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Heat Transfer Mechanisms
Thermal energy flows from warmer objects to cooler objects.
U-value: A measurement of the heat flow rate through a substance.
R-value: A measure of the heat flow resistance of a substance.
Temperature: In accordance with Federal Trade Commission regulations, R-value is tested at an average temperature of 75 degrees. R-values will change when tested under different temperatures. Regulations specify a standard temperature so consumers can evaluate R-values derived from identical test conditions.
Thickness: R-values are given for one inch of thickness. This R-value is not necessarily the R-value per inch of thickness.
Density: R-value can change with changes in density of the insulation materials
Whole Wall Systems:
Richard T. Bynum writes in Insulation Handbook,
Since the R-value of the insulation material alone does not accurately indicate the average R-value of the whole wall system, Bynum describes three methods for measuring R-values:
Bynum notes that “For some conventional wall systems, the whole wall R-value is as much as 40 percent less then the clear wall R-value.”
Go to the Whole Wall Value comparison chart for more information
Bynum, Richard, 2001, Insulation Handbook, McGraw-Hill, New York, NY