Question: I am trying to determine R-values for poured-in-place concrete walls, both above grade and below grade, at varying thicknesses. The walls I am working with vary from 6 inches thick for one wall and 10, 12, and 14 inches thick for other walls. (The walls have rebar of various diameters imbedded throughout.) Is there a table or a formula that would let me calculate R-values for each additional inch of structural concrete wall thickness?

Answer:The thermal conductivity of concrete walls varies significantly depending upon the density or pounds/cubic foot (pcf) of the concrete as well as air content, saturation, crystallinity of the aggregates, ambient conditions, and how the R-value is calculated. Typical R-values or thermal resistance for concrete are provided in the Concrete Construction Handbook, third edition, by Joseph J. Waddell and Joseph A. Dobrowolski, published by McGraw-Hill, 1993:

Unit weight, pcf (kg/m3)R-values
Concrete thickness, in. (m)
1 (0.03)2 (0.05)4 (0.10)6 (0.15)
20 (320) lightweight1.4 (0.24)2.9 (0.50)5.7 (0.97)8.6 (146)
145 (2320) normal weight0.07 (0.01)0.15 (0.03)0.3 (0.05)0.45 (0.08)

As you can see, the R-value for a typical normal-weight 6-inch-thick concrete wall is about 0.5 in this reference. A wall of greater thickness would be more optimal in terms of insulating value if it were of lightweight or low density. Low-density concretes are most effective for insulation and also soundproofing as they contain more air voids than normal-weight concrete.

The Portland Cement Association (PCA) provides R-values for concrete walls in the July 2001 publication of Concrete Technology Today and lists the R-values for concrete, including values for 8- and 12-inch walls as follows:

Concrete density kg/m3 (lb/ft3)R value, m2 x K/W (hr x ft2 x F/Btu)
Thickness, mm (inch)200 (8)300 (12)
320 (20)1.91 (10.85)2.79 (15.85)
1360 (85)0.48 (2.71)0.64 (3.64)
2300 (144)0.24 (1.35)0.28 (1.60)

The R-value listed by PCA for a normal-weight (144 pcf) 6-inch concrete wall is 1.23 and includes inside and outside air-film resistances, which contribute to overall thermal resistance. Call PCA at 847-966-6200 ext. 564 to obtain copies of this reference.