Thermal comfort and its factors

Thermal comfort is the state of mind in which a person is satisfied with the thermal environment around him or her, feeling neither hot nor cold.

Factors determining thermal comfort

1. Air temperature: the dry-bulb temperature of the air; comfort range roughly 21 to 27 °C.
2. Mean radiant temperature: heat radiated by walls, floor, ceiling and the sun. A hot ceiling makes people feel warm even in cool air.
3. Relative humidity: 30 to 60 % is acceptable. High humidity stops sweat from evaporating, and low humidity dries the skin and throat.
4. Air velocity: Air movement increases heat loss by convection and evaporation. 0.1 to 0.5 m/s is comfortable; more than that causes draught.
5. Clothing insulation (clo): Heavy clothes keep heat in.
6. Metabolic rate / activity (met): Hard work produces more heat, so a lower temperature is preferred.
7. Personal factors: age, sex, health, body size, acclimatisation, food, and the time spent in the space.
8. Other: floor surface temperature, vertical temperature difference, and the outside climate.

Factors 1 to 4 are environmental factors (controlled by the designer), and factors 5 to 7 are personal factors. The combined effect of temperature, humidity and air motion is expressed by effective temperature. The designer controls the environmental factors through orientation, insulation, shading, ventilation and, if needed, air conditioning.