Thermal Comfort Parameters

It is impossible to specify precise values for the seven comfort parameters which would give an environment suitable for everyone. The interactions between the parameters have, however, been described by a number of thermal indices (such as the optimal operative temperature, comfort zones, the predicted mean vote and predicted percentage of dissatisfied) which can be used to establish the conditions under which a percentage of occupants will be comfortable or dissatisfied. Comfort charts are also available to enable a quicker assessment of the comfort zones, for a predicted percentage of the population (typically 75%), to be made. These show given values of certain comfort parameters as a function of the other comfort parameters. Bioclimatic charts also show the influence on thermal comfort zones of changing buildingrelated parameters.

Three of the seven comfort parameters relate to the individual: metabolism, clothing and skin temperature. The other four are linked to the surrounding environment:

Metabolism is the sum of the chemical reactions which occur within the body. The aim is to maintain the body at a constant internal temperature of 36.7 degrees C. Because the temperature of the body is usually higher than that of the room, metabolic reactions occur continuously to compensate for loss of heat to the surroundings. Production of metabolic energy depends on the level of activity in which the individual is engaged. Office work, for instance, generates approximately 0.8 met whereas playing squash produces approximately 7.0 met. The met is the unit of metabolic energy and is equivalent to 58 watts per square metre. The surface area of the human body, on average, is 1.8 square metres.

The thermal resistance of ordinary summer clothing is 0.5 clo while that of indoor winter wear is 1 clo. The clo is the unit of thermal resistance due to clothes and is equal to
0.155 square metres K per watt.

Skin temperature is a function of metabolism, clothing and room temperature. Unlike internal body temperature, it is not constant.

Room temperature, measured with an ordinary dry bulb thermometer, is very important to thermal comfort since more than half the heat lost from the human body is lost by convection to the room air.

Relative humidity is the ratio (expressed as a percentage) of the amount of moisture in the air to the moisture it would contain if it were saturated at the same temperature and pressure. Except for extreme situations (when the air is absolutely dry or it is saturated), the influence of relative humidity on thermal comfort is small. In temperate regions, for instance, raising the relative humidity from 20% to 60% allows the temperature to be decreased by less than 1K while maintaining the same comfort level. Generally, the relative humidity in a room should be between 40%, to prevent drying up of the mucous membranes, and 70%, to avoid the formation of mould in the building.

The average surface temperature of the surfaces enclosing a space is the mean radiant temperature. As a simplification, this can be taken to be the mean of the temperatures of the surrounding surfaces in proportion to their surface areas. If a building is well insulated, the temperature of the internal surface of the outer walls is close to room temperature. This reduces the radiative heat losses and therefore increases the feeling of thermal comfort. It also diminishes the occurrence of convective draughts.

The velocity of the air relative to the individual influences the heat lost through convection. Within buildings, air speeds are generally less than 0.2 metres per second. The relative air velocity due to the individual’s activity can vary from 0 to 0.1 metres per second for office work to 0.5 to 2 metres per second for someone playing squash.

It is crucial to remember when designing spaces for human occupancy that people are not best suited to entirely “comfortable” conditions. In fact, we are conditioned to adapt to quite major changes in our environment, and the absence of these can create a feeling of discomfort. The pattern of variation is also important. People are more tolerant of changes which they understand, such as a sunbeam or a draught, and particularly those which can be controlled. Causes that are not obvious, or with which the occupant has little sympathy, such as those caused by a faulty air conditioning system, cause the most stress. Thus, it is more important to design spaces in which people can influence the conditions they experience that to try to maintain complete stability.
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