thermal comfort: concepts, measurements and...
TRANSCRIPT
Thermal Comfort:
Concepts, Measurements and Standards by
Shirish M. Deshpande
Theme
• Buildings
• Comfort : Facts
• Losses
• Parameters affecting
• Standards
• Measurements
Shelter …need
• Ever since we left the caves, buildings have been our bastions
against the natural elements
• Every culture has evolved its architecture to suit the local
environment
• In North America / Europe, buildings needs have to be heated to
guard the occupants against the biting cold outside; thus the
buildings are insulated to prevent cold drafts and heat loss
• In an Indian context, for more than 80% of the country, air-
conditioning is cooling
• In Indian summers, our worry is how to keep the heat OUT
and many a times we also need to deal with Humidity
Buildings
• Visual form (external)
• Designed for People (Use)
• To Provide Visual and Thermal Comfort for the occupants
• These buildings should be responsive, flexible and adaptive to
the changing demands / needs of its users
• These buildings support services:
• People (users / services)
• Processes (automation and control systems)
• Places (building fabric, structure and facilities)
Thermal Comfort
A state of mind, that expresses satisfaction with the thermal
surroundings
Influencing factors:
• Activity (metabolic rate)
• Clothing
• Air temperature
• Mean Radiant Temperature
• Air Movement
• Humidity
Thermal Discomfort
Thermal Discomfort
Thermal Comfort : Facts
• You feel comfortable when metabolic heat is dissipated at the
same rate it is produced
• The human body needs to be maintained at a 36 ± 0.5 °C
regardless of prevailing ambient conditions
• Air movement is essential for comfort as it enhances heat
transfer between air and the human body and accelerates
cooling of the human body
• Air movement gives a feeling of freshness by lowering skin
temperature, and the more varied the air currents in velocity and
direction, the better the effect
• A draught is created when temperature of moving air is too low
and / or the velocity is too high
Thermal Comfort : Facts
• At comfort room temperature (23 to 26 °C), acceptable air
velocity range is 0.15 to 0.50 m/s
• Higher the space %RH, the lower the amount of heat the human
body will be able to transfer by means of perspiration /
evaporation
• If indoor air temperature is high and absolute humidity is high
(above 11.5 g vapour per kg dry air), the human body will feel
uncomfortable
• Generally, %RH for indoor comfort conditions should not
exceed 70 %
Architect’s Role
• An architect’s primary functions as part of the design team is to
“create an environment”
• This environment has both a psychological and a
physiological effects on the occupants, which in turn, impacts
human productivity, visual and thermal comfort of the occupants
• Elements to be addressed while designing:
• Site location,
• building orientation and geometry,
• building envelope,
• arrangement of spaces, and
• local climatic characteristics
Solar Passive Design
• The basic idea of passive solar design is to allow daylight, heat
and airflow into a building only when beneficial
• The objectives are to control the entrance of sunlight and air
flows into the building at appropriate times and to store and
distribute the heat and cool air so it is available when needed
• Many passive solar design options can be achieved at little or
no additional cost. Others are economically viable over a
building’s life-cycle
Solar Passive Design
• Passive building design starts with consideration of siting and
day-lighting opportunities and the building envelope; then
building systems are considered.
• Almost every element of a passive solar design serves more
than one purpose; e.g.
• Landscaping can be aesthetic while also providing critical shading or
direct air flow
• Window shades are both a shading device and part of the interior
design scheme
• Masonry floors store heat and also provide a durable walking surface.
Sunlight bounced around a room provides a bright space and task
light
Envelope Design
• The building envelope, or “skin,” consists of structural materials
and finishes that enclose space, separating inside from outside.
This includes walls, windows, doors, roofs, and floor surfaces
• The envelope must balance requirements for ventilation and
daylight while providing thermal and moisture protection
appropriate to the climatic conditions of the site
• Envelope design is a major factor in determining the amount of
energy a building will use in its operation
• Also, the overall environmental life-cycle impacts and energy
costs associated with the production and transportation of
different envelope materials vary greatly
Envelope Design
• In keeping with the whole building approach, the entire design
team must integrate design of the envelope with other design
elements including material selection; day-lighting and other
passive solar design strategies; heating ventilating, and air-
conditioning (HVAC) and electrical strategies; and project
performance goals
• One of the most important factors affecting envelope design is
climate:
• Hot and Dry, Warm and Humid, Temperate, Composite or Cold
climates would have different design strategies
• Specific designs and materials can take advantages of or
provide solutions for the given climate
Envelope Design
• A second important factor in envelope design is what
occurs inside the building
• If the activity and equipment inside the building generate a
significant amount of heat, the thermal loads may be primarily
internal (from people and equipment) rather than external (from
the Sun), this affects the rate at which a building gains or loses
heat
• Building volume and siting also have significant impacts upon
the efficiency and requirements of the building envelope
• Careful study is required to arrive at a building foot-print and
orientation that work with the building envelope to maximize
energy benefits
Envelope Design
• In general, build walls, roofs and floors of adequate
thermal resistance to provide human comfort and energy
efficiency
• Roofs especially are vulnerable to solar gain in summer and
heat loss in winter
• If the framing system is of a highly conductive material, install a
layer of properly sized insulating sheathing to limit thermal
bridging
Envelope Design: Moisture Prevention
• Prevent moisture build-up within the envelope
• Under certain conditions, water vapor can condense within the
building envelope
• When this occurs, the materials that make up the wall can
become wet, lessening their performance and contributing to
their deterioration
• To prevent this, place a vapour-tight sheet of plastic or metal
foil, known as a vapour barrier, a near to the warm side of the
wall construction as possible
• For example, in areas with meaningful heating loads the vapour
barrier should go near the inside of the wall assembly. This
placement can lessen or eliminate the problem of water-vapour
condensation.
Envelope Design : Material Properties
• Consider the reflectivity of the building envelope
• In regions with significant cooling loads, select exterior finish
materials with light colours and high reflective envelope may
result in a smaller cooling load, but glare from the surface can
significantly increase loads on and complaints from adjacent
building occupant.
Upto 25% Cooling can
be lost through Roof
Upto 30% Cooling can
be lost through un-
insulated Walls
Losses
Increase internal loads
also add to cooling
load
Unshielded Glazing can
increase cooling load
Modes of Heat Transfer
External Climate : Solar Radiation
The energy content of a substance depend on its
temperature, mass and specific heat
Thermal Resistance of Clothing
1 clo ≈ 0.155m2 0k/W
• For a naked body ≈ Zero
• For a full 3-piece suit ≈ 1.5 clo
• Average Indian cloths ≈ 0.7clo
Emissivity (Ɛ) is the ratio of the thermal radiation from unit area
of a surface,
to the radiation from unit area of a black body at the same
temperature
Emissivity
Specific Heat
Specific heat of a substance is the amount of heat energy needed to raise the
temperature of a unit mass of a substance by 1 0C
The unit of specific heat is Joules/ kg deg C or Joules / kg deg K
Substance Specific Heat Cp
(cal / gm oC) (J / kg oC)
Air, dry (sea
level) 0.24 1005
Asphalt 0.22 920
Bone 0.11 440
Ice (0oC) 0.50 2093
Granite 0.19 790
Sandy clay 0.33 1381
Quartz sand 0.19 830
Water, pure 1.00 4186
Wet mud 0.60 2512
Wood 0.41 1700
Specific heat of air
is low because of
mass – for same
volume, the mass is
very low
Thermal Capacity & Calarofic Value
Thermal Capacity of a body is the product of its mass and the
specific heat of its material. It is measured as the amount of heat
required to cause unit temperature increase of the body. It is
measured in J/ 0C
Calorific value is the amount of heat released by unit mass of a
fuel or food material by its complete combustion and it is measured
in J/kg
Glass
Glass as a construction material need to be understood well before it is put to
use
• Need to be understood well for its thermal behaviour,
before it is incorporated in the design
• Glass needs to be placed in such a manner that, it
would bring in adequate day-light without brining in
much heat
• ECBC restricts window-to-Wall ratio (WWR) to 60%
• Glass needs to be appropriately shaded to improve its
Solar Heat Gain Coefficient (SHGC)
• In Indian context, SHGC is more significant
component than the assembly U-value
Thermal Gains : Conduction
• To reduce thermal transfer from conduction, develop details that
eliminate or minimize thermal bridges
• To reduce thermal transfer from convection, develop details that
minimize opportunities for air infiltration or exfiltration
• Plug, caulk or putty all holes in sills, studs, and joists. Consider
sealants with low environmental impact that do not
compromises indoor air quality
Envelope : Moisture Protection
• Although a primary function of the building envelope is
protecting the building interior and its occupants from inclement
weather...
• In the Western countries, 80% of insurance claims against
architects are related to moisture intrusion through the
building envelope
• Moisture intrusion is a leading cause of sick-building syndrome
• Water can enter through the building envelope by three
methods:
• direct rainwater intrusion,
• water vapour transmission, and
• negative pressurization (unwanted infiltration)
Vapour Transmission
• Often overlooked in design is water vapour transmission into
and across the building envelope
• Appropriate members of the design team should examine each
proposed building envelope assembly type and conduct a
vapour transmission analysis for each
• Calculation methods for evaluating vapour transmission and
determining the likelihood of moisture collecting within the
building envelope can be found in the ASHRAE Handbook-
Fundamentals
Vapour Transmission
• While negative pressurization of a building in an arid climate
generally has little air quality impact, IAQ problems can result
when it occurs in a Warm and Humid and sometimes even in a
moderate-climates
• The resulting infiltration of humid air, in addition to being an
added air-conditioning cost, can result in condensation in
unexpected and sometimes at unseen-places
• The ensuing problems (such as mold mildew, spore production,
etc.) can be so severe as to result in building evacuation and
extensive remedial costs, sometimes even exceeding the
original cost of the building
• Having to build a building twice is not sustainable !!!
Adaptive Comfort
• Architecture and engineering journals have been increasingly
attentive to innovative non-residential buildings designed with
operable windows. Such buildings may rely exclusively on
natural ventilation for cooling, or may operate as mixed-mode,
or “hybrid” buildings that integrate both natural and mechanical
cooling
• Architects who want to incorporate natural ventilation as an
energy-efficient feature need to collaborate closely with HVAC
engineers
• Unfortunately, engineers often need to veto such natural
approaches, citing their professional obligation to adhere to
thermal comfort standards such as ASHRAE Standard 55 or
ISCO 7730
Adaptive Comfort
• In their current form, these standards establish relatively tight
limits on recommended indoor thermal environments, and do
not distinguish between what would be considered thermally
acceptable in buildings conditioned with natural ventilation v/s
air conditioning
• In other words, engineers have not had a suitable tool to
help decide when and where full HVAC is required in a
building, and under what circumstances they can
incorporate more energy-conserving strategies without
sacrificing comfort
Adaptive Comfort
• Architecture and engineering journals have been increasingly
attentive to innovative non-residential buildings designed with
operable windows. Such buildings may rely exclusively on
natural ventilation for cooling, or may operate as “mixed-
mode”, or “hybrid” buildings that integrate both natural and
mechanical cooling
• Architects who want to incorporate natural ventilation as an
energy-efficient feature need to collaborate closely with HVAC
engineers
• Unfortunately, engineers often need to veto such natural
approaches, citing their professional obligation to adhere to
thermal comfort standards such as ASHRAE Standard 55 or
ISCO 7730
Adaptive Comfort
• While ASHRAE Standard 55 was originally intended to provide
guidelines for centrally controlled HVAC, its broad application in
practice is hindering innovative efforts to develop more person-
centred strategies for climate control in naturally ventilated or
mixed-mode buildings
• Such strategies may hold great social and environmental
benefits, reducing energy consumption and increasing
occupant satisfaction, especially in office buildings
Adaptive comfort
• It advocates for a more flexible thermal comfort standard have
long argued that the primary limitation of ASHRAE Standard 55
is its “one-size-fits-all” approach, where clothing and activity
are the only modifications one can make to reflect
seasonal differences in occupant requirements
• The standard was originally developed through laboratory test
of perceived thermal comfort, with the limited intent to establish
optimum HVAC levels for fully climate controlled buildings.
Clothing … to suit Climate
Clothing … changing with Season
C
• Such issues have particular relevance with regards to naturally ventilated
buildings, where occupants are able to open windows, creating indoor
conditions that are inherently more variable than buildings with centralized
HVAC systems. In such settings, an alternative thermal comfort standard
based on field measurements might be able to account for contextual and
perceptual factors absent in the laboratory setting. Toward this end,, the
research began by focusing on three primary modes of adaptation:
physiological, behavioural and psychological.
• Physiological adaption, also known as acclimatization, refers to biological
responses that result from prolonged exposure to characteristic and
relatively extreme thermal conditions. One example in hot climates is a fall
in the set point body temperature at which sweating is triggered, leading to
an increased tolerance for warmer temperatures. Laboratory evidence
suggests, however, that such acclimatization does not play a strong role in
subjective preferences across the moderate range of activities and thermal
conditions present in most buildings.
Adaptive Comfort
• In naturally ventilated buildings, where occupants are able to
open windows, creating indoor conditions that are inherently
more variable than buildings with centralized HVAC systems
• In such settings, an alternative thermal comfort standard based
on field measurements might be able to account for contextual
and perceptual factors absent in the laboratory setting
• Lot of research is being done or going on by focusing on three
primary modes of adaptation:
• physiological,
• behavioural and
• psychological
• Behavioral adaptation refers to any conscious or
unconscious action a person might make to alter their body’s
thermal balance
• Examples include changing clothes or activity levels turning
on a fan or heater, or adjusting a diffuser or thermostat
• Behavioral adjustments offer the best opportunity for people
to participate in maintaining their own thermal comfort
• Providing ample opportunities for people to interact with and
control the indoor climate is an essential strategy in the
design of naturally ventilated buildings
Adaptive Comfort
PMV: Air-Conditioned Buildings
• In the air-conditioned buildings (Figure 1), the observed (dotted ) and
predicted (solid) lines appear very close together, demonstrating that
PMV was remarkably successful at predicting comfort temperatures in
these buildings
• A corollary of this finding is that, in air-conditioned buildings,
behavioural adjustments to clothing and room air speeds fully explain
the relationship between indoor comfort temperature and outdoor
climatic variation
PMV: Ventilated Buildings
• In the context of naturally ventilated buildings, where the observed
responses show a gradient almost twice as steep as the PMV model’s
predicted comfort levels
• By logical extension therefore, it appears that behavioural adjustments
(clothing and air velocity changes) may account for only half of the
climatic dependence of comfort temperatures within naturally ventilated
buildings
Adaptive Comfort
Adoptive Comfort
• Having accounted for the effects of behavioural adaptations,
physiological (acclimatization) and psychological components of
adaptation are left to explain the divergence
• However, existing literature suggests that acclimatisation is
unlikely to be significant factor
• This leaves psychological adaptation as the most likely
explanation for the difference between field observations and
PMV predictions in naturally ventilated buildings
• This means the physics governing a body’s heat balance must
be inadequate to fully explain the relationship between
perceived thermal comfort in naturally ventilated buildings and
exterior climatic conditions
An Adaptive Comfort Standard
• Using ASHRAE Standard 55 to determine acceptable indoor temperature
ranges requires one to know, or at least anticipate, the average metabolic
rate and amount of clothing worn by people in a building, regardless of
whether that building is already built or occupied
• In contrast, an adaptive model relates acceptable indoor temperature
ranges to mean monthly outdoor temperature (in this case, defined as the
arithmetic average of mean monthly minimum and maximum air
temperature)
• This is a parameter already familiar to engineers and can be found easily
by examining readily available climate data
• Because the adaptive model is based on extensive field measurements,
the relationship between expected clothing and outdoor climate already is
built into the empirical statistical relationship
Some Findings
• The research has demonstrated that occupants of buildings with
centralized HVAC systems become finely tuned to the very
narrow range of indoor temperatures presented by current
HVAC practice
• They develop high expectations for homogeneity and cool
temperatures, and soon became critical if thermal conditions do
not match these expectations
• In contrast, occupants of naturally ventilated buildings appear
tolerant of – and, in fact, prefer – a wider range of temperatures.
This range may extend well beyond the comfort zones
published in Standard 55-1992, and may more closely reflect
the local patterns of out-door climate change
Some Findings
• The research has demonstrated that occupants of buildings with
centralized HVAC systems become finely tuned to the very
narrow range of indoor temperatures presented by current
HVAC practice
• They develop high expectations for homogeneity and cool
temperatures, and soon became critical if thermal conditions do
not match these expectations
• In contrast, occupants of naturally ventilated buildings appear
tolerant of – and, in fact, prefer – a wider range of temperatures.
This range may extend well beyond the comfort zones
published in Standard 55-1992, and may more closely reflect
the local patterns of out-door climate change
Some Findings
• In many climatic settings, the practice of maintaining a narrowly
defined, constant range of temperatures in fully air conditioned
buildings is unnecessary, and carried as high-energy cost
• However, the research showed that the PMV model could not
predict people’s thermal preferences in naturally ventilated
buildings. This would seem to indicate the PMV model is an
unsuitable guide when deciding whether air conditioning is even
necessary in a particular building
• Adaptive model of thermal comfort may usefully augment
laboratory based predictive models in the setting of thermal
comfort standards
The conditions for transfer of energy through the building fabric and for
determining the thermal response of people are local and site specific.
These conditions are grouped together under the term of “Microclimate”
which includes wind, radiation, temperature and humidity experienced
around the building.
The Microclimate of a site is affected by the following factors:
• Landforms
• Vegetation
• Water bodies
• Street width and orientation
• Open space and Built form
Micro-climate
Creating Micro-climate in the Open Areas
Double Screen Facade
Open Grid Pavers
Credit
ASHRAE 55
ASHRAE Book of Fundamentals
ASHRAE Journal
My energy audit experience
Thank You !
Radiant Heat Gain
• Radiant gains can have a significant impact on heating and
cooling loads. A surface that is highly reflective of solar
radiation will gain much less heat than one that is adsorptive.
In general, light colours decrease solar gain while dark ones
increase it.
• This may be important in selecting roofing materials because
of the large amount of radiation to which they are exposed over
the course of a day; it may also play a role in selecting thermal
storage materials in passive solar buildings.
• Overhangs are effective on South-facing facades while a
combination of vertical fins and overhangs are required on
East and West exposures
Monthly Diurnal Averages
Temperature Range
Monthly Temperature Range
Solar Radiation Range
Sky Cover Range
Monthly Wind Velocity
Ground Temperature
Dry Bulb v/s %RH
Dry Bulb v/s Dew Point
Ancient Methods
• In India, mostly thermal comfort meant avoiding heat stress
• Our master builders of yore evolved an elegant three pronged
formula for thermal comfort:
• Raise barriers against Sun-light
• Use mass to delay heat transmission
• Drain out the residual heat to flowing water and to the sky by
radiation, mostly at night
• None of these processes needed external energy
Ancient Techniques
• Firstly the barriers comprised of trees, shaded
verandas and carved stone screens
• The trees also kept the ground shaded, besides the
walls
• They are, of course, the best air fresheners and
evaporative coolers
• The builders of our heritage structures have used
mass quite effectively
• The Sun would take a lot longer to heat a massive
heritage building wall than a thin modern building
Envelope Design: Hot & Dry • In Hot and Dry climates, use materials with high thermal
mass
• Buildings in hot/dry climates with significant diurnal temperature
swings have traditionally employed thick walls constructed from
envelope materials with high mass (masonry)
• These buildings will lessen and delay the impact of temperature
variations from the outside wall on the wall’s interior
• The material’s high thermal capacity allows heat to penetrate
slowly through the wall or roof
• As the temperature in Hot and Dry climates tends to fall
considerably after Sunset, the result is a thermal flywheel
effect – the building interior is cooler than the exterior during the
day and warmer than the exterior at night
Envelope Design : Warm & Humid
• In Warm and Humid climates use materials with low thermal
capacity
• In these climates, night-time temperatures do not drop
considerably below day-time highs, thus, light materials with little
thermal capacity are preferred.
• In these climates, masonry materials also function as a
desiccant, to absorb moistures
• Roofs and walls should be protected by plant materials or
overhangs
• Large openings protected from the summer Sun should be
located primarily on the North and South sides of the envelope to
catch breeze
Envelope Design : Temperate
• In temperate climates, select materials based on location
and the heating / cooling strategies to be employed
• Determine the thermal capacity of materials for buildings in
temperate climates based upon the specific locale
• Walls should be well insulated
• Openings in the skin should be shaded during hot times of the
year and un-shaded during cool months
• This can be accomplished by roof overhangs sized to respond
to solar geometries at the site or by the use of awnings
Envelope Design: Cold
• In colder climates design wind-tight and well-insulated
building envelopes
• The thermal capacity of materials used will depend upon the
use of the building and the heating strategy employed
• A building that is conventionally heated and occupied
intermittently should not be constructed with high mass
materials because they will lengthen the time required to reheat
the space to a comfortable temperature
• Where solar gain is not used for heating, the floor plan should
be as compact as possible to minimize the area of building skin
Conduction
It is one dimensional heat flow (q) through planer building
component
•Heat flow from indoor air to indoor surfaces
•Heat flow through the component
•Heat flow from outdoor surface to outdoor air
Conduction
• Conduction occurs when two bodies of different temperatures
are put in contact
• As faster molecules collide with slower ones, they lose their
energy in the process leading to a convergence of two
temperature levels
• Some materials such as materials are good conductors while
others such as wood are poor conductors – poor conductors
act as insulators
Surface conductance
• In addition to the resistance of a body to the flow of heat, a resistance will
be offered by its surfaces, where a thin layer of air film separates the body
from the surrounding air
• A measure of this is the surface of film resistance, denoted thus, 1/f (m2
deg C/W), where f being the surface or film conductance ( W/m2 deg C )
• Surface conductance includes the convective and the radiant components
of the heat exchange at surfaces
• The magnitude of surface or film-conductance is a function of surface
qualities and of the velocity of air passing the surface
Surface Resistance, Conductance
The rate of conduction (or
vibration speed of
molecules) or conductivity
varies with different
materials and is described
as property of material
Conduction
Thermal conductivity (λ) is measured as the rate of heat flow (flow of energy per unit time) through
area of unit thickness of the material, when there is unit temperature
difference between the two sides
It is measured in Watt/ m. deg Kelvin (equivalent to Watt/ m. deg Celsius)
Thermal Bridges
IR Thermography
77
Computer Simulation
• The reciprocal of air-to-air resistance is the air-to-air
transmittance
• Its unit of measurement is the same as for conductance – W/m2
deg C
• the only difference being that here the air temperature difference
( and not the surface temperature difference) will be taken into
account
• This is the quantity most often used in building heat loss and
heat gain
• problems as its use greatly simplifies the calculations
Transmission
Coefficient of Heat Transfer: U-value
• It is a measure of the rate of non-solar heat loss or gain through
a material or assembly
• It gauges how well a material allows heat to pass through
•U-value ratings generally fall between 0.20 and 1.20
• The lower the U-value, the greater a product's resistance to
heat flow and the better its insulating value.
• The inverse of the U-value is the R-value or Resistivity of the
material
•U-value is expressed in units of W/m2 °C or Btu/hr ft2 °F
• In the US, values are normally given for NFRC / ASHRAE winter conditions of 0°F (-18°C) outdoor temperature, and for
70°F (21°C) indoor temperature, with 15 mph of wind speed,
and no solar load
• U-values are often quoted for windows and doors
• In the case of a window, for example, the U-value may be
expressed for the glass alone or for the entire window
assembly, which includes the effect of the frame and the
spacer materials
Coefficient of Heat Transfer: U-value
IAQ Problems
• At AHU, we spend energy to remove moisture and then add
moisture and hazardous chemicals to the supply air…