A few years ago, New England contractor Wade Paquin of WKP Construction introduced me to the technique of using closed-cell spray foam (CCSF) below a slab. He was insulating the below-grade slabs of the new homes he was building by spraying a couple inches of CCSF over a gravel bed before pouring the concrete. I have since had the opportunity to try this insulation method on a few of my own projects. I’ve concluded there are benefits to this approach.
Standard regional practices
Frost-protected slab-on-grade construction has been common in my market for several years. I collaborated with my concrete contractor to design an insulation system for a frost-protected shallow foundation (FPSF) that we used on several projects. I have not had any call-backs, so I assume everything is working as designed. The drawback is the amount of time it takes to do a good job installing the insulation and then detailing the polyethylene sheeting used for air, vapor, and radon control. It can take multiple days, especially on larger projects.
The photo to the right shows a small guest house I built in 2011. The project took two days to install and seal the sub-slab insulation. There are 2 in. of XPS foam under the slab and footings, 2 in. covering the slab edge—cast in place during the concrete pour—and 4 ft. of 2-in. XPS used as a wing for the perimeter foundation insulation. (See the IRC Table R403.3(1) for local requirements).
An inherent problem
With blower-door testing, I have determined that penetrations in slabs are a source of air leakage. I believe the air is coming down the radon pipe on the roof, entering the radon rock bed, and finding a path either along a pipe, a cast-in-place tub box, or other floor penetration. Often the…
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