Peter Fusaro is building a high-efficiency house on spec, and his plans include insulating the foundation walls. The question is how.
One option is to apply rigid foam to the outside of the foundation. That would leave roughly 1 ft. of insulation above grade, and Fusaro is concerned about how durable the foam would be.
Another possibility is to use a sandwich of 2 in. of foam between two outer faces of concrete, each 4 in. thick, making an assembly with both structural and thermal properties. He’s been told a wall built that way would have an effective R-value of 19.27.
In a Q&A post at GreenBuildingAdvisor, Fusaro looks for advice. That’s the subject of this week’s Q&A Spotlight.
R-value is inflated
To GBA senior editor Martin Holladay, what Fusaro is describing sounds like the Thermomass Building Insulation System . These panels consist of Dow Styrofoam polystyrene insulation and two outer layers of concrete held together with a grid of fiber-composite fasteners. According to the company, the system is designed to minimize thermal bridging that would degrade the performance of the insulation.
The problem is that 2 in. of extruded polystyrene foam has an R-value of only 10. Concrete contributes very little thermal insulation to the assembly, so the R-19.27 Fusaro cites would seem to be little more than wishful thinking.
“The claim that the thermal mass of the concrete wall increases the effective R-value of the wall is false, although you hear it often,” Holladay writes.
“In some climates, especially with dependable nighttime temperatures below 72 degrees and dependable daytime temperatures above 72 degrees, walls with a lot of thermal mass can lower the energy used for heating or cooling. However, let’s not confuse thermal mass effects with R-value.
“In most parts of…