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Spray foam air gap

jeames21 | Posted in General Questions on

I and insulating a year round camp that is built on piers and 2×10’s and it is only 12″ off the ground. I plan to use closed cell spray foam in between all the joists and spray from the top of the joists and spray against some rigid 1″ foam attached to the bottom of the 2×10’s. The question I have is since the 2×10’s are 9″ high and I only plan to spray 3″ of foam is the 6″ of air gap a problem from the foam to the subfloor?

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  1. Expert Member
    Dana Dorsett | | #1

    The gap between the closed cell foam and subfloor is a potential thermal bypass of the insulation for air leaking at the band joists. If you go that route, insulate & air seal the band joists at the same time.

    Filling the 6" with blown fiberglass may be worth it in cooler locations. With 1" rigid and 3" ccSPF you're looking at maybe R22-R25. The IRC code calls out R30 minimum for US climate zones 4C, 5, & 6, and R38 minimum for zones 7 & 8. Installing 6" of low to mid density fluff adds another R20-R23 at comparatively low cost, and impedes lateral air movement, mitigating the thermal bypass issue. Depending on your location and the type of rigid foam you might just skip the closed cell foam (beyond what it takes to air seal) and install 9.25" of blown or mid-density batt fiber insulation.

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