THERMAL COMFORT
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James Marston Fitch
American Building: The Environmental Forces that Shape It
“The fundamental thesis of this book
is that the ultimate task of
architecture is to act in
favor of man: to interpose itself
between man and the natural
environment in which he finds
himself, in such a way as to remove
the gross environmental load from
his shoulders. The central function of
architecture is thus to lighten
the very stress of life.”
James Marston Fitch (1909–2000) was an architect
and a Preservationist, one of the founders of the
Graduate School of Architecture, Planning, and
Preservation at Columbia University in 1964. (wikipedia)
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“Removing the Load” in Context
context is critical: bus shelter versus elementary school versus hospital operating room
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Thermal Comfort
• Why?
– Why should we worry about thermal comfort?
• What?
– What is thermal comfort?
• How?
– How is thermal comfort addressed during
design?
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Why Worry About Thermal Comfort?
• Designers have an ethical responsibility to
cause no harm – my personal opinion
• Gripes about thermal comfort are consistently
the number one complaint heard by building
managers/owners
• Comfort decisions can have substantial energy
and resource consumption implications
• Comfort is the basis for a substantial investment
(in a climate control system)
• Green design demands (sort of ) thermal comfort
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Two Ways to Look at
Thermal Comfort
As a psychological phenomenon
As a physical phenomenon
Both views are valid, both must be
tempered by statistics, and both views
are important to building design efforts
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What is Thermal Comfort?
“That condition of mind which expresses
satisfaction with the thermal environment
and is assessed
by subjective evaluation.”
ASHRAE Standard 55-2010
Thermal Environmental Conditions for Human Occupancy
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ASHRAE?
Reminder  The American Society of
Heating, Refrigerating and Air-Conditioning
Engineers, Inc.
(www.ashrae.org)
A key developer and promulgator of building design
standards and guidelines.
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Condition of Mind?
• Thermal comfort is defined as being an opinion
(essentially an individual perception)
• This opinion will be influenced by the
environmental conditions surrounding a person
and by his/her interpretation/impression of those
conditions
– Physical conditions are both group and individual
• Room air temperature is typically common to a group
• MRT (mean radiant temperature) is often experienced
differentially by those in the same space
– Interpretation is solely individual
• Nevertheless, a group consensus might be ascertained
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Condition of Mind?
• A perception (condition of mind) is best assessed
by asking people (occupants) how they feel
• A critical note: one can only ask people who exist
and occupy the environment of interest—thus,
“asking” is a POE (post occupancy evaluation)
activity … there is no one to ask about conditions
in a space until that space is built and occupied
• Information from existing situations, however, can
be collected, collated, and provided as guidance
(or precedent) for future designs
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Subjective Evaluation (Asking)
1. The traditional 7-point “status” scale:
cold | cool | slightly cool | neutral | slightly warm | warm | hot
2. An alternative “action” scale:
Would you prefer: to be warmer | no change | to be cooler
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Uses for a “condition of mind”
(psychological) approach
• As a post-occupancy evaluation (POE) tool
– If people are in the space being evaluated
• Not directly usable as a design tool
– There is no occupied space during design
• But ... POE/laboratory studies can provide
useful information (trends and patterns) to
help develop a design tool
– Such as a “comfort chart”
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ASHRAE Thermal Comfort Chart
(from Standard 55-2010)
comfort
zone(s)
addressing operative temperature, relative humidity, and occupant clothing
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Midstream Thoughts
• Thermal comfort is an important design intent (in my
opinion) and can be easily benchmarked via criteria
• Thermal delight might be an even better intent here
and there (see Lisa Heschong: Thermal Delight in
Architecture)
• Thermal comfort is not typically required by building
codes (it is rarely required, in fact) so it will become a
design issue mainly via intent/criteria (or through
general practice or by accident)
• Thermal comfort as a psychological-statistical
concept should be straightforward
• But …. we need to understand the physical basis of
comfort to really use it in design (since environmental
elements will need to be manipulated)
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humidity
Mapping Physical & Psychological
Comfort Territories
temperature
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Physical Context of Thermal Comfort
-- dishealth
-- dishealth
conditions
the body’s response
see required reading for more details on these responses
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Physical Basis of Thermal Comfort
Fundamentally, comfort involves a heat balance
(a thermal equilibrium) … where:
heat in
≈
heat out
where “heat in” is provided by metabolism,
radiation, conduction, convection
where “heat out” is via radiation, conduction,
convection, evaporation
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Heat Flow
Mechanisms
three external “to” mechanisms; four “from” mechanisms
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Heat Flow to/from Human Body
Sensible Heat
– Flows via conduction, radiation, and convection
– Flow rate is generally related to space temperatures
Latent Heat
– Flows via evaporation
– Flow rate is generally related to space humidity
Total Heat Flow = sensible + latent flows
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Heat Flow to/from Human Body
Conduction (sensible)
Convection (sensible)
Radiation (sensible)
Evaporation/Condensation (latent)
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Conduction
The flow of heat between two adjacent
and touching solids (or from one part to
another part within an object) by direct
interaction between molecules
example: walking on a beach in your bare
feet
for comfort, the key environmental variable
is: SURFACE TEMPERATURE
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Convection
The flow of heat from the surface of a
material to/from a surrounding fluid
(usually air); the free motion of molecules
of the fluid is very important in promoting
heat flow
example: fanning yourself with a
newspaper
for comfort, the key environmental variables
are: AIR TEMPERATURE | AIR SPEED
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Radiation
The flow of heat between objects that are
not in direct contact—but that can “see”
each other via electromagnetic radiation;
the objects may be a few inches or a
million miles apart
example: warming yourself in front of a
fireplace
for comfort, the key environmental variable
is: SURFACE TEMPERATURE
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Evaporation
The flow of heat that must be provided as
a material changes state (from a liquid to
a gas); this heat represents the energy
required to break molecular bonds (called
the latent heat of vaporization)
example: feeling cool coming out of a
swimming pool on a breezy day
for comfort, the key environmental variables
are: RELATIVE HUMIDITY | AIR SPEED
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The Mechanisms Adapt
the body automatically adapts to surrounding environmental conditions in its quest for
thermal equilibrium; under high temperatures, evaporation becomes critically important
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Environmental Comfort Factors
•
•
•
•
Air temperature (dry bulb – deg F)
Relative humidity (%)
Air speed (ft per min)
Radiant conditions
– Mean radiant temperature [MRT] in deg F
– or other radiation value in Btuh per sf
These factors are controllable through design – a passive system
should control all four factors; an active (HVAC) system is expected to
control the first three (with “architecture” controlling the fourth)
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Measuring Environmental Factors
air temperature,
RH, wind speed
air speed
data logging
surface
temperature
wet and dry bulb temperatures
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MRT
MRT stands for mean radiant temperature
MRT is the (hypothetical) uniform
temperature of surrounding surfaces with
which the human body would exchange the
same heat by radiation as occurs in an
actual (non-uniform) environment
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MRT
Surface temperatures in a typical room are
often not all the same (for example, cold
window glass, warm radiators); the human
body will radiate to/from these different
surfaces. MRT is the temperature (if all
surfaces were at this one temperature) at
which the body would exchange the same
heat by radiation as occurs in the messy,
many-temperature real space.
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Estimating MRT
MRT = (angle 1)(temp 1) + (angle 2)(temp 2) + ….
360 deg
repeat for two additional space views … and then average
2
1
2
1
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Personal Factors Affecting Comfort
• Physical
– Clothing (specifically its insulation value in “clo”)
– Activity level (specifically metabolic heat
production in “met”)
• Mental
– State of mind (experiences, expectations,
influences of other conditions, …)
These factors are not controlled through design,
but must be understood by a designer as they
will affect occupant thermal comfort responses
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Typical clo Values
http://www.homeenergy.org/
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Typical met Values
http://www.homeenergy.org/
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Physical Basis of Thermal Comfort
The potential for thermal equilibrium is:
– Influenced by environmental factors
• Often common to all occupants in a space
• Designer must control these conditions
– Influenced by personal physical factors
• Individual to each occupant in a space
• Designer must be aware of and consider these
conditions
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The Designer’s Job
Understand the physical basis of thermal
comfort and related variables
Appreciate the influence of the psychological
aspects of thermal comfort
Use this understanding and appreciation to
design spaces that building users will decide
are thermally comfortable
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www.wbdg.org/images/acoustic_4.jpg
Are We Happy Yet?
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