Insulation

By Herold A. Knopp, Vice-Piesident The Celotex Corporotion

The verb "to insulate" is derived from the Latin word Insula, meaning island. As defined in most dictionaries, "to insulate" means to place in a detached state or to isolate. Strictly speaking, this definition does not ap- ply to commercial heat insulations, since no material is an absolute heatstop. This definition would therefore apply to an ideal or perfect heat insulation, if such a material existed.

Definition:

In the more practical sense and in the sense implied in this discussion,

Harotd A. lncpp a heat insulation is a material or substance which will effectively retard the passage of heat from the warmer to the coofer level. Unfortunately, there is no word in the English languase which exactly applies to this definition of hlat insu6tio-n. A commercial insulation is generally understood to mean a material which has a high degree ol heat resistance per unit of thickness. When installed in a wall, an insulatioh effectively retards the passage of heat through the wall. In other words, a smaller amount of heat will pass through the wall in a given period .of tinie, say one hour, than-if it were not insulated, and consequently less heat or refrig- eration is required to maintain the desired temperature in the building.

Technically, a corrlmercial insulation is a material which has a conductivity .not -to exceed 0.4O Btu per hour per square foot per degree Fahrenheit per inch thickness, ind which .has a total internal resistanCe of at least 1.0.

Examples in Nature:

There are many examples of the principle of insulation in nature. Fur is one of the best insulations known; hence furrovered animals 'may be considered to be insulated. If it rvere not for the fur covering of the polar bear he could not withstand the cold blasts of the arctic regions, sleep on icy floes and bathe in icy water. Featheri provide I degree of insulation and therefore birds are insulated.

A'side from purposes of modesty, the insulation idea is the main reason for wearing clothing. The purpose of cloth: rr:r8, of course, is to retard the loss of heat from the body. The proper amount of clothing from the insulation standpoint is that amount which will permit heat to be dissipated from the body at the rate at which it is produced. Of course, in the warm weather, the problem is tb dispose of body heat and the less insulation (clothing) the better.

Even insulated dwellings are really not ne\r. The thatched hut of norihern Europe was for all practical pufposes insulated because it was built with walls of.clay and stone several feet thick and a roof of straw two feet thick woven together. The stone castles of the middle ages had walls of sufficient thickness in many cases to justily their being called insulated, since heat resistance is a function of thickness as well as of type of material. The Spanish Mission houses of the southwest desert. where the temperature sometimes rises to l4O degrees in the daytime, were always marvelously cool because the wells were built with three feet of clay and straw. It should be understood, however, that while an insulated dwelling can be obtained by the use of ordinary ,materials in. sufficient thicknesses, commercial heat insulations are such because they have a high degr€e of heat resistance per unit of thickness.

Mass Insulationg:

Commercial insulating materials are frequently classified as -(1) rigid or semi-rigid insulations, (2) flexible insulations and (3) fills and these together are known as mass insulations, as distinguished from reflective materials, which are of an entirelv different classification.

The addition of anv material of finite thickness to a wall or roof structure wilf reduce the heat loss through the wall or roof to one degree or another. But such material may not necessarily be classified as an insulation unless it is a poor conductor of heat and thereby provides a high degree of heat resistance per unit of thickness in accordance with the definition of an insulatiori. The substitution of a mass insulation of a given thickness for any material having a lesser total heat resistance will likewise reduce the heat loss.

Heat Resisting Factors:

The efiectiveness of a mass insulation such as Celotex in reducing heat transfer is due to the fact that such material has a low rate of heat conduction and practically no transfer of heat by convection and radiation, and these characteristics in turn are due to the condition of subdivision or density of the material. The minute air voids within the bagasse fibers and the interstices between these fibers contain still or dead air because these air cells are too small to permit any circulation of air in or between them, thus eliminating convection. Radiation does'not take place within these voids or interstices because there are no measurable temperature differences between the infinitesimally small bounding surfaces-and radiant heat transfer requires appreciable temperature differences, in accordance with the Stefan-Boltzmann law.

The moleculary theory explains why materials such as Celotex have a lower rate of heat conduction than hard, dense materials such as steel, concrete and glass. If the molecules of a material are packed together more solidly as in the case of steel or concrete, any vibration or motion of the molecules is readily communicated from one part of the substance to the other. Thus in the case of a metallic rod, if heat is applied at one end of the rod the motion of the molecules at this end is increased, and by impact with adjoining molecules is rapidly passed down the rod to the cold end. With lighter and more porous materials such as Celotex, any motion of molecules due to heat applied to one side, is not readily transmitted through to the other side by impact with adjoining molecules because the molecules are less compact.

The amount of heat (Btu) which will pass through any material depends on several factors including the temperature difference, surface area, thickness, the number of hours, and the characteristics of the material. The heat transfer through a compound wall or roof structure depends on these and other factors.

Coefficients:

For convenience, however, it is customary to state the rate of heat transfer through building materials and through

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