A thermal insulator is essentially a material having a large percentage of relatively small voids containing air. Little, if any, convection can take place within such a material, and the solid portions effectively screen off the radiation, so that the low conductivity of air is utilized to a much greater extent than in an air space. Since every known solid material has a greater thermal conductivity than air, it is evident that the conductivity of air fixes the lower limit of the conductivity of insulating materials containing air. By exhausting the air from an insulating material the conductivity can be materially reduced, and although this principle is made use of in certain types of thermos bottles and jars, it is impracticable on a large scale.
The application of the term thermal conductivity has thus far been restricted to uniform or homogeneous materials. Insulating materials are obviously not homogeneous in the microscopic sense, but in a practical sense they may be considered homogeneous, since their structure is finegrained in comparison with the size of the specimens ordinarily dealt with. Bearing this in mind, we may use the terms thermal conductivity and insulating value in the same sense as they have been used in the case of homogeneous materials.
Investigation has shown that the differences in the respective thermal conductivities of the various light fibrous or cellular materials are not very great. The conductivities of most materials manufactured and sold primarily as insulators fall within the range 0.25 to 0.35 B.t.u. per hour, square foot, and temperature gradient of 1° F. per inch thickness. Of such insulators less than 1½ inches of the poorest material is equivalent in insulating value to 1 inch of the best. The better insulators approach fairly closely to the ideal limit, since the thermal conductivity of air is only slightly less than 0.2 B.t.u.
Commerical insulating materials can be divided into two general groups - (1) fibrous materials either in loose form or fabricated into soft flexible quilts confined between relatively thin layers of paper or textile and (2) more or less rigid boards in which the components are bonded together in some way. The differences in the respective insulating values of materials within each group are usually so small that the average purchaser can neglect them. In fact, the tabulation of these small differences often tends to obscure other far more important facts. In general, the lighter the material per unit total volume the better its insulating value per inch of thickness. Stiff fibrous insulating boards having considerable structural strength are somewhat poorer insulators than lighter and looser materials. Dense highly compressed wall boards made of wood or other organic fiber are not as good insulators as less compressed boards of the same general character. Heavy wall boards containing plaster in one form or another are relatively poor insulators, although they are very useful building materials, and, like building paper, may be valuable in reducing infiltration of air through an otherwise porous wall.