Limestone aggregate and R-value

Concrete Construction, June, 2003

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 [ft.sup.2][degrees]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, though, 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 ASHRAE (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.