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Open-cell spray foam under roof deck (not vented)

Dean2710 | Posted in Green Building Techniques on

I am considering insulating under my roof deck with open cell foam. I am in zone 5. It is a hip roof so the space between the second floor ceiling and the rood deck is at most 5 feet and in some areas it is vaulted (skylights in main bath and above stair well). I have read that in zone 5 using open cell foam  under roof deck un-vented a moisture retarder is needed. Would applying paint with  a moisture retarder to the  second floor ceiling be good enough or should I apply a moisture retarder directly under the foam. I am considering using MemBrain Continuous Air Barrier and Smart Vapor Retarder by CertainTeed ? Also is there any reason why a dehumidifier  should not be placed in the conditioned area above the second floor ceiling?


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  1. Expert Member
    Dana Dorsett | | #1

    Since the air leakage between the ceiling and attic space is an unknown (especially over the long term), it's safer to go with a smart vapor retarder or gypsum board painted with vapor barrier latex primer directly up against the open cell foam.

    Installing a dehumidifer in the attic is fine. It'll heat up the attic, turning the heat of vaporization of the moisture removed into sensible heat (hot air), which may not be exactly what you want to do in the summer.

  2. GBA Editor
    Martin Holladay | | #2

    First of all, can you tell us your name? (I'm Martin.)

    Vapor retarder paint cannot be applied directly to cured spray foam. It won't work unless it is applied to drywall (or a drywall-like surface).

    Using open-cell spray foam on the underside of roof sheathing is more risky than using closed-cell spray foam. Open-cell spray foam is associated with moisture accumulation in roof sheathing. For more information on this issue, see "High Humidity in Unvented Conditioned Attics."

    1. Expert Member
      Dana Dorsett | | #3

      >"Vapor retarder paint cannot be applied directly to cured spray foam. It won't work unless it is applied to drywall (or a drywall-like surface)."

      Which seems very much like...

      >"... applying paint with a moisture retarder to the second floor ceiling..."

      ... the fly in the ointment being the uncertain air tightness between the second floor and attic.

  3. Dean2710 | | #4

    Hello my Name is Dean, Thanks for replys. If I go with open cell under the deck I will apply the smart Membrain directly under it and will not go with the dehumidifier but will keep an eye on the humidity levels with a sensor. However I just met with a contractor who is proposing a vented blown in cellulose solution and will seal the perimeter where the roof deck meets the wall with closed cell foam and of course thoroughly seal the second floor ceiling.

    I also have a great room that is vaulted at the widest space from the bottom of the ceiling joist to the roof deck is 12 inches and the near the walls it goes down to 8 inches. Each side spans about 14 feet from wall to beam so not a lot of room to vent so I am leaning towards an un vented solution again considering open cell foam. In this set up I will have drywall primed with a moisture retarder covering the foam.

    The cellulose contractor also proposed an un vented solution with a combination of closed cell where the roof meets the wall, densely packed cellulose at the narrow sections then loosely packed where the space widens up at the top. He also said he could build customs vents if I wanted a vented solution it will add to the cost. He did think the un vented solution would work an may also through in a vapor retarder. Any thoughts on which way I should go?


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