I’m a project manager for a spray-foam insulation contractor in New England. A good insulation package is a must in our climate, and we’re always looking for sensible, practical applications for spray foam. Insulating under foundation and basement slabs using SPF (spray polyurethane foam) offers certain advantages over the more common method of using rigid foam board. As with any new technique, however, there are some considerations to take into account before using this approach for the first time.

Two-pound foam. Only closed-cell SPF can be used in a subslab installation; the open-cell foam does not have adequate compressive strength. Two-pound closed-cell foam has a compressive strength of 20 to 30 psi (depending on the specific brand used), which is generally acceptable for a typical nonload-bearing residential slab. Should there be a...

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