lnsulation

(Continued from Page 26) compound structures on a unit basis, such as one square foot of area, one hour of time and one degree temperature difference. Heat transfer constants, when stated on a unit basis in this manner, are known as "coefficients," such as the "coefficient of conductivity,l' the "coefficient ot conductance" and the "coefficient of transmission." In some cases, it is customary to drop the word "coefficient" and to refer onli' to the last word in the term, such as conductivity or conductance.

The coefficient of conductivity is used to represent the rate oI heat transfer through building and insulating materials. 'When, in the case of'certain materials, it is not logical to state the rate of heat transfer in terms of a one inch thickness of the material, the term conductance is used instead of conductivity, as explained in the following paragraphs. The term "coefficient of transmission" is used in connection with simple or compound wall, floor, roof or ceiling structures.

Conductivities, Conductances:

The conductivity of a building or insulating material is the amount of heat (Btu) which will pass through one square foot of the material one inch thick in one hour for a one degree temperature difference between the two surfaces. Even though a material may be installed in thicknesses other than one inch, this thickness is used for comparing the heat-resisting properties of materials which are homogeneous or alike throughout.

If t-he material is heterogeneous in character, such as hollow tile, the heat transfer may be stated on the basis of the thickness or type of material involved, as, for example, 4-inch hollow tile. This quantity is called the "con-. ductance" of the material. Both conductivity and conductance refer to the transfer of heat from one surface of the material to the other (not air to air), the essential difference being that conductivity refers to a one inch thickness whertas conductance refers to some other thickness, but the thickness or construction must always be stated when giving the conductance value of a material. It is also possible to state the conductance of two or more materials in combination such as wood lath and plaster, for which the conductance is 2.50 Btu per hour per square foot per deglee Fahrenheit difference in temperature, for a total thickness of % inch.

Conductivitics not Additivc:

Conductivities and conductances are not additive. Two or more c.onductivities or conductances cannot be added together to obtain any rational or significant result. For eximple, if the conductivity of Celotex (0.33) is added to that of gypsum plaster (3.30), the sum of these two figures would be 3.63 which of course is greater than either conductivity value alone. It is obvious that any two materials used together would have a lower total heat'transfer than either material by itself. It is only by applying the proper formula in each case that the combined effect of two or more materials can be obtained.

Air Spaces:

The conductance of an airtpace is designated by the letter "a" and is the amount of heat transmitted by radiation, convection and conduction in one hour through an air space having an area of one square foot for a temperature difference of one degree Fahrenheit. The conductance of an air space depends on the temperature, width and position (whelher horizontal or vertical) and the character of the materials enclosing it. An air space % inch or greater in width and bounded by ordinary building materials has a conductance of about 1.10 Btu per hour per square foot per degree Fahrenheit temperature difference, at a mean temperature oI 4O degrees, and this value is commonly used for heat transfer calculations

Surfacc Conductances:

Every material has a thin layer of air in contact with the surface which offers a certain resistance to the passage of heat. For this reason, the amount of heat which will pass from one surface of a material to the other surface is greater than the amount of heat which will pass, in a given period of time, from the air on the one side of the material to the air on the other side.

The amount of heat (Btu) transmitted from a surface to the air surrounding it, or vice versa, in one hour per square foot of the surface for a difierence in temperature of one degree Fahrenheit between the surface and the surrounding air is known as the "Surface conductance" of the material. This'conductance is dependent upon the character of the surface, the velocity of air passing over it, the temperature of the surface and the difference intemperature between the surface and surrounding air. To differentiate between inside and outside surfaces, f is used to designate the inside film or surface conductance and f the outside film or surface.conductance. If the air is still (no wind), then for the same material f' and f are the same. and f ' : fo; but if the outside air is in motion, then f is always greater than f' and will increase as the wind velocity increases.

Values for ft and f have been determined for various building materials at the University of Minnesota, under a cooperative research agreement with the American Society of Heating and Ventilating Engineers. The range of values for ordinary building materials, however, is comparatively small and for practical purposes may be assumed constant for either still air or for any given wind velocity. The average value of f' for ordinary surfaces and for still air is about 1.65. In determining basic heat transmission values for building construction, it is customary to use that value of f which will occur when a 15-mph wind blows parallel to the outer surfaces considered. The average value of fo for a wind velocity of 15 miles per hour is 6.00 Btu per hour per square foot per degree Fahrenheit difference in temperature between the surface and the surrounding air.

Coefficients of Transmission:

Although conductivities and conductances are useful in comparing the heat transfer through materials based on the surfice to surface transfer, the ultimate objective in most cases is to ascertain the over-all or air to air transfer of heat through the wall, floor or roof structure. This quantity is known ai the coefficient of'transmission and is designated by the letter "IJ." The coefficient of transmission is the amount of heat (Btu) transferred through one square foo-t of the wall, floor, roof, or glass in one hour per degree difference in temperature between the air on the two sides of the structure.

It should be understood that the conductivity of a material refers to the heat transfer through it mainly by conduction. This is true also of conductance of materials except in the case of materials containing ,air_ spaces such -as holtow tile, there is also a transfer through air spaces by radiation and convection. The coefficient of transmission of a structure involves all three methods of heat transfer whether or not there are materials containing air spaces, inasmuch as heat is carried to or from the wall or other surface by radiation, convection, and conduction.