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Community and Q&A

Foam skim-coat on attic floor in new construction?

B Snyder | Posted in Energy Efficiency and Durability on

I am considering skimming the entire attic floor (top side of the drywall as well as the horizontal chord of each roof truss) with a half-inch or so of sprayed-in foam before adding the deep blanket of blown-cellulose. The location is mid-Michigan, and the rest of the house will be fairly tight. Is this a wise approach (structurally, financially, etc) or am I headed in the wrong direction? Thanks for your help.

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  1. Riversong | | #1

    If you've done a reasonable job of air-sealing the ceiling then the cellulose alone should be sufficient to control convection and you'll save a lot of money.

    Given the propensity for roof trusses to uplift during the winter (and crack ceiling-to-wall drywall joints) because of differential moisture content in the top and bottom chords, it may be wise to allow the bottom chords to breathe.

    Make sure the cellulose contains only borates for fire retardant, since the ammonium sulfate that some manufacturers use can corrode metal truss plates.

  2. GBA Editor
    Martin Holladay | | #2

    B Snyder,
    I think to get the benefit of the air-sealing properties of the foam, you would want at least 1 in to 1 1/2 in. And you should be sure to use closed-cell foam rather than open-cell foam.

    There are cheaper ways to air seal an attic, by just using the air-tight drywall approach and minimizing penetrations. Then all you might need is two or three cans of spray foam, and you're done.

    It's probably a better use of your money to air seal the usual way, and use the extra money to buy deeper cellulose.

  3. B Snyder | | #3

    Thank you, both of you, for the excellent information.

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