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Insulation Basics

Heat Transfer Mechanisms

  • Convection: When a liquid or gas is heated and moves upward as a result of the heat. Warm air rising and cooler air falling is a good example.

  • Conduction: The transfer of heat through a solid substance. Heat transfer through metal is an example.

  • Radiation: The transfer of heat via electromagnetic waves. The sun transfers heat to the earth using radiation.

Heat Flow

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.

Test Conditions

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,

 “Currently, most wall R-value calculations are based on experience with conventional wood frame construction, and they do not factor in all the effects of additional structural members at windows, doors and exterior wall corners. Thus they tend to overestimate the actual field thermal performance of the whole wall systems.”

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:

  • Clear wall R-value: This is the R-value of a wall with just studs and does not include the framing included in windows, doors and exterior corners.

  • Center of Cavity R-value: This is the R-value estimate of the area of the cavity space between studs that contains the most insulation.

  • Whole wall R-value: This is an R-value estimation that includes both the clear wall estimate of R-value and takes into account additional framing like windows, doors and exterior corners

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

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