Industry Guide to Selecting the Best Residential Window

Options for the Florida Climate

Authors Ross McCluney and Paul Jindra

Original Publication

McCluney, R., Jindra, P., “Industry Guide to Selecting the Best Residential Window Options for the Florida Climate”.

Publication Number

FSEC-PF-358-00

Copyright

Copyright © Florida Solar Energy Center/University of Central Florida 1679 Clearlake Road, Cocoa, Florida 32922, USA

(321) 638-1000 All rights reserved.

Disclaimer

The Florida Solar Energy Center/University of Central Florida nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Florida Solar Energy Center/University of Central Florida or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the Florida Solar Energy Center/University of Central Florida or any agency thereof.

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This document has been prepared to help retailers, builders, distributors, and other window industryprofessionals in Florida assist homeowners and other residential window purchasers in choosing the bestwindow options for Florida homes. Consumers may find it useful in obtaining detailed informationconcerning proper window placement and selection for Florida’s generally hot, humid climate. A com-panion publication, “Homeowners’ guide to selecting the best residential window options for the Floridaclimate” is also available from the Florida Solar Energy Center.®

Ross McCluney and Paul Jindra

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It used to be said that “windows are little more than holes in theinsulation.” It is true to some extent that poorly insulated and draftywindows in certain climates defeat the purpose of wall insulation. How-ever, modern high performance windows are almost as good as opaqueinsulated wall sections, at least in terms of total energy savings over longperiods of time. In some cases, they can be shown to actually out-performinsulated walls. Of course, they have the priceless additional benefits ofproviding views to the outdoors and natural daylight illuminationindoors — important issues of quality and comfort.

The main purpose of a building and its windows is to provide comfortto the occupants — as the sun moves through the sky, as the outdoor airtemperature and humidity vary, and as the wind and rain come and go. Ifcomfort can be achieved while reducing the building’s energy use andlowering monthly utility bills, so much the better.

This Energy Guide focuses on choosing window options for residentialbuildings in hot climates. Many of the principles offered here apply as wellto non-residential buildings. However, there are major differences in thetypes of windows available for these two building classes, and there arenormally major differences in their building occupancy schedules. This isimportant because unoccupied buildings don’t need illumination. Non-residential buildings are most generally occupied during daylight hours.This provides a greater opportunity for the use of daylighting in thesebuildings to displace electric lighting, saving energy in the process.

Residential buildings are usually less occupied during daylight hours, sothere is less chance to save energy by using daylight to displace daytimeelectric lighting. With its relatively higher fraction of retirees, however,Florida offers numerous exceptions to this general rule. Proper use ofwindows for daytime lighting of residential building interiors can displacethe electric lighting that might otherwise be needed.

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Windows are not directly energy-consuming devices.Sometimes they cause the building to use more energythan would be the case without them, and at other timesthey actually reduce the building’s need for energy.When windows let heat escape on cold winter nights(causing the heating system to use more energy) andwhen they admit solar radiant heat on hot summerafternoons (causing the air cooling system to use moreenergy) they increase the building’s energy costs.

How can they reduce energy use? One way is to admitsolar radiant heat into the building on a cold winter day.The solar radiation directly heats the building’s interior,and this heat is used to displace energy that wouldotherwise be purchased for heating the house. Whendaylight illumination enters through a window, it di-rectly illuminates the interior of the building. If thisdaylight displaces electric lighting which would otherwisebe on, there are energy savings from not having to turnon the lights.

Windows are seldom selected by consumers on thebasis of their energy performance alone. Instead, appear-ance, the operating mechanism, color, and price domi-nate selection features. Energy impacts have been lessimportant, due primarily to a lack of objective informa-tion on the window itself, or available at the sale site,about energy performance. Furthermore, it is not as easyto compare windows on the basis of energy efficiency, aswith refrigerators, microwave ovens, and automobiles.To partially alleviate this difficulty, the National Fenes-tration Rating Council (NFRC) has developed a systemfor the certification, rating, and labeling of windows forenergy performance. This system is slowly entering useby code bodies in the U.S. The NFRC label is intendedto help the purchaser and code officials determine theenergy performance of windows for which labels areissued.

Even when using the NFRC labeling program,reducing the energy costs associated with windows while

increasing their human comfort without excessive priceincreases, is a real challenge to the building designer.Achieving an acceptable design depends upon many factors.

� Which way the window faces relative to thedirection of the sun

� How much outside shading is planned or ispresent for the window

� How bright the exterior scene is (the brighter thescene, the greater the potential for glare)

� How dark the interior is (the brighter the interiorsurfaces, the less the window glare)

� The homeowner’s willingness to operate shadingdevices to achieve their best performance

� The homeowner’s desire (or code-requirement)for impact resistance (the impact can come fromstorm-blown objects or intruders)

� How critical it is to maintain an unobstructedview to the outside (gorgeous vistas shouldn’t bepermanently blocked by window shades or otheradd-ons, although temporary blocking for privacy orsolar heat rejection may be desired at times)

� The homeowner’s desire for acoustic isolation innoisy environments

� Whether the window can meet aesthetic desiresfor appearance and quality

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There are some basic principles to keep in mind whenselecting windows for Florida residences. The winterspace-heating season is short and not severe. Thus, thereis not a strong need to insulate a window, at least incomparison with the much greater need to protect itfrom direct solar radiant heat gain. It is true thatinsulating the window with multiple panes, insulatinggases, and a special coating can reduce the size of the airconditioner needed to meet peak loads and that thissmaller size will reduce both heating and cooling costs aswell. Reduced heating and cooling equipment size couldsave enough construction dollars to pay for the extra costof the window insulation, but this should be proven by

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calculations before it is accepted as truth. (Computertools are available, such as the program RESFEN, formaking these calculations. See the web sites mentioned atthe end of this publication for more information aboutsuch programs.)

For retrofit applications — replacing windows in anexisting building — there is seldom the chance to savedollars on the air conditioner, unless it happens that thehomeowner needs to replace the air conditioner at thesame time that the windows are replaced.

If the homeowner decides to install insulatedwindows — for whatever dominant reason — a fewextra comfort benefits can be expected too. There willbe less transfer of sound through the window, anadvantage in urban settings with frequent road or aircraftnoise, but a possible disadvantage for rural sites where theoccupant might enjoy hearing better the sound of thewind or the chirping of birds. Insulated windows haveless tendency toward condensation and the resultantgrowth of mold and mildew. On the infrequent coldFlorida winter nights and excessively hot summerafternoons, an insulated window will be morecomfortable to sit near.

In trying toprevent unwantedsolar radiant heatgain duringsummer months,it is important torealize that thesun rises north ofeast and setsnorth of westduring these

months (as shown in Fig. 1). It rises due east and sets duewest only on the equinoxes, near the 21st of March andSeptember. Thus, whenever possible, it is best tominimize window exposures toward the east andnortheast and toward the west and northwest. This canbe accomplished by the design and orientation of thebuilding and by shading the window, as discussed below.

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It is far better, for heat gain prevention, to block thesunlight before it reaches the window, thereby dissipatingthe absorbed heat outside where it can be carried away byair currents. This means that simple shade trees and otherexterior shading methods can be very effective, both insaving air cooling energy and in blocking the strong glarewhich direct sunlight can produce if allowed inside.

If the owner and building designer cannot avoidwindows facing east or west, then the use of exterioroperable shading devices should be considered to protectthe windows from the sun. In this case, shades can bepulled down to reduce solar radiant heat gain along withits glare and higher energy costs. They can be openedwhen the sun is not shining directly on the window, toprovide both good exterior views and interior daylightillumination. A variety of exterior shading devices isillustrated in Fig. 3 (page 4).

Note that in winter, the sun rises south of due eastand sets south ofdue west (Fig. 2).Advantage canbe taken of thisfact by puttingexterior verticalshades on east-and west-facingwindows (Fig. 4,page 4). Properlydesigned, vertical protruding shades on one side of thewindow can allow sunlight to enter from the southeast toeast through east-facing windows in winter, and from thesouthwest to west through west-facing ones, whileblocking the sun at other seasons of the year. With thisstrategy of putting a “wing wall” on the window (alongwith a properly sized roof overhang), the worst of thesolar gain conditions could be avoided, while stillproviding a view and daylight illumination without theneed to operate exterior shades. With Florida’s warmwinter climate, admission of any direct solar gain, even inwinter, may not be desirable for everyone, so othershading strategies should also be considered.

S

W

E

N

Solar noon

Sunrise

Sunset

Sunpath on summer solstice at southern latitude

86˚

Figure 1. Summer solstice sunpath.

S

W

N

E

38˚

Solar noon

Sunset

Sunrise

Altitudeangle

Sunpath on winter solstice at a southern latitude

Figure 2. Winter solstice sunpath.

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If exterior shades are not wanted, consider the use ofwhite or otherwise highly reflecting interior shades. Forbest performance, they should be operable. When theyare closed, they can reflect solar radiation back throughthe window to the outside. They can be drawn closedwhen the sun is strong and not wanted, and then openedwhen the sun is not shining on the glass or when access toa good view and daylight is desired.

Exterior window shading strategies

Bahamashutters

Exteriorroll blind

Sarasotashutters Sun screen

Slattedaluminum

Venetian awning(east or west exposure) Trellis & vinesPorch

Hoodawning

Gambrel awning (for casement windows)

Trees

Solid aluminumawning

Roller awning(self-storing)

Figure 3. A variety of exterior shading devices is available to block the sun before it reaches the glass.

East-facingWest-facing

Figure 4. Exterior vertical shades in the form of “wing walls.”

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If the use of exterior and interior shades isunacceptable, there are otheroptions that can be selected inthe window itself to reduce(though not necessarilyeliminate) solar gain effects.These options can be effectiveeven if interior shades areinstalled too.

One way is to choose a“spectrally selective” glazingsystem for the window, usingspecially tinted glass orcoated glass that blocks muchof the solar gain withoutadversely affecting the viewthrough the window. Tounderstand this option requiressome knowledge of the solarspectrum.

An early experiment inoptics (Sir Isaac Newton,about 1723), diagramed inFig. 5 revealed that whitelight is composed of arainbow of colors, spanningwhat is called the “visibleportion of the solarspectrum.” The completesolar spectrum is plotted inFig. 6 , showing that thevisible portion contains lessthan half the energy of thetotal solar spectrum. All ofthe solar spectrum producesheat when absorbed byinterior surfaces, but only thefairly narrow visible portionproduces the sensation ofvision. The rest is invisible

radiation. This includes what are called infrared andultraviolet radiation. Neither of these wavelength bandscontribute to vision and cannot be called light.

The visible transmittance of glass is simply thefraction of incident light transmitted by the glass to theinterior. It has the symbol VT (or Tv) in window productliterature and is a number ranging from 0% to 100% (or0 to 1.0 in fractional terms). “Spectral selectivity” denotesthe ability of glass (or its coating) to transmit radiation ina tailored way over the spectrum. In this document, theprincipal applications of spectral selectivity are two: thetransmittance might be high over the whole solarspectrum but low outside this region, or it might be highover just the visible spectrum and low everywhere else.

BlueGreen

YellowOrange

Red

Glass prism

Figure 5. Colors ofthe visible spectrum.

Solar spectrum

Human eye sensitivity

(visible portion of the

spectrum)

Near infrared (NIR)

Wavelength in nm

Ultraviolet (UV)

UV VIS NIR

0.2

0.0

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 1000 1500 2000 2500500500

BlueGreen

YellowOrange

Red

Spec

tral

irra

dian

ce in

W/ (

m

nm)

2

Figure 6. Comparison of the solar spectrum to the human eye color sensitivity.

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The transmittance of low-e coated glass is high overthe solar spectrum to capture the maximum solar heatpossible, but low (with high reflectance) “beyond theupper limit” of the solar spectrum, shown in Fig. 6(page 5). In a cold climate, radiation from the warminner pane of a two-pane window to the colder outer

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The total energy spectrum extends well beyond thesolar spectrum limits, and all warm objects radiate someenergy outside the solar spectrum. A special kind of glasscoating called low-emittance or “low-e” was developed forcold climates, to exploit the difference between thewavelengths of incident solar radiation and those muchlonger wavelengths of radiation emitted by warm interiorobjects, as shown by the blue boundary in Fig. 7.

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.00.5 1 5 10 50

Wavelength in micrometers

Hot climatetransmittance

Cold climatetransmittance

Solarspectrum

Human eyeresponse

23.9 Cblackbody

Spec

tral

irra

dian

ce in

W/ (

m

nm)

2

Figure 7. Solar spectrum at a window, blackbody radiation spectrum from the interior, and twoidealized window transmittance spectra.

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Cold-climate low-ecoated windows

Hot-climate coated windows

Low-emissiveconfiguration

a. b.

d.c.

Or

High-reflectiveconfiguration

Absorptive longwave conversionOr

Solar direct reflectionOr

* *

*

Long-wavelength IRSolar near IRSolar near IR absorber (longwave convertor)

Cold-climate low-e coatingHot-climate solar near IR reflective coatingSecond pane optional in principle

Cold ColdWarm Warm

Hot HotCool Cool Cool CoolWarm Warm

Figure 8. Low emittance coatings for radiative energy trapping or rejection.

pane has wavelengths much greater than those of the solarspectrum. The laws of physics tell us that this long-wavelength infrared radiation propagates between theglazings in both directions, but that the net energy flow isfrom the warm inner glazing across the gas space to thecold outer pane, where it is absorbed by the bare glass andthen re-emitted to the outdoors, causing a loss of thebuilding’s interior heat.

The low-e coating was originally developed to stopthis wintertime heat loss. The coating’s physicalproperties allow this in either of two differentconfigurations, emissive or reflective (see Fig. 8, a and b.A warm inner pane with a low-e coating (8a) is a pooremitter of longwave radiation to the cold outer pane, andthence to the cold outside air, thus trapping this radiantheat indoors.

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infrared” (NIR) radiation that extends from the edge ofthe visible portion of the spectrum to the end of thesolar spectrum.

Windows with conventional, cold-climate low-ecoatings effectively trap radiant heat inside the building,making it warmer than it otherwise would be withoutthe coating, and reducing the need for supplementalheating. This works great for cold northern climates, butnot in sunny Florida! We might call the conventionallow-e coating a “northern low-e” one, or more accuratelya “cold-climate low-e coating” (to include our southernhemisphere neighbors in this discussion, where southernlatitudes are the colder ones). Another name for thiscoating is “high solar gain low-e.”

In summary, typical cold-climate low-e coatings emitpoorly and reflect well in the far infrared region of thespectrum; their reflectance is made to change from highto low between the solar spectrum and the FIR,admitting solar radiation to warm the interior whiletrapping radiant heat inside.

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The low-emittance principle can also be applied to a“hot-climate glazing system.” (The “system” in this casecombines a specific glass, plus coatings.) The newerglazing system has been developed to meet the needs ofFloridians and others living in hot climates (who, on anannualized basis, want to exclude solar energy, not trap itinside).

Ideal hot - and cold - climate spectral transmittancesare contrasted in Fig. 7 (page 6). Crucial to the differencebetween them is the choice of the exact wavelength atwhich the transmittance departs from its “low” value atlonger wavelengths (to “high” at shorter ones). Thisillustrates an application of the general spectral selectivityprinciple. The end effect is to reduce the transmittance ofnear infrared radiation, lowering solar gain, but without aloss of visible transmittance.

The hot-climate transmittance is high only over thevisible portion of the solar spectrum. The remaininginfrared radiation, which makes up over half the totalsolar radiation, is blocked from entering the building.

The terminology in this case is “hot climate glazingsystem with low-e coating(s).” Another generic term usedfor this class of glazing system is “low solar gain low-e,”but this terminology can be misleading because the“low-e” quality extends over different spectral regions inthe two different low-e coating types, as described in thefollowing two sections.

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One way of achieving hot-climate solar NIR rejectionis by spectrally selective absorption, as illustrated in Fig. 8c.An uncoated sheet of glass is selected that has highabsorptance over the near infrared portion of the solarspectrum but good transmittance over the visibleportion. As a result, such glass will absorb much of thesolar NIR radiation and heat up. But being spectrallyselective, much more solar energy is absorbed outside thevisible spectrum than inside it.

In order to protect the interior of the building fromthe heat conducted and convected away from this hotglass, a second pane must be added on the interior side,with an insulating gap in between. The result is adouble- pane insulated glazing unit (IGU) with aspectrally selective absorber for the outer pane. It helps ifa cold-climate low-e coating is added to the outer pane’sinner surface. The reason for this apparent paradox of

This same energy trapping function is retained if themanufacturer places the low-e coating on the outer pane,facing inward (8b), even though that glass is colder thanthe inner one. The explanation here depends on the highlongwave reflectance associated with a typical low-e coating.The longwave radiation reflects back toward the warmerglass. A common name for this longwave radiation is “farinfrared” (FIR) radiation, because its wavelengths are farfrom the visible spectral region, farther than the “near

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mixing cold- and hot-climate techniques is to reduce theinfrared re-radiation from the hot outer glass to thecooler inner one. The result of this combination (Fig. 8c)is just the opposite of that for the cold-climate low-ecoating; it is not to trap heat inside the building but toprotect the inner pane of glass from the heat of the outerone, thus keeping solar heat out of the building. If aninsulating gas is also used between the panes, the systemworks even better. This arrangement offers a good wayof achieving a high overall spectral selectivity forhot climates.

In two-pane glazing systems, the four glass surfaces areconventionally numbered from the outside in. Thus, thecoating can be placed on either surface 2 or surface 3 asshown in Fig. 8c. (Note that if this absorbing type ofhot-climate glazing system is flipped over, with the insidenow facing the outside, the result is a good window for acold climate. For more information about this cleverwindow design, see the sidebar on flip windows (page 12).

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There is a secondbasic way of rejectingthe solar NIR radiation— by working directlyon this radiationwithout converting it toheat first. In thisapproach, the rejectionis achieved by a newtype of coating ratherthan by absorption inthe glass. This coating ismade to have a highreflectance over the solarnear IR. The transmittance of this coating is high overthe visible portion of the spectrum for good view anddaylight admission, but low over the NIR to reduce solarheating of the interior. See Fig. 9.

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The best-performing of the hot-climate coatings ismade with a process that results in the coatings beingrelatively soft. In consequence, they cannot be placed onthe outside of the window glass — exposed to rain andweather — nor on the inside of the uncovered glass, sincewashing the windows could damage them. As a result,these highly spectrally selective soft coatings are put insidea two-pane sealed glazing unit, to protect them fromdamage. The added insulation of the permanentlytrapped gas in the gap has other benefits as well. Thesebenefits are described subsequently, as are the liabilities.

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Some manufacturers sell tougher coatings (nowtypically manufactured by a lower-cost pyrolytic process),but their spectral performance is generally not quite asgood as that of the softer coatings. With just a littlesacrifice on the energy performance, however, choosing asingle-pane window with a hard coating is a good optionin the Florida climate. Single pane windows are not ascostly as double pane ones. As the technology of makingthese coatings advances, so too will their energyperformance, causing them in many ways to approachthe ideal.

If one of these hard coatings is used, then the secondpane becomes functionally optional — it is not requiredfor this principle to work. But because present-day hardcoatings have far from ideal NIR reflection characteristics,the glass under them gets somewhat hot and a secondclear pane is still added to isolate this heat from theinterior and its occupants.

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The term “low-e” clearly needs qualification, especiallywhen its hot-climate applications are added. For the cold-climate version, it is only required that the coating’semittance be low (Fig. 8a), or equivalently, its reflectancehigh (Fig. 8b), over the far infrared portion of the

Terminology - continued on page 15

Reflected infraredradiation fromthe sun,keeping it outdoors

Transmitted visible radiation from the sun, sky and surrounding objects,

providing view and daylight illumination

Window for hot climates

Figure 9. A window withspectrally selective opticalproperties for hot climates reflectsthe NIR component of incidentsolar radiation while admittingvisible radiation.

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One can expect to pay more for an energy efficientwindow or shading system. The extra initial cost will beoffset by savings on the monthly energy bill. After-the-fact window upgrades always entail a price premium.The payback time is the time it takes for these savings toequal or exceed the extra cost of the better windows.(Dividing the initial cost by the annual dollar savingsyields the simple, as opposed to discounted, paybacktime in years.) Payback times of only a few years can bevery compelling to a homeowner, especially when it ispointed out that the window lasts, and continues to savemoney, for many more years into the future. One caneven get special mortgages for energy-efficient homes andspecial home improvement loans for energy efficientreplacement windows.

Other incentives are available from time to time.Sometimes electric utility companies offer financial orother incentives for selecting energy-efficient windows.Whatever discounts, subsidies, or rebates are available, thetotal net cost to the homeowner, including installationcosts, should be divided by the anticipated monthlyenergy savings when computing payback time. Moresophisticated measures of life-cycle costs are available, buta good simple payback time is likely to be reflected asalso favorable when other, more accurate, life-cycle costsare calculated.

As mentioned, better windows and shades can make itpossible to install a smaller air conditioner (A/C). In thiscase, some or all of the extra cost of the better windowcan be offset by the reduced air conditioner cost. Thereare cases where the reduced A/C cost more than pays forthe extra window costs, making the payback time zero.The window has paid for itself the instant it is installedin such cases, and all future energy savings are completelyfree of cost.

The homeowner pays energy bills monthly. Duringtimes of peak electrical demand, such as during anighttime winter cold front or on a very hot summerafternoon, the homeowner pays the same rate per

kilowatt hour of electricity as at other times.It costs the electric utility more, however, to producepeak load power. Some utilities provide incentives tohomeowners for installing devices or using strategies thatreduce their peak electrical demand. For windows, thisusually means insulated windows that also limit solargain. The energy economics of windows, therefore, is noteasily determined by the homeowner alone. Someassistance in determining window costs and savings isprovided on the Efficient Windows Collaborative website listed at the end of this publication. After-the-factwindow upgrades always entail a price premium.

Peak load is a relative term. It does not occuruniformly for all members within a power sharingcommunity. Your domestic peak load may not coincidewith your community’s peak load, especially in mixedresidential/commercial regions. Air conditioner sizing isrelative too. It need not amount to the same heatpumping requirement from identically built houses. Airconditioner sizing depends on your lifestyle.

Only the utility itself experiences the aggregate peakdemand of the local area. (Minor peaks may appear inaddition to the major one.) During Florida’s relativelyfew winter cold snaps, efficient hot climate windowsslightly worsen the daytime peak load to the utility byadmitting less radiant solar heat to warm the interior.

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Energy costs are not the whole story, since a windowprovides far more than just energy control. The valuableassets of better visual and thermal comfort, aestheticdesign, natural daylight, and acoustic isolation are oftenmore important to the homeowner than just a window’senergy costs. These features, not easily given dollar values,are difficult to combine with a window’s cost accountingin the decision-making process. The homeowner knowstheir value, however, and may even be able to assign apersonal monetary value to them. Though most Floridahomeowners are very conscious of initial price, they canoften be persuaded to pay the extra cost of a better

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window when it combines lowered energy bills alongwith the other less tangible assets. Fortunately, mostcomfort and aesthetic features of a window arecompatible with good energy conservation as well.

It is important to realize that a window can beexpected to last many years. Future energy prices cannotbe predicted with clarity, though few believe they willdrop. An investment today in energy efficient windowscan be a valuable hedge against energy price inflation,sheltering the homeowner against future price increases.Installing better windows can be thought of as a form ofinsurance. They protect us from future economicfluctuations that might make a home much moreexpensive to operate.

Providing the homeowner with an energy-efficientwindow that also lowers the electric load of the homeduring peak hours benefits everyone. The window sellerbenefits through the higher profits generated by sellingmore expensive windows. The homeowner benefitsthrough lowered electric bills. The utility benefitsthrough lowered peak loads. Society benefits throughreduced air pollution emissions at the power plant andfrom the resulting cleaner air for all of us.

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The solar heat transmittance of a window is measuredby the solar heat gain coefficient, or SHGC. This is thefraction of incident solar radiation that enters thebuilding as heat gain by all mechanisms. Also, thefraction of sunlight, skylight, and reflected daylightincident on a window that enters as light is measured bythe visible transmittance. Hot-climate glazing systems arenormally are intended to make the window pane high inVT while low in SHGC. The ratio of these twoperformance indicators is called the light-to-solar-gainratio, or LSG

In general, the higher the LSG, the better the hot-climate performance, with the emphasis being onlowering the SHGC value. LSG numbers greater than1.0 should be chosen, and those exceeding 1.5 offer thebest protection from the heating rays of the sun whilestill providing good views of the outdoors and letting inplenty of daylight.

The LSG value is seldom published by manufacturers,nor does it appear directly on window energy labels. Tocalculate it, divide the visible transmittance of the glazingplus frame by its solar heat gain coefficient — twocharacteristics specified by the NFRC that are publishedby reputable window manufacturers.

If a window manufacturer does not publish SHGCvalues, but instead the older shading coefficient, SC, theresults usually will be little different if SHGC is replacedby SC in the above formula. There is a caution in this,however. The NFRC values for both the VT and theSHGC apply to the whole window, including opaqueframing elements. It is an area-weighted sum of thetransmittances of all parts of the window, including glass,opaque frame, and semi-opaque parts. Thus, the visibletransmittance value used in calculating the LSG aboveshould be the NFRC standard one (for the wholewindow). When using SC in the denominator of theLSG equation, however, the numerator should containthe visible transmittance of the glass only.

High-LSG glass provides the best energy and comfortperformance in Florida buildings. The extra cost for suchglass can often be offset by smaller air conditioners(reduced peak load) and lower monthly electric bills(reduced average energy use). Since high-LSG glass ismost often offered only in double pane models, theextra benefits of double pane windows are a bonus.

The solar gain could always be reduced still further bylowering VT, while keeping LSG the same. This shouldnot be overdone. A VT below about 0.3 to 0.35 wouldlook somewhat extreme. VT

SHGCLSG = .

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The double pane absorptive glazing system forhot climates has a useful feature for regions withpronounced seasonal swings in climate: if thepositions of the two glass panes are flipped overfrom their summertime positions during the coldwinter, the system converts to a solar radiantheater. In the cold-day position, solar radiationpasses through the clear outer pane and is absorbedby the inner one, which heats up and dumps mostof its heat to the interior, warming the building.The low-e coating on the inner pane now reducesthe radiation of heat from this hot inner pane tothe cold outer one, trapping the heat inside.Flipping it back over makes it a hot-climateglazing system since the solar heat is now absorbedin the outer pane of glass which is insulated fromthe interior of the building. (The glazing serves aseither a hot- or a cold-climate system dependingon its orientation.) A measure of performance fora flip window is the asymmetry index, the ratio ofthe winter and summer SHGC’s

“Flip windows” are manufactured and sold inseveral areas, including Europe, mainly to aid incleaning the outside of a window from the inside,especially useful in high-rise apartments. Such aflippable window, equipped with a hot-climateglazing system based on spectrally selectiveabsorption in one of the two panes, can saveenergy in both hot or cold seasons. The sub-tropical Florida peninsula does not experience trueextremes in summer/winter climates, but theworldwide application for flip windows issubstantial. J-values of 2.3 have been achieved.

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Damage from hurricanes and other severe weather hasled some Florida counties to require impact-resistantwindows. The new Florida unified statewide buildingcode requires impact resistant fenestrations in countieshaving highest susceptibility to damage.

One way of achieving this goal is to use laminatedglass panes. Such panes consist of two sheets of glassstuck together with a layer of polyvinyl butyral. At leastone manufacturer offers such a glazing system with aspectrally selective (not a conventional low-e) hot-climatecoating sandwiched in the middle of the laminated glass.Other manufacturers are expected to follow with similaror alternate means of reducing the SHGC (withoutoverly reducing the VT). In addition to the damageprotection provided, this is an excellent way to achievesome of the benefits of a spectrally selective coatingwithout having to resort to full double-pane windowsjust for solar gain control.

The other accepted way to meet the damageprotection requirements of the building code is withexterior shades and shutters that can be drawn or rolledtightly over the window, protecting it in times of severeweather. This option has the benefits for energyconservation detailed previously for shading devices.

Since some form of damage protection is required inmuch of Florida, alert homeowners will compare therelative costs of damage-resistant glazing versus attachedexterior shades or shutters. This comparison, to becomplete, should include the price of reduced airconditioner installation and the annual saved energy costsfor the two as well.

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The windows chosen for a house should reflect theindividual outdoor conditions in the directions they face.An exterior that is generally dark, for example, withreduced views of the sky and the sun, probably requireshigh visible transmittance glass to provide good view andto admit reflected daylight. Although a low SHGC isgenerally desirable, it is less important in this case due tothe lesser solar radiation incident on the window.

If, on the other hand, the window looks out ontoclear open sky and brightly reflecting surroundingobjects, it is best to have a lower visible transmittance,and as low a SHGC as possible. (High-LSG glass is notthe primary goal here.) Again, adding shades to thewindow can moderate this sky brightness, reducing theneed for the low transmittance and low SHGC glass.Well-shaded windows probably do not benefit greatlyfrom high-LSG glass, so the extra cost of this glazingfeature can be avoided with such windows. Whenchoosing operable shades, however, it is best to selecthigh-LSG glass for protection during the times whenoccupants forget to, or choose not to, close the shades.

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Many consumers seem willing to pay more for aproduct labeled “green.” The achievement of a claim toenvironmental friendliness, as judged by an independentagency, can do wonders in the marketing of energy-efficient windows.

The “green buildings” movement is based on apresumption that energy efficient buildings are good forthe environment. Of course if the number of buildingsincreases more than their efficiency, the net impact on theenvironment is negative. On an hypothetical basis atleast, it is clear that by saving energy, a building would beresponsible for less air and water pollution at the powerplant, less depletion of fossil fuels and its attendantconsequences (or reduced adverse consequences of nuclearpower production), and reduced emissions of certain gases.

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No matter how good the window, if it faces east orwest and is not adequately shaded, it can produce seriousglare, peak A/C loads, and localized over-heatingproblems inside the building. The localized over-heatingcan make the areas affected very uncomfortable forresidents. These are the reasons that the use of shadingdevices was discussed so extensively at the beginning ofthis Energy Guide.

As described previously, exterior shades are moreeffective than interior ones. Shades located on the roomside of the window heat up when the sun shines onthem, even if they reflect some of the incident sunshineback out through the window. In consequence, they canstill admit a lot of heat into the building, making theroom less comfortable and causing the air conditioner towork harder. Shades located outside, however, releasenearly all of the solar heat absorbed by them to thesurrounding air and outside objects. For this reason, it ismuch more effective to block incident solar heat before itreaches the window.

Interior shades are most effective if they have highreflectivity over the whole solar spectrum on theoutward-facing side. This means that the outsideappearance of such shades will be either bright white orshiny. The inward side can be just about any color,texture, and pattern wanted. If the outside appearance ofthe shade is mirror-like or excessively shiny, this can be anaesthetic problem to some people, so it is probably bestto have a “duller,” diffusely reflecting white shade.

Some shades offer a partially or fully trapped air spacebetween the outward- and inward-facing sides, andshould better keep the heat that is absorbed by theshade’s outer surface away from the interior air. Thisreduces the load on the air conditioner and makes theshade more comfortable to sit near. (For best resultswith such shades, the edges should feather seal to theedges of the window, trapping the warm air between theshade and the window. In this case, the heat so trappedhas a better chance of being conducted through thewindow to the outside.)

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In selecting windows for the Florida home, the firstthing to consider is their orientation relative to the pointsof the compass. In new construction, this leads to a fewsimple recommendations.

� Minimize east - and west - exposures, which aredifficult to shade early or late in the day.

� Don’t worry too much about north - facingwindows (the glazing recommendations belowshould be sufficient).

� Use a reasonably-sized roof overhang to protectsouth - facing windows and a modest one on thenorth side.

� Keep natural shading. Don’t cut down shadetrees prior to construction. Locate the residenceto take free advantage of tree shading, especiallyfrom their upper canopies.

Experience has shown that the best exterior shade foreast - or west - facing windows is a tall tree full withleaves. If it is some distance from the building, it willprovide a nice scene to view when the sun is on the otherside of the house. It will also block low sun early in themorning or late in the afternoon. Blocking the sun’s raysbefore they reach the window is the most effectivepreventive strategy that can be used. Nature is the bestlandscape architect. (Deciduous trees are an effectiveshading device for south-facing windows in climaticzones with substantial winter/summer temperaturedifferences. They provide good shade in summer butdrop their leaves in winter to admit solar radiation intothe dwelling. Deciduous trees are of less pronouncedbenefit for Florida, however, due to our mild winters andhigh summer sun angles: for south - facing windows,direct summer sun is easily blocked by roof overhangs.)For guidance on roof overhangs, see FSEC publication“Roof Overhangs and Solar Time,” R. McCluney,FSEC-RM-5-80, (Supplement to “Sun Position inFlorida,” FSEC-DN-4-83, Rev. March, 1985.

If east - or west - facing windows cannot be avoided,either because the building already exists or because thebest views are in those directions, then follow ourgraduated recommendations for protecting them, and theoccupants, from the heat and glare effects of direct sunentry into the house. In these recommendations, tallexterior trees, separated from the house, were emphasizedfirst; the remaining recommendations follow.

� The next best outdoor shade for east - orwest - facing windows is a tall existingbuilding, opaque fence, tall and denselyfoliated hedge row, or possibly an overheadtrellis filled with green vines. Then comesthe large variety of awnings and other exteriorshades attached to the window, as shown in

The latter means smaller contributions to the inadvertentclimate modification that has been linked to globalwarming. These benefits, therefore, would also accruefrom any energy efficient windows present in a building.

Against these benefits must be balanced the extraenergy and materials costs required to manufacture highperformance windows. One way of viewing thisenvironmental impact would be to claim that the extraenvironmental costs attributable to the manufacturing ofenergy-efficient windows are more than balanced by theirenvironmental life-cycle savings.

Such arguments are prone to errors andmisrepresentations, but are currently popular in today’sculture. If you desire to market or promote energy-efficient windows as “green,” at least be mindful of thewords of Chris Hayhurst, writing in “Look for theLabel” in the May/June 2000 issue of E MAGAZINE.

“Still, don’t just dash into the store and fall for every‘environmentally friendly’ label you see. Not allclaims of environmental or social good are legitimate.A general rule: If the label comes directly from themanufacturer or from a private trade association,proceed with caution. It may be a ‘greenwashing,’unjustified environmental claims. On the other hand,if the label has been awarded by an independent,third-party program with no vested interest in theproduct itself, it’s probably for real.”

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Fig. 2. If effective shading already exists forthe windows, uninsulated windows withsingle pane clear glass can be purchased.Exterior shades have the value that heatabsorbed by them is carried away by thewinds and radiation, and doesn’t heat up thehouse as much as an interior one might do.

� The next best shade is a brightly-reflectingoperable interior shade that can be closed toreflect unwanted solar heat and glare, andopened for good views and daylight access.

� The final choice to be made is the kind of glazingfor the window. If moderate amounts ofdirect-beam sunlight can reach this glassduring the hottest parts of the day, then high-LSG glass should be used in this window,preferably single pane, if such a product isavailable. If the high-LSG value chosen isonly available in a soft-coat IGU, thehomeowner will have no choice but to use anIGU. These double-pane insulated windowsmay be desired for other reasons, such asimproved comfort on cold nights or hot days, toreduce the size and cost of the air conditioningsystem, for acoustic isolation, or to protectagainst possible future added costs from theutility company if clear, single-pane glasswere used. (If the building was built before thenew costs or other penalties for usinguninsulated windows are imposed, thebuilding may be “grand-fathered” in —exempted from the new requirements.)

� Finally, note that although plastic, appliedwindow films are a common retrofit choice formoderating glare and overheating problemson east - and west - facing windows, they areless effective than the previousrecommendations. In order for them to blocksufficient solar heat, they generally are quite dark,and have low visible transmittances. If they are

dark enough to provide effective shieldingfrom the glare and heat of direct sunlight,they may be so dark when the sun is on theother side of the house that they badlydegrade the view. Of course, window filmmanufacturers are continuously improving thefilms’ spectrally selective performance, andwindow films applied to single pane clear glasscan have LSG ratios a little greater than 1.0, butthese latest products may not be widelypromoted by retailers — possibly for reasonsof cost.

If the homeowner is considering window film, it isimportant to realize that some window manufacturersvoid their warranties if window films are applied to theirproducts. The main reason is that most window filmsaccomplish their solar glare reduction by absorbing solarradiation inside the film, causing it to heat up. Some ofthe heat in the film will conduct to the glass on which itis applied, causing this glass to expand. If it expandsenough, stresses can build up in the window and cause itto fail, most often through a shattering of the glass.

To see comparisons of some energy-savings fordifferent window types in Miami, Tampa, andJacksonville, visit the web site of the Efficient WindowsCollaborative at http://www.efficientwindows.org.This site also offers a special fact sheet, “SelectingEfficient Windows for Homes in Florida,” that can bedownloaded and printed. Computer software formaking such calculations is described athttp://www.efficientwindows.org/software.html.

Terminology - continued from page 9

spectrum. The emittance can vary outside that particularrange. Indiscriminate use of the term “low-e” can lead tomisunderstanding because the exact lower wavelength atwhich the emittance departs from its “low” value, as thewavelength decreases, is crucial to the different cold/hotapplications of the low-e principle. (Equivalently, interms of the reflectance, for the hot-climate version thereflectance must be high over both the NIR andFIR regions.)

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“Choosing the Best Residential Window Options for the Florida Climate — Guide for FloridaHomeowners,” R. McCluney and P. Jindra, FSEC Energy Note: FSEC-PF-358-00 (to be published).Contact the Efficient Windows Collaborative, Alliance to Save Energy, 1200 18th St., N.W., Suite 900,Washington, DC 20036 (Phone: 202-857-0666) and visit the fenestrations web site of the Florida SolarEnergy Center at http://www.fsec.ucf.edu/~fen/. This publication and others can be found there as well.The following somewhat more technical publications are available from the Florida Solar Energy CenterPublications Office.

“Low-E and Other Coatings and Glass Tints for Window Glass,” R. McCluney and P. Jindra, FloridaSolar Energy Center Technical Note: (FSEC-PF-359-00) (to be published).

“Rebuilding for Efficiency: Improving the Energy Use of Reconstructed Residences in South Florida,” co-authors D. Parker, P. Fairey, C. Gueymard, R. McCluney, J. McIlvaine, and T. Stedman, December1992, prepared for U. S. Department of Energy, Florida Energy Office, and Florida Power & Light.(FSEC-CR-562-92).

“Selecting Windows for South Florida Residences,” R. McCluney and C. Gueymard, prepared for FloridaEnergy Office, 2740 Centerview Dr., Tallahassee, FL 32399, 15 January 1993, (FSEC-CR-577-93).

“Fenestration Solar Gain Analysis,” Florida Solar Energy Center, 1 November 1996. (FSEC-GP-65-96).

“Let There Be Daylight,” Window Rehabilitation Guide for Historic Buildings, The Window Conferenceand Exposition for Historic Buildings II, National Park Service, Washington, DC, 19-21 February 1997,pp. IV-45-54.

Residential Windows: A Guide to New Technologies and Energy Performance, co-authors Carmody, John;Selkowitz, Stephen; Heschong, Lisa. W. W. Norton & Company, Inc., 550 Fifth Ave., NY, NY 10110(1996). 214 pp.

“The benefits of using window shades,” R. McCluney and L. Mills, ASHRAE Journal, November 1993,Vol. 35, No. 11, pp. 20-26. (FSEC-PF-263-93).

“Choosing the Best Windows for Hot Climates,” Ross McCluney, Proc. Innovative Housing ‘93Conference, Vancouver, BC, Canada, 21-25 June 1993, pp. 366-375. (FSEC-PF-253-93)

“Daylighting in America. Some Practical Suggestions for Proper Usage,” Ross McCluney, Lighting Design& Application, Vol. 15, No. 7, July 1985, pp. 36-38. (FSEC-PF-88-85).

“A Daylighting Checklist,” Ross McCluney, Solar Age Magazine, April 1985, p. 84.

Some of these papers, and additional information including this Energy Note, are available on the FSECFenestration Web site: http://www.fsec.ucf.edu/~fen/

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