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A question about adding spray foam in a ceiling

b4316207 | Posted in General Questions on

I have a general question. We are about ready to start insulation. I have a traditional ventilated roof. I had plywood placed on the floor in the attic.

Originally, I was going to just use canned spray foam to just seal the open gaps, but someone recommend just spraying 1-2 inches of spray foam on the floor from under the ceiling of the 2nd floor.

Question: Is this a good idea or bad idea? What problems?

As stated earlier, I’m not looking for R-value. I’m dong traditional blow-in insulation in the attic.

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    Because your question is unclear, I need to ask a few questions.

    1. I assume that the blown-in insulation is for the attic floor. Has it already been installed, with a plywood floor installed above the blown-in insulation?

    2. Are you interested in sealing the seams of the plywood subfloor (as an air sealing measure)?

    3. Does your house have a drywall ceiling yet? I can't understand how you could spray foam on the underside of the plywood subfloor from below. Perhaps you are talking instead about your roof sheathing?

  2. b4316207 | | #2

    1) blow in will go in attic floor , no blow in is not placed yet , plywood is in
    2) yes seams of sub floor and any other open gaps
    3) no dry wall ceiling yet , I have trusses and was able to put plywood between trusses I now have a 3.5 inch gap in 2nd floor ceiling between plywood and where dry wall will go

  3. GBA Editor
    Martin Holladay | | #3

    If the bottom chords of your roof trusses are 2x4s, they may not be designed to bear much weight. Be careful what you put above the plywood subfloor unless these trusses were designed to bear weight.

    Most builders use the drywall ceiling as their air barrier -- and you may still be able to do that, as long as you pay attention to sealing any penetrations through the ceiling.

    If you want to use the plywood subfloor as your air barrier, several questions arise:

    1. Did you install the subfloor with construction adhesive?

    2. If so, was the construction adhesive installing consistently, in uninterrupted ribbons, to prevent air leaks?

    If the subfloor was installed without any construction adhesive, you may want to try to seal the plywood from below with spray foam. But that's an unusual approach.

    P.S. One of the main problems with using the plywood subfloor as your air barrier is that it is hard to seal the diagonal members of your roof trusses, which penetrate the subfloor.

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