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Table of Contents

Introduction to Comfort
Main Comfort Factors
Other Comfort Factors

ASHRAE research and handbooks discuss the use of operative temperature for basis of establishing comfort. Operative temperature is the average of the MRT and dry bulb.

There are six major factors that determine thermal comfort.

Air temperature (DBT)
The dry-bulb temperature is the temperature of the air around us (and is one of ) the important factors. The human body's primary response is towards the changes in (operative) temperatures (Dry Bulb + MRT) and it is this temperature that we attempt to keep within comfort conditions while designing structures for habitation. (see MRT below)

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 temperatures, 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:

1. 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.

2. 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.

The above table represents combinations of Mean Radiant temperatures and dry-bulb temperatures that will give a thermal sensation of 21.11°C (70°F). MRT is the mean of thermal radiation readings from all material around us including walls, floors, other human bodies etc. Its the kind of radiation that comes from a heated stone. There can be a difference between MRT and the temperature of the air.

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 deg C 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 deg C temperature change. Under windy conditions, or if one is engaged in heavier work, the effect would be more pronounced.

Garment Insulation Value (CLO) Description Clo Description Clo Underwear Men's brief Panties Bra T-shirt Full slip Long underwear top Long underwear bottoms 0.04 0.03 0.01 0.08 0.16 0.20 0.15 Trousers and Coveralls Walking shorts Trousers Sweatpants 0.15 0.24 0.30 Suit jackets and vests(lined) Single breasted, thin-thick Double breasted, thin-thick 0.36-0.44 0.42-0.48 Footwear Sweaters Ankle-high athletic socks Calf-length socks Panty hose Sandals/thongs Boots 0.02 0.03 0.02 0.02 0.10 Sleeveless thin-thick Long-sleeve, thin-thick 0.13-0.22 0.25-0.36 Dresses and knee-length skirts Skirt, thin-thick Long-sleeve shirtdress, thin-thick Sleeveless, scoopneck 0.14-0.23 0.33-0.47 0.23-0.27 Shirts and Blouses Sleeveless, scoop-neck blouse 0.12 thin-thick Short-sleeve, dress shirt Long-sleeve, dress shirt Long-sleeve, flannel shirt Long-sleeve, sweat shirt 0.19 0.25 0.34 0.34 Sleepwear and Robes Long-sleeve, long gown, thick Long-sleeve pajamas, thick Short-sleeve pajamas, thin 0.46 0.57 0.42

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.