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) lightweight | 1.4 (0.24) | 2.9 (0.50) | 5.7 (0.97) | 8.6 (146) |
| 145 (2320) normal weight | 0.07 (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.