There are six major factors that determine
thermal comfort.
| Environmental | Air temperature (dry bulb temperature or DBT) Humidity (relative humidity RH) Air movement (velocity v in m/s) Radiation (Mean radiant temperature MRT) |
| Individual | Activity Clothing |
Air temperature (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 (RH%)
Humidity of the atmosphere has little effect on thermal comfort sensation at or near-
comfortable temperatures unless it is extremely low or extremely high. Of the various
measures of humidity, the relative humidity (RH) is here the most immediately relevant, as
it determines the possible evaporation rate. Moisture from the skin evaporates much more
quickly in a dry than a humid atmosphere. At high temperatues, skin evaporation is the
most important heat dissipation channel. Saturated air at 100% prevents any evaporative
cooling.
Air movement (v)
The air movement can produce different thermal effects at different air temperatures,
in the following two ways:
- It increases convective heat loss, as long as the temperature of the moving air is less than the skin temperature. If this condition is not fulfilled, the air actually warms the skin.
- It accelerates evaporation, providing a physiological cooling. Its effect is insignificant at humidities lower than 30% , when there is an unrestricted evaporation even with still air, and humidities above 85%, when even air movement cannot help add vapour to the already highly saturated air.
'Pleasant' ranges of air movements induce skin evaporation, more significantly in medium (40%-50%) humidities.
Thermal Radiation (MRT)
Next to air temperature, radiation has the greatest effect on thermal sensation.
Radiation falling on the body surface activates the same sensory organs as the warmth of
the air. Falling on an intervening surface, such as clothes, the radiant heat is converted to long-wave electromagnetic radiation causing sensible heat (molecular movement), which is then conducted through that material to
the skin.
Activity (MET)
Generally speaking, the human body generates more heat when exerting physically in comparison to when at rest. A very important principle involved here is that of metabolic rate. The human body constantly produces heat, but at a varying rate. Metabolism is the term describing the biological processes within the body that lead to the production of heat.
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6.
Clothing (CLO)
The individual can exert a considerable degree of control over most forms of heat exchanges between his body surface and the environment by choosing his/her clothes. Calculation of heat transmission through clothing would be extremely cumbersome, so the clo unit has been devised to simplify the handling of this insulating cover. This corresponds to an average U-value of 6.5 W/m² degC over the whole of the body surface. Under still air conditions, when a person is engaged in a sedentary activity the variation of 1 clo would be compensated for by some 7 degC temperature change. Under windy conditions, or if one is engaged in heavier work, the effect would be more pronounced.
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While the above factors are the most important, there are some subjective, largely non-quantifiable factors too, that an architect will benefit from the knowledge of, while in the design phase.
Click to read about them
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