Human thermal comfort and thermal balance

Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment (ASHRAE). A person feels neither too hot nor too cold, and wants no change in the surroundings.

Thermal factors affecting thermal comfort

• Air temperature (dry-bulb): the main factor. Comfort range about 21 to 27 °C.
• Mean radiant temperature: radiation from the surrounding walls, roof and sun.
• Relative humidity: comfortable between 30 and 60 %. High humidity stops sweat from evaporating.
• Air movement: moving air increases evaporation and convection; 0.1 to 0.5 m/s is comfortable.
• Clothing (clo value) and activity level (met value).
• Personal factors: age, sex, health, body build and acclimatisation.

Thermal balance of the human body

The body keeps its core temperature near 37 °C. Metabolism produces heat (about 100 W at rest). The body is in balance when

$$M \pm R \pm C - E = 0$$

where $M$ is metabolic heat, $R$ is radiation, $C$ is convection and conduction, and $E$ is evaporation. In hot conditions, the skin blood vessels widen (vasodilation) and sweating cools the body by evaporation. In cold conditions, the vessels narrow (vasoconstriction), shivering raises metabolism, and heat loss is reduced.

 Metabolism (M) ----> BODY 37 C
                       |  |  |  |
          Radiation R--+  |  |  +--Evaporation E
          Convection C----+  +-- Respiration

Thermal preferences (ASHRAE 7-point scale)

The building should be designed to give a vote between -1 and +1 for most users. The comfort zone is shown on a bioclimatic chart (Olgyay or Givoni) using temperature and humidity. A combined index, effective temperature (ET), expresses temperature, humidity and air movement as one number: the temperature of still, saturated air that gives the same sensation.

Design should cut heat gain in summer (shade, insulation, ventilation) and keep heat in during winter (insulation, solar gain), so that the body stays in balance without strain.