Answer: The R-values in the table above, based on data from the 1993 ASHRAE Handbook (American Society of Heating, Refrigeration, and Air-Conditioning Engineers), are for 6-inch-thick concrete walls of differing densities.ACI 213R-87, "Guide for Lightweight Structural Aggregate Concrete," divides lightweight concretes into three categories. Structural concrete has a density ranging from about 90 to 120 pounds per cubic foot and compressive strength from 2500 to more than 6000 psi. Moderate-strength concrete has a density between 50 and 90 pcf and a compressive strength in the 1000- to 2500-psi range. Low-density concrete commonly used for insulation purposes has a density ranging from 20 to 50 pcf and a strength of 100 to 1000 psi.You might also want to consider using preformed foam. Replacing 20% to 50% of the concrete volume with preformed foam produces concrete with densities in the 75- to 120-pcf range that can reportedly be used in structural precast operations (See "Expanding Possibilities with Preformed Foam," The Concrete Producer, August 1998, pp. 565-567).
R-Values for Concrete
|Normal weight--150 pcf||1.22|
|Structural lightweight--110 pcf||1.85|
|Lightweight aggregate--80 pcf||2.47|
|Lightweight aggregate--60 pcf||3.46|
|Lightweight insulating--30 pcf||6.30|
|Lightweight insulating--20 pcf||8.35|