Insulating materials are specifically designed to reduce the rate of heat transmission through ordinary construction materials to an acceptable level. A dry material of low density is considered a good insulator; however, in addition to this characteristic, it must also have a conductivity value of less than 0.5.
As described earlier, the conductivity value of a material is a purely arbitrary one determined by the amount of heat that flows in
1 hour through a 1-inch thickness of a material 1 square foot in area with the temperature exactly 1 degree Fahrenheit higher on one side of the material than on the other.
Air spaces, or air spaces bounded by either ordinary building materials or aluminum foil, also provide some insulation, but not to the degree formerly thought possible. Dead air spaces in building walls were once considered capable of preventing heat transmission in a manner similar to the space between the walls of a Thermos bottle. Later research proved this to be a somewhat false analogy, because the air in such spaces often circulates and transmits heat by convection.
Air circulation can be checked by filling the hollow space with an insulating material that contains a great number of small, confined air spaces per unit volume. This stoppage of air circulation is what produces the insulating effect, and not merely the existence of the air space. Under these circumstances, it is obvious that the most practical method of insulation is to fill the area in the walls with a material containing these minute air spaces.
Several manufacturers produce insulating materials in a variety of shapes and forms for installation in houses and other buildings. Frequently, instructions for the installation of the products will also be provided by the manufacturer. Local building supply outlets and lumber yards will often be very helpful, too, and will usually recommend the best way to install the insulation material. Some of these materials and their applications are described in the following paragraphs.