Question: What is the R-value of concrete?

Answer: An R-value is simply one measure of a material's resistance to heat flow, which also can be thought of as its insulating value. It depends not only on the material but also on its configuration.

To calculate an R-value, you first need to know the material's resistivity, which is expressed on a per-unit-thickness basis, and assumes a constant area, such as 1 square foot. A reasonable value of resistivity to use for standard concrete with limestone aggregate would be 0.09 hr ft2 degree-F/Btu inch. Multiplying times the thickness of a concrete wall, for example, would give you its R-value. If you have a 6-inch wall, its R-value is 0.54.

However, this applies to the concrete alone. The total R-value of a system is cumulative. Obviously, insulation installed on one or both sides of the wall increases the system's R-value. But even if the concrete wall is open to the atmosphere, the thermal resistance of the air-film adjacent to the concrete surface—in a still area—can significantly increase the system's R-value. For specific values to use in such cases, refer to the American Society of Heating, Refrigerating and Air-conditioning Engineers' Web site www.ashrae.org.

Remember, too, that concrete has a significant thermal mass that provides an advantage in maintaining steady temperatures. That flywheel effect is not taken into consideration in calculating an R-value, which only refers to heat transfer per unit time.