Thermal insulation is concerned with the problem of reducing the transfer of heat from one region to another. The physical principles involved in the subject of insulation are thus identical with those involved in the subject of heat transfer. Heat is transferred by three general methods or modes, called, respectively, conduction, convection, and radiation, which may operate either separately or in combination, depending upon the particular conditions. In any case the flow of heat invariably takes place from regions of higher temperature to regions of lower temperature.
In solid materials heat is transferred by a process known as conduction, the exact nature of which is not completely understood. The amount of heat conducted from one region to another is proportional to the temperature difference between the two regions in question. The ability to conduct heat varies widely among different materials, metals being, in general, far better heat conductors than nonmetallic substances. It therefore follows that nonmetallic materials are, in general, better insulators than metals. Gases, with two exceptions, are the poorest conductors of heat, but, as will be discussed later, heat transfer through gases is usually complicated by other factors besides conduction.
The numerical measure of the ability of a substance to conduct heat is called its thermal conductivity, defined in customary units as the amount of heat in B.t.u. (British thermal units) which will flow in one hour through a uniform layer of material 1 square foot in area and 1 inch in thickness, when the temperature difference between the surfaces of the layer is maintained at 10 F. A B.t.u. is the amount of heat necessary to raise the temperature of 1 pound of water 1° F. The insulating value or thermal resistivity of a material is equal to the reciprocal (one divided by) of its conductivity.
Thermal conductivity is a property of the material itself, not depending upon the size and shape of a particular piece of the material in question, providing the latter is of uniform structure. It is therefore incorrect to speak of the conductivity of a wall or other structure but only of the conductivity of the material or materials of which the structure is composed.
When dealing with a given body, such as a building wall, its insulating value as a whole is measured inversely by a property known as conductance, defined as the amount of heat flowing through the wall per unit time and per unit area when the temperature difference between the surfaces of the wall is 1°. The insulating value or thermal resistance is equal to the reciprocal of the conductance. The conductance of a wall depends upon the conductivity, size, and arrangement of the materials of which the wall is composed. If it consists of a single uniform material, its conductance is numerically equal to the conductivity of the material divided by the thickness of the wall. If the wall is composed of parallel layers of different materials, its conductance can be easily calculated from the respective thicknesses of the layers and the conductivities of the materials composing them. The insulating value of the wall is equal to the sum of the respective insulating values of the different layers. If, on the other hand, the wall does not consist simply of parallel layers, the calculation of the insulating value from the conductivity and dimensions of the wall components is much more difficult, and will not be discussed here.