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Spray foam advice

user_953789 | Posted in General Questions on

We are working on a single-family home in climate zone 5.  I am looking at the floor assembly for an “overhanging” floor (above unconditioned space) highlighted in the section below.  We are planning to use spray foam insulation.  The code minimum is R-30, which is easily achievable because our floor framing is 9.5″ deep.  Do you think open or closed cell spray foam is more appropriate for this application?

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  1. Expert Member
    AKOS TOTH | | #1

    Either one should work well if the joists are TJIs. Open cell is better as it uses much less polymer and significantly cheaper.

    If you have dimensional lumber joist, it is better to get a layer of rigid insulation underneath them to provide a thermal break.

    With overhangs the most important part is to air seal, so make sure the spray foam covers around the perimeter not just under the subfloor.

    PS. No matter how much insulation you put, overhanging floors are always cold. For example, my bedroom is in an overhang with 16" of insulation in the floor. If I turn the floor heat off, eventually the floors become too cold to be comfortable in bare feet. I would recommend adding a bit electric resistance floor heat on the top to warm the floor a bit.

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