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Just insulated between cantilever floor joists with spray foam, what are my options for covering the area?

guylafleur | Posted in General Questions on

I recently had the underside of an extension spray foamed. The floor over that area gets a little cold on the feet in the winter so I’m looking to keep that area a little more comfortable.

The joists are old full dimension 2×10’s and the contractor sprayed about 5 inches of foam in the cavities between the joists, so there’s still 5 inches of open space between the bottom of the joists and the foam.

To reduce thermal bridging through the joists, I’d like to cap the underside with rigid insulation then a layer of plywood. Would there be any problem leaving the empty 5″ of space between the foam and the rigid insulation or should I fill that with batts?

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  1. Expert Member
    Akos | | #1

    You definitely want to fill that space with batts, it will significantly bump up your assembly R value for not much cost.

    More foam you get under the joists, the better it will be, but the floor area will never be "warm". It will definitely be significantly more comfortable than it is now, the SPF is great for air sealing but no much R value because of the thermal bridging of the studs, which is where the foam really helps.

    About the only way to get overhangs not cold is with supplemental heat (floor heat, baseboards, or extra register blowing across the floor).

    1. guylafleur | | #2

      There was originally about 3" of sawdust, so hopefully the batts+foam+rigid insulation will be better!

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