- Q. What is the R-value of normal weight concrete made with limestone aggregate? Is it expressed per inch of concrete thickness?
A. The resistance of a material to conduct heat energy from one surface to another is its thermal resistivity, which for a given area is expressed on a per-unit-thickness basis. For what you describe, a workable resistivity would be 0.09 hr ft2 deg. F/Btu in. Multiplying that per-inch value times the thickness of the concrete would give you an R-value for the concrete. Assuming a 6-inch wall, that would yield an R-value of 0.54.
Remember, the total R-value is affected by whatever is adjacent to each side of the concrete. If it is open to the atmosphere, for example, the thermal resistance of the air-film adjacent to a concrete surface in a still environment can significantly increase the system’s R-value.
For specific values, you can contact the American Society of Heating, Refrigeration and Air-Conditioning Engineers at www.ashrae.org. Also note that the R-value only refers to heat transfer per unit time and does not take into account the flywheel effect of concrete’s thermal mass.