a conservatory or not, it will still have to conform to requirements forventilation, means of escape, etc.

Definition and exemptionsAccording to the Manual to the BuildingRegulations 1985 for England and Wales, aconservatory means - for the purposes of theBuilding Regulations - a conservatory with atransparent or translucent root Any glazedextension to a house that has an opaque roof isnot, for the purpose of the Regulations, treated asa conservatory but as part of the rest of the house,

Under Regulation 9, the Building Regulations donot apply to any work described In Class VII ofSchedule 3, This Class Includes an extension to abuilding by the addition at ground level of agreenhouse or conservatory whose floor area doesnot exceed 30m2,

While this regulation makes it clear thatconservatories of over 30m2 (whether extensionsto existing buildings or constructed as part of anew dwelling) are not exempt, it remains unclearwhether conservatories of less than 30m2 areexempt when built with a new house, Advice onInterpretation should be sought either from thelocal authority or, If necessary, from theDepartment of Environment.

Conservatories and houseventilationPart F of Schedule 1 to the Building Regulations1985 requires a means of ventilation In dwellings sothat an adequate supply of air may be provided forpeople In the building, The Approved Document toPart F shows provisions meeting the requirement.

Where a conservatory covers a window or doorwhich would ventilate a habitable room, thehabitable room may be ventilated through theconservatory, provided that• there is an opening (which may be Closable)

between the habitable room and theconservatory with an area which is equal to atleast 1/20th of the combined floor areas of theroom and the conservatoryand

• there Is a ventilation opening area In the roomand the conservatory together, or in theconservatory alone, which area Is equal to atleast 1/2Oth of the combined floor areas of theroom and the conservatoryand

• some part at least of the ventilation openingarea is at least 1,75m above the floor level.

110

Cm/b'l/Atofl/IitrU. c) j

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Exposed elements in ahouse

Diagram A1

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Figure 122Conservatories and theconservation of fuel and powerPart L of Schedule 1 of the Building Regulations1985 states the requirement for the resistance tothe passage of heat in dwellings and gives themaximum rate of heat loss permitted In theexposed elements of the fabric. The ApprovedDocument shows procedures for calculating therate of heat loss.

A ventilation opening includes any means ofventilation (whether it Is permanent or closable)which opens directly to external air, such as theopenable parts of a window, a louvre, or alrbrick. Italso Includes any door which opens directly toexternal air, as long as the room or space also hasan area of ventilation opening (equal to at least1OOmm x 100mm) which can be opened withoutopening the door.

The required performance can also be met byfollowIng the relevant recommendations of BS5925:1980 Code of practice for design ofbuildings: ventilation principles and designingfor natural ventilation of which the relevantclauses are numbers 11 to 15.

Figure 122 shows the requirements.

The Appendix to the Approved Document, whichshows examples of exposed elements, containsDiagram Ai - reproduced as Figure 123, Theaccompanying text states that, in Diagram Ai (b),the car port Is open at one end to give apermanently open ventilation area in excess of 5percent of the area of walling enclosing the carport. Section AS is therefore an exposed element.It would appear from this diagram that if thecarport were a conservatory which did not nothave a permanently open ventilation area in excessof 5 percent of the area of walling section AS wouldnot be an exposed element. It would therefore notneed to comply with the insulation valuedemanded of exposed elements.

;=-B_~C

garage

A(a) Garage

Figure 123

D(b) Carport

A

B

car­port

Despite this, in view of the expected monthlyaverage temperatures in a conservatory (see page91), it is desirable that any dividing wall is insulatedto reduce heat loss to the conservatory. For thesame reason, large areas of single glazing betweena house and its conservatory, while permittedunder the RegUlations, will lead to excessive heatloss into the conservatory.

111

Conservatories and means ofescape in case of firePart B of Schedule 1 of the Building Regulations1985 requires means of escape in case of fire froma building to a place of safety outside the buildingwhich is capable of being safely and effectivelyused at all material times. This demand may onlybe met by complying with the relevantrequirements of The Building Regulations 1985·Mandatory rules for means of escape in case offire, published by HMSO.

These state that a dwelling house shall beconstructed in accordance with therecommendations in the relevant clauses of BS5588 Fire precautions in the design andconstruction of buildings, Section 1.1: 1984Code of practice for single family dwellinghouses, These requirements apply only to todwellings of three or more storeys,

One and two storey housesThese are excluded from the requirements of theRegulations for means of escape, But there arerecommendations in BS 5588 that apply to oneand two storey houses which need to beconsidered,

In BS 5588, a window is used for rescue or escapewhen a room (other than kitchen, utility room,bathroom, dressing room or wc) is an "inner" room. .This is a room from which escape Is possible onlyby passing through an "access" room. And this, Inturn, is a room that is the only escape route froman Inner room, as opposed to a passageway thatleads to an external door. In other words, if astaircase lands within a downstairs room (such asa living or dining room), then an alternative meansof escape or rescue must be provided from thewindows of upstairs habitable rooms,

In the house designs illustrated in Figure 124, bothof which are featured on page 91, the stairs landwithin downstairs rooms which are thereforedefined as "access" rooms. If a fire were to start inthe room containing the staircase, the alternativemeans of escape from the bedroom(s) on theupper floor would be via the bedroom windows,These should meet the recommendations forescape or rescue:• the unobstructed opening should be not less

than 850mm high by 500mm wide

• the bottom of the opening should be notmore than 1.1 m above floor level (and this willdetermine the height of the conservatory roof).

112

Figure 124

Firrr finlY

In the case of the south facing bedrooms, escapeor rescue would be over the glazed conservatoryroof. The practical implications of this escape routeshould be borne In mind.

Conversely, a two storey conservatory whichencloses a first floor window, while perhaps aidingescape from the bedroom, might also trap smokefrom the fire and cause it to penetrate the upperfloor. Since BS 5588 does not refer to escape orrescue via a conservatory, it may be necessary toseek advice from the local fire authorities wherethis option is contemplated.

Three storey housesThe recommendations In BS 5588 becomemandatory under the Building Regulations forhouses of three or more storeys.

Windows in the top storey of such dwelling are notrequired for escape or rescue because analternative escape route or protected stairway hasto be proVided. A conservatory roof could thereforebe sited under a top floor bedroom window withoutraising difficulties about escape or rescue. Unless aprotected stairway Is provided to the first floor, firstfloor windows have the same requirements as theydo in two storey houses. So the alternative meansof escape given In the examples above for twostoreys apply. In practice, It Is unlikely that theopen plan arrangement illustrated would be used ina three storey house, as a protected staircaseleading directly to an outside door will be the usualmethod of meeting the requirement.

Four storey housesThe additional requirements here are for alternativemeans of escape for the upper floors other than asingle protected Internal stairway. Conservatoriesfor such houses need to be designed taking theserequirements Into consideration.

Flats and maisonettesBS CP3 Chapter IV, Code of basic data for thedesign of buildings, Is the relevant code ofpractice. Here windows are not used as means ofescape because protected passage ways arerequired. Balconies may be used as escape routes.Care should be taken that conservatories adjoiningsuch escape routes do not obstruct them.Conservatories may be constructed on balconiesthat are not used as escape routes.

113

Conservatories and fire: internaland external fire spreadFor conservatories which fall within the scope ofthe regulations, (ie, those over 30m2 or that do nothave a transparent or translucent roof), some ofthe requirements in Part B of Schedule 1 of theBuilding Regulations 1985 which relate to internaland and external fire spread are relevant. Theserequirements may Influence the choice of both roofand external wall materials,

Figure 125 summarises the requirements,

Roofs: internal spread of flameAny wall which is at an angle of less that 70' to thehorizontal is classified by the Regulations as a roof,Glazed conservatory roofs are not specificallymentioned In the Approved Document except inTable B1 of Appendix B,

Where a conservatory roof Is within 6m of aboundary, its internal surface should have a Class1 surface spread of flame, see Table 1.4 of theApproved Document to Part B, Georgian wiredglass and clear annealed glass achieve this rating,PVC and polycarbonate sheet can also do so,Table B1 of Appendix B of the Approved Documentshows concessions for rooflights of plasticsmaterials under certain circumstances, Forconservatories of less than 40m2

, rigid PVC may beused,

Roofs: external spread of flameTable 1,3 of the Approved Document shows thelimitations placed on roof construction, Where aconservatory Is less than 6m from a boundary, onlyroof coverings that are rated AA, AB or AC may beused, Both 6mm georgian wired glass and 3mmwired PVC can achieve this rating, Annealed glass,unwired PVC, polycarbonate or acrylic sheetcannot. However where the conservatory is lessthan 40m2 in area, annealed glass or unwired PVCmay be used for the roof but polycarbonate oracrylic sheet cannot.

Walls: internal spread of flameTable 1.4 of the Approved Document states thatwalls should have Class 1 surface spread of flame,This would include wired glass, clear annealedglass (Which being non-combustible Is classified asClass 0), and may include rigid PVC andpolycarbonate under some circumstances,

Walls: external claddingThe external cladding of walls that are less than 1mfrom the boundary should be of Class 0combustibility, This Includes glass but excludesthermoplastics materials,

114

!<..oofJ Wa(,fy

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A1~'HJU#I /f!I/XlrnttlY! rot~1ItJ1fft1 of tiY't~ Iffi} ~ A {,(;'J,,PyorI-Ctt) "I"t4.

Walls: limits of unprotected areasA glazed conservatory wall must comply with therequirements for an unprotected area of wall. Anunprotected area of wallis one with less than therequired degree of fire resistance and Includeswindows and doors. The danger from unprotectedareas is that fire will spread by heat radiation fromone building to another.

Appendix J of the Approved Document describesthree alternative methods of calculating thepermitted extent of unprotected areas. Method 1applies to residential accommodation less thanthree storeys high and 24m in length which Is notless than 1m from the relevant boundary.Unprotected areas are not permitted less than 1mfrom a boundary. The diagram shown as Figure126 Illustrates the permitted extent of unprotectedareas according to Table J1. It can be seen that afUlly glazed conservatory may be limited in size,depending on its proximity to the boundary, or thatsome opaque walling will be necessary nearboundaries.

Junction of separating wall and roofAdjoining conservatories in terrace andsemi-detached houses are covered by therequirements for separating walls In Appendix C ofthe Approved Document. The requirements are toprevent the spread of fire from one dwelling toanother. Either the wall should be taken up abovethe roof or the roof and its covering should resistfire penetration and spread.

If AA, AS or AC rated roof glazing is used (eg.wired glass, clear glass, or wired PVC), the glazingmaybe carried over the top of the separating wallproviding it Is adequately flrestopped with mortaror similar material. For other types of glazing theseparating wall must project 375mm above theroof covering.

<

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115

116

SECTION 6Heating systems and solar energy

~DDn ~r

117

Space heating and solar gain

Solar gains are an important consideration when calculating the heat balance of awell·insulated house and influence the choice of its space heating system. Likewisethe responsiveness of the system and its controls will help to determine whethersolar gains that do occur are, in practice, useful.

Solar energy and space heatingSolar gains may be considered to fall into twocategories:• useful soiar gains which reduce the need for

an input from the space heating system(called auxiliary heating)and

• excess solar gains which raise roomtemperatures above demand temperature.

To maximise the usefulness of solar gains andreduce the likelihood of overheating, it is essentialthat the auxiliary heating system can respond to:• the solar gains

and• the internal heat gains (from cooking,

occupants and appliances)that occur within a house.

In the past, when houses were poorly insulated, theinfluence of solar and internal gains was small incomparison with fabric and ventilation losses. Soboth barely figured In calculations of auxiliaryheating demand.

In low energy houses which incorporate high levelsof insulation and sophisticated glazing systems,their Importance has to be recognised. Not only Isheat loss through adventitious ventilation (cracksaround windows and doors) reduced but the rateof heat loss through the fabric is also much lower.Thus the energy inputs required to balance heatlosses are lower. In these circumstances, solar andinternal gains are more significant. On their own,they can constitute a relatively large proportion ofthe energy Inputs required to bring a house up toits demand temperature.

On an hourly or daily basis, it Is possible that (evenduring the heating season) solar gains may exceedthe space heating requirements of a house,especially If they occur at the same time as otherlarge internal gain such as cooking. Even if they donot exceed the total requirements, solar gains(alone or with internal gains) may, on occasion,cause excessive temperatures in the rooms on thesunny side of a house.

118

For these reasons, solar and internal gains have tobe treated as Important factors when calculatingthe energy balance for well-Insulated houses, Theyhave to be taken into account both in the overalldesign of a space heating system and In thespecification and location of its controls,

Heating load calculationsThe overall energy balance of a house comes fromthe simple equation

energy input = energy output.

Energy inputs are made up of:• delivered energy (the purchased fuels)

• metabolic gains from the occupantsand

• solar gains.

These are put to various uses:• space heating

• water heating

• cookingand

• lights and appliances.

Solar and metabolic inputs are usually counted ascontributions to the space heating.

Energy outputs refer to how energy Is lost:• conduction losses through the fabric

• ventilation losses both deliberate (throughopened windows) and adventitious (throughcracks)

• losses from hot water that goes down thedrain

• losses from flues and chimneys

Figures 127 and 128 shows examples of the resultsof calculations using a computer program calledEnergy Designer, based on the BRE DomesticEnergy Model (BREDEM}, to analyse the inputs,uses and outputs for a low energy house. 1

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Figure 127

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Figure 128

Milton Keynes Development Corporation, 1986, Energy World exhibition oa.talogue, MKDC, Milton Keynes.

119

In these figures the notional output term - excessenergy - refers to energy surplus to requirements,ie. energy which would take a house above itsdemand temperature (21°C). The largestcontribution to this category wiii come from solarand other Internal gains. In practice, it is assumedthat some people will take this heat as a bonus,while others wlli open the window to ventilate itaway.

The model attempts to represent the exchangesbetween different categories of energy. Forexample, some of the energy used for waterheating is lost as 'water loss'. But some of thisgoes into the air from:• the hot water cylinder

• the pipesand

• hot water while standing in sihks etc.

This heat Is iost through the fabric or by ventilation.

The total capacity of a system should be designedmatched to the heat load of the house. Oversizing,though frequent in practice, reduces the efficiencyof some types of heating system which operatemost efficiently at maximum output.

The responsiveness of a heatingsystemA system should be chosen to match theconstruction of a house, particularly in terms of thethermal response of its fabric. Both should matchthe anticipated heating reglme(s) of occupants.Conventional wisdom Is that a lightweight buildingoffers a quick thermal response so that, togetherwith a warm air system, It will be well suited to twoperiod Intermittent (morning and late afternoon toevening) heating regimes or to partiai househeating. A heavyweight building heats up andcools down much more slowly. It should be fittedwith a system more suited to delivering continuousor one-period intermittent (early morning toevening) heating regimes and to whole househeating.

ZoningIn well-insulated houses, it may be possible toachieve adequate comfort conditions withouthaving heat emitters in every room. Similarly theuse of localised heat sources may haveadvantages over the installation of a centraiheating system.

120

---------------------------

Several options are possible where there areuneven heat requirements throughout a house:• local control in each room, for example by

thermostatio control of each heat emitter

• zoning of the system, for example, into livingroom and bedroom zones, or into southfacing and north facing zones

• redistribution of heat may be possible withwarm air systems from, for, example southfacades to north facades.

Thermostats should be located to avoid positionswhich suffer from draughts, or receive heat from aradiator, or are in direct sunlight.

The location of heat emittersIn the past when houses were poorly insulated withleaky single glazed windows, it was essential forheat emitters to be placed beneath windows tocounteract cold downdraughts caused by the coldsurface of the glass. But, where window systemsare used that have a higher energy performance,this may no longer be necessary. Heat emitterscan be placed on internal rather than externalwalls, so making greater use of the heat emitted towarm the Internal structure of the house rather thanjust its external envelope.

Boller energy manager systemsA further level of control over the heating systemmay be achieved by the use of a boiler energymanager system. Electronic sensors measure bothoutdoor and boiler temperatures, and thesemeasurements are used to calculate the optimalratio of the boiler 'on' and 'off' cycles necessary tosatisfy the heat demand. Such systems reduceboiler cycling and maintain boiler efficiency at athigh level irrespective of heat load.

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121

122

SECTION 7References and index

123

Useful organisations

A full list of organisations providing professional, technical or trade advice orinformation about building design and the construction industry can be found inBuilding Design's Easibrief.1

Those listed below are 'government' organisationswith responsibility for producing information drawnupon in this handbook.

British Standards Institution2 Park StreetLondon W1A 2BSTelephone: 01 6299000Telex: 266933Fax: 01 6290506

Building Research Advisory ServiceBuilding Research EstablishmentGarstonWatford WD2 7JRTelephone: 0923 664664Telex: 923220Fax: 0923664010

BRECSU Enquiries BureauBuilding Research Energy Conservation UnitBuilding Research EstablishmentGarstonWatford WD2 7JRTelephone: 0923894040Telex: 923220Fax: 0923664010

General EnquiriesEnergy Efficiency OfficeDepartment of EnergyThames House SouthMillbankLondon SW1 P 40TTelephone: 01 211 6774 or 6811

International Energy Agency2 rue Andre Pascal75016 ParisFranceTelephone: 010 33 45 24 8200

Meteorological OfficeLondon RoadBracknellBerkshire RG 12 2SZTelephone: 0344420242

Renewable Energy Publicity OfficeEnergy Technology Support UnitBuilding 156Harwell LaboratoryOxon OX11 ORATelephone: 0235834621 extension 3467Telex: 83135Fax: 0235 432923

Haverstock, H, and Heard, H., 1987, The Building Design Easlbrlef: a concise reference book for building designers,Morgan~Gramplan, London,

124

Sources of figures

The following figures have been redrawn, or the information taken, from thesources listed.

1: Hawkes, 0., 1986, Modern oountry homes in England: theArts and Craft architecture of Barry Parker, CambridgeUniversity Press, Cambridge.

2: Royal Institute of British Architects, 1933, The orientation ofbuildings, RIBA, London,

4: Steadman, P" and Brown, F., 1987, Estimating the exposedsurface area of the domestic stock, in Hawkes, D., Owers, J.,Rlckaby, p, and Steadman, P., Energy and urban built form,Butterworths, London,

5,6,8,10,11,12,20: Jones, R, W, and McFarland, R. D" 1984,The sunspace primer: a guide for passive solar heating, VanNostrand Reinhold, New York.

7,9,16,76,92: Achard, P, and Gicquel, R., (editors), 1986,European Passive Solar Handbook, CEe, Brussels.

13, 14, 21: British Standard Draft for Development 0067: 19S0,Basic data for the design of buildings - sunlight, BSI, London.

15: UKclnternatlonal Solar Energy Society, 1976, Solar energy:a UK assessment, UK-ISES, London.

18: Oppenheim, D., 1981, Small solar buildIngs in coolnorthern climates, Architectural Press, London.

19: Bartholomew, D" 1984, Possiblllties for passive solardesigns In Scotland, ~nergy Teohnology Support UnitOocaslonal Paper, referenoe ETSU L14, ETSU, HarwelL

27,28,29,106,107,108,111: ie Paige, M" Gratia, E, and deHerde, A., 1986, Guide d'aide a la conception blocllmatique,Services de programmation de la politique scientifique,Brussels.

30: Milbank, N" 1986, The potential for passive solar energy inUK housing: PO 102/86, Building Research Establishment,Garston, Watford.

31: Holtz, M, J, and Place, W" 1979, A classification schemefor the common passive and hybrid heating and coolingsystems, prooeedlngs of the third National Passive SolarConferenoe, San Jose, California, USA.

32,37: NBA Teotonlos, 1986, A study of passive solar housingestate layout, report to the Energy Technology Support Unit,reference ETSU-S.1186, Harwell.

33,34,58: Lowe, R., Chapman, J. and Everett, R., 1985, ThePennyland projeot, report to the Energy Technology SupportUnit, referenoe ETSU-S~ 1046, Harwell.

35,36: Tabb, P" 1984, Solar energy planning, McGraw Hill,New York.

38: House at Milton Keynes Energy World exhibition, 1986,designed by PCKO Architects, developer D. F, W. Golding(Hertford) Limited,

39: House at Milton Keynes Energy World exhibition, 1986,designed by Max Lock Easton Perlston & King, developer S &SHames,

40,47,48,55,68: Button, D, 1982, Energy options for housingdesign: glazing options, Journal of the Royal Institute of BritishArohlteots, vol, 89, no. 10,

41: British Standard BS 6375 Performance of windows, Part 1:1983 classifioation for weathertightness, BSI, London,

42: Green, J" Innes, W., Maby, C. and Osbaldeston, J., 1987,Heating advioe handbook, London Energy and EmploymentNetwork, London.

45: Chartered Institution of Building Servioes Engineers, CIBSEnergy Code: Part 1: Guldanoe towards energy oonservingdesign of bulldings and services, 1977, CIBSE, London.

46: Pilkington Environmental Advisory Servioe, 1988, Glassand transmission properties of windows, Pilkington Glass Ltd,St. Helens.

49,56: Owens, P., 1984, Low emissivity ooatings on glass Inwindows, UK-ISES Conference Proceedings (C~8): Coatingsfor energy efficiency and solar applications, pages 11-21.

50,51,52,53: Wilberforce, R., 1976, The energy balanoe ofwindows, Building Services Engineer, vol. 43, no. 12, pages241-243,

54: Energy Technology Support Unit, 1984, Passive solar studygroup: windows, ETSU, Harwell.

62: Davis, Belfield & Everest Consultancy Group, 1986,Newbuild design studies: summary of quantitative data fromthe oonst analysis project, Report to the Energy TechnologySupport Unit, DB&E, London,

63,64,65: Cost Information prepared·for this handbook byDavis, Belfield & Everest Consultanoy Group,

67,71: Bloomfield, D., 1987, lEA Task VIII ~ Dooumentation ofparametric study, Building Researoh Establishment, Garston,Watford,

69: House at Milton Keynes Energy World exhibition, 1986,designed by Feilden Clegg Design, developer Haslam Homes,

70: Greenberg, S. and Hawkes, D., 1984, Report on Wlmpeypassive design study, ETSU Passive Solar Programme,Greenberg & Hawkes, London.

72: Harris, N, C., 1985, Solar energy systems design, Wiley,New York.

73,74,81,82,83,84: Hastings, R. S. and Crenshaw, R., 1977,Window designs to conserve energy, NBS Building ScienceSeries 104, National Bureau of Standards, Washington DC,

77,86,87,89,90,91: Ruyssevelt, P. and Littler, J., 1985,Movable window insulation, F!nal report to the CEC, referenoeEUR 9996 EN, Brussels,

78,79,85: Energy Monitoring Company, 1987, Studies Inpassive solar test cells, Test cell studies 1: Solar distribution,solar lost and window coverings, report to the EnergyTechnology Support Unit, reference ETSU-S-1162, Harwell.

80: Tomany, R., 1982, The measurement of window treatmenteffeotiveness in reducing residential heating and oooling costs,ASHRAE Transactions, vo1.88, no, 2, pages 235-248,

88: Everett, R" 1980, Passive solar in Milton Keynes,EnergyResearch Group, Report ERG 031, Open University, MiltonKeynes,

125

93: Balcomb, J, D" Jones, R, W" McFarland, R. D, and Wray,W, 0,,1984, Passive solar heating·analysis: a design manual,Los Alamos National Laboratory/ASHRAE,

94: Pitts, G. 1982, Energy options for housing design: buildingfabric, Journal of the Royal Institute of British Arohitects, vol.89, no. 10.

95: Based on constructions illustrated in Kok, H, W, M" 1987,Construction issues: Design Information Booklet 5,International Energy Agency Task VI1I.

96: House at Milton Keynes Energy World exhibition, 1986,designed by Phippen Randall and Parkes, developer JohnMowlem,

97: Tressider, J, and Cliff, S" 1986, Living under glass,Thames and Hudson, London,

98~ 120: Penz, F" 1986, The energy implications of keepingplants In conservatories, Eclipse Research Consultants,Cambridge,

99,100,101,119,121: Ford, B" 1983, Thermal performanoemonitoring of a terrace house with conservatory, NewBradwell, Milton Keynes, report to the Energy TechnologySupport Unit, reference ETSU~S·1056B, Harwell.

102,103,115,118,117,118: James, R. and Dalrymple, G.,1985, Modelling of conservatory performance, in Norton, B"(editor), Greenhouses and conservatories, UK-ISESConference Proceedings (C39), UK·ISES, London,

104,105: Penz, F" 1983, Passive solar heating in existingdwellings, unpublished PhD thesis, University of Cambridge,

109: Baker, N" 1983, Sunspaces· heat transfer, modelling andauxiliary heating, in Yannas, S, and MoCartney, K" SunspaceRetrofit Studies, unpublished report by the ArohiteoturalAssooiation to ETSU,

114: Based on examples In the Milton Keynes Energy WorldOfficial Guide, 1986, MKDC.

122, 124: NBA Teotonlcs, 1985, A study of institutionalconstraints on passive solar design, report to the EnergyTechnology Support Unit, referenoe ETSU~S·1143, Harwell.

123: Building Regulations 1985, Approved Document to Part L:Conservation of fuel and power, HMSO, London,

126: Building Regulations 1985, Approved Dooument to PartB: Fire, HMSO, London,

127,128: Mllton Keynes Energy World Official Guide, 1986,MKDC.

126

Index

Blindssee shading devices

British standardsenergy efficient housing: BS 8211 4code of practice for glazing: BS 6262 40, 96sunlight: Draft for Development DD67 28, 44windows and rooflights - sound insulation:BSCP 153 36window performance: BS 6375 38window testing: BS 5368 38ventilation principles: BS 5925 111

Building Regulationsconservatories

exemptions 110fire spread 114-115house ventilation 110means of escape (fire) 112-113procedure for calcuiating heat loss 111

windowsarea 50prescribed U-values 50procedures for calculating trade-offs 50specification 36, 50

Conservatoriespoints to remember 89, 97, 99, 102, 104, 109advantages and disadvantages 87attached greenhouses

internal temperatures 88fuel costs 89

buffer zone 99Building Regulations 110-115comfort conditions 91

measured results 91computer simulations 92

evidence of energy savingsmeasured results 105simulated results 106

frames 96glazing 96greenhouse effect 100heat transfer 101-104heating 89, 97orientation 95, 97, 99, 102seasonal shift 95soiar collection 100-102thermal mass 109ventilation 108

pre-heating 103-104shading 108

Costsbuilding elements 51heated conservatories 88window systems 52

Energybalance

shutters (internal) 76windows 44-47east-facing glazing 47north-facing glazing 46south-facing glazing 46west-facing glazing 47

efficient design 4and solar gain 10Twenty steps to 6

savingsmeasured 76predicted 43, 54-56, 72

Glazingpoints to remember 43, 49, 56area 50-56cavity width 41-42coated 42

advantages and disadvantages 43comfort 42double

advantages and disadvantages 42effective U-values 47gas-filled 43Glass and Glazing Manual 40low emissivity 40,41orientation 43-47percentage external wall 9shading co-efficients 40solar transmittance 40sound Insulation 36standards 40systems 36-43

Heating systemssee space heating

Housesenergy balance 119external envelope

rate of heat loss 8space heating 118-121

127

Insolationangle of Incidence 20greenhouse effect 13vertical surfaces 12, 17

Insulation devices (glazing)external

shullersadvantages and disadvantages 68

internalblinds

shading co-efflclents 73U-value 75advantages and disadvantages 76

curtains 72-73advantages and disadvantages 73

draughtstrlpplng 71shullers

advantages and disadvantages 78energy balance 76

venetian 74advantages and disadvantages 76

Orientationpoints to remember 49conservatories 95-102RI BA Housing Design Brief 5Interaction with obstructions 32, 48site planning 29windows

area 52-56energy balance 44-47energy consumption 48optimum orientation 48

Overshadingpoints to remember 33Interaction with orientation 32, 48Interaction with microclimate 33prediction techniques

graphical methods 31heliodon 32photographic methods 32shadow prints 31shadow masks 32

rule 01 thumb 31roof pitches 30-31shadow prints 31

Passive solar designadvanced 1complex 24simple 24

128

Room arrangementpoints to remember 33

Shading devicesawnings

orientation 67solar transmission 66

blinds (external)solar gain factor 67

blinds (internal)solartransmlsslon 73

(venetian) blindsshading co-efflclents 74solar transmission 74

curtainsshading co-efficlents 72

external 64-68advantages and disadvantages 66, 68shading co-efficients 63

Information for occupants 60-62Internal 70-78

advantages and disadvantages 73, 76,78effectiveness 70shading co-efflclents 73, 74

overheating 58roof overhangs

solar geometry 65graphical method 65

use by occupants 60

Shutterssee insulation devices

Site planningpoints to remember 33house spacing

privacy distances 30obstruction 30overshading 29-30roof pitch 30standards 30

orientation 29overshading 29-31solar access 30-33solar radiation 29-30sunlight 4-5, 28

Solar accesssee Site planning

Solar apertureangle of incidence 22exposed area 22

Solar gaingreenhouse effect 23useful heat 23

Solar radiationangle of incidence 22diffuse 11direct 11eX1raterrestrial 11global 11

seasonal variation 12greenhouse effect 23maps 16regional variation 16-18site planning 29-30useful heat 23wavelengths 10

Solar transmissionblinds (eX1ernal) 66-67blinds (internal) 73curtains 72glass 20-21

Sound insulation 36

Space heatingpoints to remember 121capacity 120heat emitters 121solar gain 118zoning 120

Sunaltitude 12azimuth angle 12equinoxes 12path 12

diagrams 12stereographic projections 14-15

solstices 12sunlight 4, 28, 43

Thermal masspoints to remember 84effect on comfort 82-83effect of construction 83-84effect on fuel consumption 83and solar radiation 80, 81, 82

location 83materials

absorptance 80emissivity 80storage capacity 80thickness 82

overheating 44, 81strategy for avoiding 58

storage of solar gain 81temperature swings 44, 81, 84

Ventilationconservatories 103, 105, 108, 110mould growth 39trickle ventilators 39

Windowspoints to remember 43, 49, 56air tightness 38area 50-56

balance north/south 55energy savings 56optimum north-facing 46optimum south-facing 53-54

Building RegUlations 36, 50cavity width 41-42effective U-values 47energy balance 44-47energy consumption 48exposure rating 37-38frames 38-39heat gain 44-47heat loss 37, 40-43, 44improving thermal performance 41low emissivity 41-42multipie glazing 41-43orientation 44-48shading co-efficients 40solar transmittance 40sunlight 44systems 36-43U-values 37, 40ventilation 39weathertightness 38

129