Thermal comfort is a very subjective feeling affected by the physical laws of heat transfer. While we cannot strictly define what exact combination of environmental factors will produce comfort for all, we can define what the major contributing factors are and act to alter their influences.
The human body contains two sets of heat-sensing organs in the skin. One senses the outflow of heat from the body to objects of lower temperature. These sensors lie close to the skin's surface, concentrating in the fingertips, nose and bends of the elbow. The other set of sensors responds to the inflow of heat to the body. They lie deeper in the skin and are concentrated in the chest, upper lip, chin, nose and forehead. Both sets of sensors trigger body responses which control blood circulation through the skin.
The human body has its own source of thermal heating through metabolic activity and is able to adapt to a wide range of environmental conditions. The body's thermostat is located in the hypothalamus, a gland at the base of the brain just above the pituitary gland. This thermostat, set close to 98.6oF (37oC), monitors changes in blood temperature caused by thermal conditions within the body and the change of temperature across the skin. When the hypothalamus detects a body temperature less that the set point, it initiates physiological responses to increase the temperature. A warmer body temperature causes responses which act to decrease body temperature. In this way, the body attempts to maintain its thermal equilibrium or heat balance, resulting in thermal comfort.
While the body's ability to acclimatize to ambient temperatures is fairly flexible when in a good state of health, a person's thermal comfort is also dependent upon psychological factors and conditioning. If you are not used to or cannot produce a high indoor temperature, you do not expect it and therefore can acclimatize to a lower indoor temperature. Psychological and social conditions also effects how people perceive their alternatives to thermal discomfort, especially clothing.
Thermal Comfort Factors
Thermal equilibrium and its resulting thermal comfort are achieved by physiological and behavioral responses to the heat produced by the body and the amount of heat gained or lost to the environment. The five factors important to achieving this equilibrium are: body metabolic activity, conduction, convection, radiation and evaporative exchange.
Body Metabolic Activity: The cereal you ate for breakfast, the sandwich for lunch, the pasta for dinner, all are turned into energy for growth, tissue regeneration and the operation of your body and its physical activity. These are metabolic processes which have an efficiency of only about 20 percent, the remaining 80 percent is converted into heat, most of which is rejected by the body to maintain the equilibrium temperature. The rate of metabolic heat production is primarily controlled by the rate of body activity. In general, the more physically active, the higher the metabolic rate. Other factors which influence an individual's metabolic rate include: body weight, sex, age and state of health.
To maintain thermal comfort, we must balance the heat generated by metabolism with the heat lost to or gained from the environment. Heat is primarily transferred away from the body by one of four methods: conduction, convection, radiation or evaporative cooling. Conduction: Conduction moves heat energy through a substance by transfer between molecules or atoms. Conduction of heat is analogous to transportation by land. It can be slow and its speed depends on the medium of transport. Air is a poor conductor of heat, transporting it slowly — like walking. Metals are the best conductors, transferring heat like a fast car or train. A heating pad or hot water bottle heats by conduction. We lose or gain heat by conduction when we are in direct contact with an object.
Convection: Convection moves energy by the movement of molecules or atoms in a fluid, i.e. a gas or liquid. It is analogous to movement by flight. In convective cooling, the fluid in contact with the object first gains heat by conduction, radiation or evaporation and then removes it from the site by fluid motion. The heat transfer rate varies with the medium in which the object is present. Generally, the more rapid the movement of the medium, e.g., a strong wind or water current, the greater the rate of heat loss. Convective heat loss is the process behind the wind-chill factor so common in cold season weather forecasts.
Radiation: Radiation moves energy by electromagnetic waves whose closest transportation analogy would be a Star Trek-type transporter beam. Objects exchanging heat energy through radiation need not be in contact but simply in sight of one another. All objects lose heat energy through radiation at a rate proportional to their temperature, some more efficiently than others. A heat lamp uses radiation as its primary method of heat transfer, and, of course, the sun heats us by radiant energy.
Evaporative Heat Loss: When liquid water evaporates from a surface, heat is required to change the water from the liquid to a gaseous state. This heat usually comes at the expense of the surface on which the liquid was present. The human body uses evaporation as a major cooling mechanism in hot environments. Perspiration is the most obvious form of evaporative heat loss from the body, but the lungs and respiratory passages continually lose heat through evaporative cooling. Evaporative heat loss is greatest in hot, dry environments.
Describing the Indoor Environment
Our homes, whether a house or some form of apartment, lose heat mostly through conduction, convection, and radiation. We attempt to maintain them, or specific rooms within them, at an ideal temperature for our bodily comfort which varies with room usage, our level of activity and personal preferences. Heat is usually added to our home by some form of heating device fuelled directly or indirectly by electricity, gas, oil, or wood — the building equivalent to metabolism.
Four factors can be used to describe the indoor environment relative to thermal comfort: air temperature, mean radiant temperature, air movement and relative humidity. Air velocity and relative humidity are more important during the warm season when we are trying to increase the rate of heat loss from our bodies.
Air temperature is the usual defining parameter most people think of in conjunction with thermal comfort. Air temperature directly effects convective and evaporative heat loss and indirectly affects conductive heat loss through its influence on the surface temperature of objects in a room.
Less known but equally important is mean radiant temperature. As mentioned earlier, all objects emit radiant energy at a level proportional to their temperature. Thus, when we are in a room, we radiate out to all surfaces and objects and they radiate back. The mean radiant temperature is a measure of the radiative effects arising in a room from all objects and surfaces. Large cold surfaces such as cold walls or windows can greatly reduce the mean radiant temperature of a room, causing significant thermal discomfort. For example, a poorly insulated home has cold interior walls, and bodies within its rooms continually lose heat to these cold surfaces. To compensate, room air temperature must be raised significantly, even as high as 27oC (80oF), before occupants feel comfortable.
Relative humidity is another often neglected indicator of comfort, especially in the warm season when high humidity reduces the rate of evaporative cooling from the skin and lungs. In most homes, especially those heated by forced-air systems, the cold season is characterized by dry air conditions unless moisture is added through humidification. Low humidity may cause physical discomfort by drying skin, nasal passages and eyes. By adding moisture to the indoor air in winter, we can reduces the discomforting effects of relative humidity and raise the degree of thermal comfort. The effects of low humidity are most notable when stepping from a bath or shower. If the air is very dry, we will quickly feel the chilling effect of evaporative cooling. Higher humidity in the bathroom will reduce this effect.
Air movement is again not as important in the cold season when windows and door are closed and fans not generally used for cooling. Higher air movement increases the rate of convective cooling when air blows across the body, what we term in the cold season as feeling drafty.
Some level of air movement is, of course, necessary to remove excess moisture and odours, a process termed ventilation. In a well-designed home, the ventilation is optimized to provide the maximum freshness to the interior air with the minimum loss of heated air to the outside.
In contrast, air infiltration is the unwanted flow of air from the cold outdoors into the building through various cracks and openings in the house's exterior shell. The greatest air infiltration generally occurs around window and door frames and can be eliminated by caulking and weatherstripping. Air infiltration is greatest when the wind is strongest with maximum entry on the windward side of the building.