Determining Coefficients of Heat Transmission
The overall coefficient of heat transmission, or U-value, as it is commonly designated, is a specific value used for determining the amount of heat lost from various types of construction. It represents the time rate of heat flow and is expressed in Btu per hour per square foot of surface per degree Fahrenheit temperature difference between air on the inside and air on the outside of a structural section. Furthermore, the U-value is the reciprocal of the total thermal resistance value (R-value) of each element of the structural section and may be expressed as
U = R
Professional societies such as the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) have already determined the U-values for a wide variety of floor, ceiling, wall, window, and door construction. Tables of these U-values are made available through ASHRAE publications (e. g., the latest edition of the ASHRAE Handbook of Fundamentals) found in many libraries. Many manufacturers of heating equipment also provide tables of U-values in their literature. When calculating heat loss, you have the option of selecting U-values from the tables provided by manufacturers and certain professional associations or computing them yourself. The latter method, if done correctly, is the more precise one.