Figure 1: Environmental factors affecting thermal
comfort
THERMAL COMFORT
1.0 Introduction
The first major issue about climate is the comfort level. Thus, one of the main functions of a
building is to provide to its occupant’s protection against external conditions of the environment.
The human body operates at optimum temperature of about 37°C; a reasonable change in this
value can lead to illness or even death.
Thermal comfort basically has to do with the temperature that the occupants of a building
consider as comfortable to stay in. Indoor thermal comfort is achieved when occupants are able
to pursue without any hindrance, activities for which the building is intended (ISO 7730 as well
as ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers).
Thermal comfort is defined as that condition of mind which expresses satisfaction with the
thermal environment. Having good thermal comfort is very necessary as it promotes good living,
increase building performance etc.
Factors affecting Thermal Comfort
The factors affecting Thermal comfort can be grouped into two categories: Environmental and
Personal Factors. Environmental factors are the components of the thermal environment, they
include air temperature, mean radiant temperature, relative humidity and air velocity. Fig. 1
shows the environmental factors affecting thermal comfort. On the other hand, personal factors
refer
AIR to the characteristics of the occupant and they are clothing and metabolic
TEMPERATURE activities
MEAN RADIANT
TEMPERATURE
THERMAL COMFORT
AIR VELOCITY HUMIDITY
Environmental factors
Air Temperature
Air temperature (dry bulb temperature or DBT). The dry-bulb temperature is the temperature of
the air around us and is the most important of all of the above factors. The human body's primary
response is towards the changes in temperatures and it is this temperature that we attempt to keep
within comfort conditions while designing structures for habitation.
Humidity
Humidity of the atmosphere has little effect on thermal comfort sensation. Of the various
measures of humidity, the relative humidity (RH) is here the most immediately relevant. Relative
humidity is the ratio of the amount of water vapor actually present in the air to the maximum
amount that the air could hold at the same temperature, expressed as percentage. The higher the
relative humidity of a space, the lower the air temperature should be. Relative humidity is more
critical at high temperatures than within the normal temperature range. High humidity
environments have a lot of vapour in the air, which prevents the evaporation of sweat from the
skin. In hot environments, humidity is important because less sweat evaporates when humidity is
high (80% +).
The evaporation of sweat is the main method of heat loss in humans. The human body has
sensors that are fairly efficient in sensing heat and cold, but they are very effective in
detecting relative humidity. Relative humidity creates the perception of an extremely dry or
extremely damp indoor environment. This can then play a part in the perceived
temperature and their thermal comfort. The recommended level of humidity is in the range
of 30-60%.
, Air Velocity
Air motion increases heat loss by convection and evaporation. The cooler the moving air stream
is, relative to the room air temperature, the less velocity is should have. Air motion is especially
helpful for cooling evaporation in hot, humid weather.
Mean Radiant Temperature (Radiation) [P/a]
Mean radiant temperature (MRT) is important to thermal comfort since the human body receives
radiant heat from or loses heat by radiation to the surrounding surfaces if their MRT is
significantly higher or lower than the air temperature. The higher the MRT of the surrounding
surfaces, the cooler the air temperature should be. MRT has about 40% more effect on comfort
than air temperature. In cold weather, the MRT of the interior surfaces of exterior walls should
not be more than 9 degrees below the indoor air temperature. The mean radiant temperature
cannot be measured directly, but it can be approximated by globe temperature measurements.
Personal Factors Affecting Thermal Comfort
Clothing Insulation
Clothing to a very large extent influences levels of thermal comfort in humans, this is because it
alters the heat loss and consequently the thermal balance. In cold weather, layers of clothing can
help keep a person warm by preventing heat loss. Also, if a large amount of work is be
performed by the person, thick layers of clothing can prevent heat loss and in turn lead to
overheating of the body. The thicker a garment is, the greater insulating ability it has. The
insulating ability of the material used in making the clothing can be affected by; air movement
and relative humidity.
The amount of clothing is measured against a standard amount that is roughly equal to a typical
business suit, shirt, and undergarments. Clo is used to measure the thermal insulation of clothes.
The standard amount of insulation required to keep resting person warm in a windless room at
70˚F is equal to one Clo. 1 Clo is equal to 0.155m2 · K/W. ASHRAE 55.
Metabolic Rate
This describes the heat we produce inside our bodies as we carry out physical activities. The
more physical work we do, the more heat we produce. If this heat is not released out properly,
overheating could occur. A person’s physical characteristics should be considered assessing
his/her thermal comfort level. Factors such as size, weight, age, fitness level and sex can all have
impact on how they feel, even if all other environmental factors are constant. People who occupy
the same room can experience different levels of thermal comfort due to their rate of metabolism.
This makes it difficult to set an optimal temperature for everyone in a given location. Food and
drink habits also affect metabolic rates, which indirectly influences thermal preferences.
Other factors which affect thermal comfort or thermal sensation of different individuals in the
same environment include:
(i) Acclimatization: The body has a way of adjusting to new set of climatic conditions. This
process is achieved fully in about 30 days. By the stated time the thermal reference of the
individual will change.
(ii) Food and drink: Foods and drinks of certain kinds may affect metabolic rate. Heavy foods
require more muscular activity than light foods which require less. More muscular activity means
greater metabolic heat production.
(iii) Body Size and shape: A thin person would have a greater surface-to-volume ratio than
someone with a more rounded body shape, so a proportionately greater heat exchange with the
environment. The more rounded person would prefer a lower temperature, partly because of the
lower surface-to volume ratio, but also because subcutaneous fat is a good insulator.
(iv) State of health: The state of health of an individual also influences thermal requirements.
Metabolic rates may increase or decrease as a result of improper functioning of the regulatory
mechanism.
(v) Age and gender: Age and sex also affect thermal preferences; older people tend to have a
narrower comfort range. (Andris et al). The metabolism of older people is slower, hence their
preference to higher temperatures. Women also have slower rates of metabolism than men; with
a preference of 1˚C higher than that of men on the average.
(vi) Skin Colouration: It has been demonstrated that the lightest skin reflects about three times as
much as solar radiation as the darkest. Light skins are however vulnerable to sunburn and other
comfort
THERMAL COMFORT
1.0 Introduction
The first major issue about climate is the comfort level. Thus, one of the main functions of a
building is to provide to its occupant’s protection against external conditions of the environment.
The human body operates at optimum temperature of about 37°C; a reasonable change in this
value can lead to illness or even death.
Thermal comfort basically has to do with the temperature that the occupants of a building
consider as comfortable to stay in. Indoor thermal comfort is achieved when occupants are able
to pursue without any hindrance, activities for which the building is intended (ISO 7730 as well
as ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers).
Thermal comfort is defined as that condition of mind which expresses satisfaction with the
thermal environment. Having good thermal comfort is very necessary as it promotes good living,
increase building performance etc.
Factors affecting Thermal Comfort
The factors affecting Thermal comfort can be grouped into two categories: Environmental and
Personal Factors. Environmental factors are the components of the thermal environment, they
include air temperature, mean radiant temperature, relative humidity and air velocity. Fig. 1
shows the environmental factors affecting thermal comfort. On the other hand, personal factors
refer
AIR to the characteristics of the occupant and they are clothing and metabolic
TEMPERATURE activities
MEAN RADIANT
TEMPERATURE
THERMAL COMFORT
AIR VELOCITY HUMIDITY
Environmental factors
Air Temperature
Air temperature (dry bulb temperature or DBT). The dry-bulb temperature is the temperature of
the air around us and is the most important of all of the above factors. The human body's primary
response is towards the changes in temperatures and it is this temperature that we attempt to keep
within comfort conditions while designing structures for habitation.
Humidity
Humidity of the atmosphere has little effect on thermal comfort sensation. Of the various
measures of humidity, the relative humidity (RH) is here the most immediately relevant. Relative
humidity is the ratio of the amount of water vapor actually present in the air to the maximum
amount that the air could hold at the same temperature, expressed as percentage. The higher the
relative humidity of a space, the lower the air temperature should be. Relative humidity is more
critical at high temperatures than within the normal temperature range. High humidity
environments have a lot of vapour in the air, which prevents the evaporation of sweat from the
skin. In hot environments, humidity is important because less sweat evaporates when humidity is
high (80% +).
The evaporation of sweat is the main method of heat loss in humans. The human body has
sensors that are fairly efficient in sensing heat and cold, but they are very effective in
detecting relative humidity. Relative humidity creates the perception of an extremely dry or
extremely damp indoor environment. This can then play a part in the perceived
temperature and their thermal comfort. The recommended level of humidity is in the range
of 30-60%.
, Air Velocity
Air motion increases heat loss by convection and evaporation. The cooler the moving air stream
is, relative to the room air temperature, the less velocity is should have. Air motion is especially
helpful for cooling evaporation in hot, humid weather.
Mean Radiant Temperature (Radiation) [P/a]
Mean radiant temperature (MRT) is important to thermal comfort since the human body receives
radiant heat from or loses heat by radiation to the surrounding surfaces if their MRT is
significantly higher or lower than the air temperature. The higher the MRT of the surrounding
surfaces, the cooler the air temperature should be. MRT has about 40% more effect on comfort
than air temperature. In cold weather, the MRT of the interior surfaces of exterior walls should
not be more than 9 degrees below the indoor air temperature. The mean radiant temperature
cannot be measured directly, but it can be approximated by globe temperature measurements.
Personal Factors Affecting Thermal Comfort
Clothing Insulation
Clothing to a very large extent influences levels of thermal comfort in humans, this is because it
alters the heat loss and consequently the thermal balance. In cold weather, layers of clothing can
help keep a person warm by preventing heat loss. Also, if a large amount of work is be
performed by the person, thick layers of clothing can prevent heat loss and in turn lead to
overheating of the body. The thicker a garment is, the greater insulating ability it has. The
insulating ability of the material used in making the clothing can be affected by; air movement
and relative humidity.
The amount of clothing is measured against a standard amount that is roughly equal to a typical
business suit, shirt, and undergarments. Clo is used to measure the thermal insulation of clothes.
The standard amount of insulation required to keep resting person warm in a windless room at
70˚F is equal to one Clo. 1 Clo is equal to 0.155m2 · K/W. ASHRAE 55.
Metabolic Rate
This describes the heat we produce inside our bodies as we carry out physical activities. The
more physical work we do, the more heat we produce. If this heat is not released out properly,
overheating could occur. A person’s physical characteristics should be considered assessing
his/her thermal comfort level. Factors such as size, weight, age, fitness level and sex can all have
impact on how they feel, even if all other environmental factors are constant. People who occupy
the same room can experience different levels of thermal comfort due to their rate of metabolism.
This makes it difficult to set an optimal temperature for everyone in a given location. Food and
drink habits also affect metabolic rates, which indirectly influences thermal preferences.
Other factors which affect thermal comfort or thermal sensation of different individuals in the
same environment include:
(i) Acclimatization: The body has a way of adjusting to new set of climatic conditions. This
process is achieved fully in about 30 days. By the stated time the thermal reference of the
individual will change.
(ii) Food and drink: Foods and drinks of certain kinds may affect metabolic rate. Heavy foods
require more muscular activity than light foods which require less. More muscular activity means
greater metabolic heat production.
(iii) Body Size and shape: A thin person would have a greater surface-to-volume ratio than
someone with a more rounded body shape, so a proportionately greater heat exchange with the
environment. The more rounded person would prefer a lower temperature, partly because of the
lower surface-to volume ratio, but also because subcutaneous fat is a good insulator.
(iv) State of health: The state of health of an individual also influences thermal requirements.
Metabolic rates may increase or decrease as a result of improper functioning of the regulatory
mechanism.
(v) Age and gender: Age and sex also affect thermal preferences; older people tend to have a
narrower comfort range. (Andris et al). The metabolism of older people is slower, hence their
preference to higher temperatures. Women also have slower rates of metabolism than men; with
a preference of 1˚C higher than that of men on the average.
(vi) Skin Colouration: It has been demonstrated that the lightest skin reflects about three times as
much as solar radiation as the darkest. Light skins are however vulnerable to sunburn and other
