Cornell University Ergonomics Web

DEA3500: Ambient Environment: Thermal Comfort 

DETERMINATION OF THERMAL COMFORT TEMPERATURE

The major aim of comfort research is to find comfort temperature for an individual or group. However, 6 major variables determine how warm or cold a person feels.

Environmental factors

• air temperature
• air speed
• humidity
• mean radiant temperature

Individual factors

• activity
• clothing insulation
• Possibly also individual differences, and recent thermal history.

Individual factors:

• Activity (met)
• Clothing insulation (clo unit) Research on clothing insulation originally conducted for military purposes. 1 clo = 0.16oC .m2/w (1 m2 = 11 ft2). The clo is a measure of thermal resistance and includes the insulation provided by any layer of trapped air between skin and clothing and insulation value of clothing itself. 1 clo unit will maintain a sedentary man at 1 met indefinitely comfortable in an environment of 21oC (69.8oF), 50% RH, and .01 m/s (20 ft/min) air movement. Assuming no wind penetration and no body movements to pump air around, clothing insulation = 0.15 x weight of clothes in lbs. (i.e., 0.15 clo per lb of clothes). So 10 lbs. clothes = 1.5 clo.

  Tog = European unit of thermal insulation

  1 tog = 0.645 clo.

Factors reducing clothing insulation:

• Wind speed  - increased wind speed decreases clothing insulation value mainly because of breakup of air layer.
• Body movements (pumping action breaks up air layer):
  • Chimney effect - loosely hanging clothing leads to ventilation of air layers
  • Bellows effect - vigorous body movements increases ventilation of air layers.
• Water vapor transfer - clothing offers a resistance to passage of water vapor and decreases heat loss by evaporation from skin (note condensation on inner surface of plastic containers). 
• Permeation efficiency factor - Another effect of clothing can be to absorb liquid sweat by capillary action known as wicking, and therefore not all latent heat of evaporation is available for cooling skin. These effects of clothing are described by the permeation efficiency factor (Fpcl). This runs from zero to unity (inverse scale!). zero = completely impermeable fabrics unity = absence of clothing. The thickness and porosity of a given textile will affect its Fpcl and if the textile gets wet, porosity decreases but heat loss from the body increases by direct conduction, thus decreasing thermal resistance and increasing Fpcl.

Thermal Comfort Indices and Standards

The combined effects of environmental and individual factors can be represented on a psychrometric chart. Basically, the psychrometric chart gives air temperature (dry bulb) along the X axis at 0% RH and then curves of equal humidity. The comfort zone =  the combinations of temperature and humidity where people report comfort. If conditions are to the right of the comfort zone then need to increase cooling say be increasing wind. If conditions are to the left of the comfort zone then need to increase heating say by increasing sun. Superimposed on this chart are lines of effective temperature i.e. lines of equal thermal sensation. Different charts for different levels of activity and different comfort zones for different clo values and different air velocities.

• Comfort Indices (ET) - ET is "an arbitrary index which combines into a single number the effect of dry-bulb temperature, humidity, and air motion on the sensation of warmth or cold felt by the human body".
• Standard Effective Temperature (SET) - For sedentary activity (1.1 met), light clothing and low air speed SET = ET* SET = ET* extended to incorporate different levels of activity and clothing SET is the temperature of an isothermal environment which has air and mean radiant temperature equal to each other, RH of 50%, still air, and in which a person with a standard level of clothing insulation would have the same heat loss at the same mean skin temperature and the same skin wettedness as the person has in the actual environment with the clothing insulation under consideration. The activity level is assumed to be the same in both environments. SET calculation involves: For a given level of activity, clothing, and air speed - psychrometric chart--lines of constant SET. It has also been suggested that SET links directly to sensation rather than air temperature. SET is the most comprehensive comfort index, yet while in its restricted form (sedentary activity--effective temperature (ET)) it has been adopted by ASHRAE in its comfort standard, in its more general form it has yet to be widely used.

COMFORT STANDARDS

ASHRAE 55-1981

• Activity - 1.2 met, mainly sedentary 
• Winter - 0.9 clo (sweater, long sleeve shirt, heavy slacks), air flow -0.15 m/s (30 fpm) 
• Optimum operative temperature = 22.7ºC (71ºF) (globe temperature, MRT) 
• Summer - 0.5 clo (light slacks, short sleeves or blouse), air flow -0.25 m/s (50 fpm)
• For each increased degree Kelvin up to maximum 28ºC at 0.8 m/s air, need to increase air motion by 0.275 m/s. OR by 30 fpm for every degree Fahrenheit up to 82.5ºF at 160 fpm. Optimum operative temperature = 24.4ºC (76ºF) With minimum clothes (0.05 clo) top = 27.2ºC (81ºF)

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