The transfer of heat in a liquid or gas is usually complicated by other factors besides conduction. Conduction is always present, but the heat transfer is ordinarily greatly increased by fluid motion called convection, set up either automatically by reason of temperature differences or by means of mechanical stirring or blowing. The former is called natural or free convection and the latter forced convection. The exchange of heat between the air in a room and the inside surface of the external wall is one of the simplest examples of natural convection. The phenomenon of convection is rather complicated and cannot be accurately expressed in terms of simple laws like those of conduction; but as an approximation, heat transfer by convection can be regarded as proportional to the temperature difference.
The transfer of heat from one solid body to another through the intervening air or other fluid medium is still further complicated by radiation, which results in a heat transfer practically independent of the presence of the air. The process is the same as the transfer of heat from the sun to the earth through the intervening space devoid of matter. Everybody is familiar with the radiation of heat from an open fire, but it is not so generally recognized that radiation plays a very important role in heat transfer at ordinary temperatures. In fact, about one-half of the heat transfer from a heated room to the inside surface of the exterior walls takes place by direct radiation from interior objects and partition walls. The other half is the result of convection in the air near the exterior walls.
Although air is a very poor conductor of heat, the insulating value of an ordinary air space is rather small, on account of the large transfer of heat by convection and radiation. Radiation is largely responsible for the ineffectiveness of air spaces bounded by ordinary building materials, such as are found in frame or other hollow walls. The low insulating value is often erroneously attributed to convection; but, as a matter of fact, from 50 to 80 per cent of the heat transfer across air spaces of ordinary sizes takes place by radiation. If the air spaces were bounded by bright metallic surfaces, the transfer of heat by radiation would be greatly diminished, since clean metallic surfaces are much poorer radiators than nonmetallic surfaces, such as brick, stone, glass, wood, plaster, paper, etc.
The terms conductance and resistance (insulating value), as already defined, can be applied to an air space as well as to a slab of solid material. On account of the large effects of radiation and convection, however, the insulating value of an air space is not proportional to its width (thickness), as would be the case with a slab of uniform solid material. Furthermore, the insulating value varies considerably with both mean temperature and temperature difference. For spaces more than about three-fourths inch wide the insulating value is practically constant, independent of the width. Narrower spaces have less insulating value, and below about one-half inch the insulating value is approximately proportional to the width. Under average conditions the conductance of the vertical air spaces commonly found in building walls is about 1 B.t.u. per hour, per square foot, and per temperature difference of 1° F. It will be seen later that this figure corresponds to an insulating value approximately equivalent to a one-third inch thickness of average insulating material.