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…