Another mathematical expression used in thermal quantification, and the most common reference used by the insulation industry, is R- value, or resistance to heat flow. Since the R-value is the measure­ment of a product’s resistance to heat flow, the higher the R-value, the better is the resistance to the flow of heat (expressed in British thermal units). Insulation is rated in terms of thermal resistance, called R-value, which indicates the resistance to heat flow. The high­er the R-value, the greater is the insulating effectiveness of mass – type insulations.

R-values are measured by testing laboratories, usually in something called a guarded hot box. Heat flow through the layer of material can be calculated by keeping one side of the material at a constant tempera­ture, say, 90°F (32°C), and measuring how much supplemental energy is required to keep the other side of the material at a different constant temperature, say, 50°F (10°C). [This process is defined in great detail in American Society of Testing and Materials (ASTM) procedures. The result is a steady-state R-value. It is called steady state because the dif­ference in temperature across the material is kept steady.2]

To ensure that consumers are provided with accurate information regarding R-values, the Federal Trade Commission (FTC) in 1980 established a rule that mandates that specific R-value information for home insulation products be disclosed in certain ads and at the point of sale. The purpose of the FTC R-value disclosure requirement for advertising is to prevent consumers from being mislead by certain claims that have a bearing on insulating value.3

In the flow of heat through a solid body to air, it was observed that the passage of heat into the air was not accomplished solely through con­duction. Instead, it occurred partly by radiation and partly by free con­vection. A temperature difference existed between the hot solid and the average temperature of the air. In this case, the resistance to heat trans­fer cannot be computed using the thermal conductivity of air alone. Instead, the resistance has to be determined experimentally by mea­suring the surface temperature of the solid, the temperature of the air, and the heat transferred from the solid to air. The resistance computed is the combined resistance of conduction, free convection, and radiation.

R-value requirements for a specific house design in a certain locale are mandated by the IECC, formerly the Model Energy Code. Many state agencies simplify this process for residential design by outlining general rules of thumb. Manufacturers often provide general planning guidelines as well (Fig. 3.1). Always verify specific thermal require­ments with the aforementioned organizations or local building officials.

R-values are reported for 1" of thickness and are not necessarily per inch of thickness (for residential construction only). R-values usually are reported at mean temperatures of 75°F per FTC regulations. The R-value per inch of a specific material is not necessarily always the same. It can be affected by several factors, including temperature, den­sity, and thickness.