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Community and Q&A

Polyethylene Sheet Foam for Sealing Tongue-and-Groove Ceiling

Lindaloowho | Posted in General Questions on

Hi All,

We have tongue and groove ceiling in a few rooms of our old home in Southern Ontario, which I love the look of, but know are prone to air leaks. Some I can see easily, others I’m sure I cannot.

I’m in the process of air sealing from the attic side, but I don’t want to use more spray foam (I think I’m developing a sensitivity) and I can’t see me caulking every joint of the sealing.

Since traditional poly plastic sheet is not recommended due to its moisture trapping tendencies, what about Polyethylene Sheet Foam? A product is available, used for vinyl floor underlayment. It’s very thin, and pliable, and closed cell, right?

If this is indeed a good idea, how would I apply it to achieve the best success? Use a caulking sealant where the attic floor joist and ceiling meet, as well as the perimeter of the attic? Then drape the sheet over the attic floor, push it down to fit tightly over joists and into the sealant? I would overlap seams and seal there too.



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  1. Expert Member


    There us no reason not to use poly on the top side of a ceiling in a vented attic in Ontario. That is how virtually every house around you is built.

  2. Lindaloowho | | #2

    Hi Malcolm,
    That’s true even if we are shy of 8000 HDD in this area? And we may air condition at some point?

    Using poly would be simpler, that’s for sure. Can I use the method described above? Drape and seal over the attic floor joists? Or do I have to cut and seal the poly to lay in the joist bays so it does not drape over the joists?


  3. Expert Member
    BILL WICHERS | | #3

    The polyethylene isn't a problem here because the vented attic will allow any mositure to escape, so you won't have mositure accumulation issues. This is different from a wall where there typically is very little air movement and thus FAR less drying potential then you have in any kind of vented attic.

    You could put poly into each joist bay and just caulk the edges. It might be easier to do it from the other side, even if that means removing and reinstalling the T and G planks though. It all depends on how accessible the attic side is, and how much stuff is in the way (like insulation), that you'd have to remove and replace.

    I'm not a fan of draping poly over the joists. I think that could pose a risk to the joists themselves over the long term, although I can't point to any specific examples where that's been a problem.

    I would suggest a possible simple alternative: try cutting pieces of 1/2" polyiso to fit between the joists against the top of the T and G planks. Seal the polyiso to the joists using canned foam. That will give you a good air barrier, should install fairly easily, and will even add a bit of insulation to the assembly which is an added bonus.


    1. Lindaloowho | | #4

      Thanks Bill!
      Was it you who asked for a quick and dirty method to seal T&G at one point? How did the polyurethane work out?

      1. Expert Member
        BILL WICHERS | | #5

        Yep, that was me! I had thought "maybe if I just prime it and paint it a few times, it will be "sealed", at least better than it was". Seemed like maybe a good idea I thought but... Stuff came up and I didn't have time and then I had a plank near a wall fall out so my thinking changed to "I have a 15 gauge finish nailer that can tack these up pretty quick". sooo...

        I'm going to pull down the T and G planks, put up a taped layer of 1/2" polyiso, then reinstall the T and G planks over the polyiso. This is a better way to go anyway, and not a huge amount of work since my room is only about 12x14 feet or so. We notice a draft in that area, and frost patterns on the roof (frost patterns on roofs are a great diagnostic tool this time of year!) shows that this room is the leakiest part of the entire house. I'm hoping to get this work done this or next week in fact.

        I always knew painting the T and G wouldn't be a great long-term solution, but after a plank fell out, I just abandoned that plan since I don't trust the other planks to stay put. I also have one can light to seal off that isn't connected anymore, so the polyiso takes care of that too.


        1. Lindaloowho | | #6

          What did you think of my idea? To use thin polyethylene foam sheet, which comes in a roll, as a quick way to air seal T&G?

          This sheet is used as underlayment for engineered flooring. Could it not be used on the attic floor and draped over the joists, providing an an air barrier, but allowing moisture to dry to the vented attic, or to the interior of the home?

          1. Expert Member
            BILL WICHERS | | #7

            I don’t see any issue with that product aside from wrapping the joists with it. Wrapping the joists on three sides concerns me a little, although I can’t point at any particular thing and say “aha, that will be a problem!”

            Basically I think you MAY be adding a small amount of risk with the joists with your plan, but how much risk I can’t really say.


  4. charlie_sullivan | | #8

    Some suggestions to consider.

    1. Cut sheets of 1/4" plywood, lay them in the cavities, and tape the edges with air sealing tape.

    2. I worry that the material you are suggesting will get little tears in it too easily, which will result in air leaks. I would think that a good quality WRB (water resistive barrier) material would work better. Wrapped over joists or cut to lay in cavities with the edges taped.

    3. For better air sealing you could use "blueskin" self adhesive WRB. That might be hard two work with in that space.

    4. A fluid applied WRB material might work really well and might be reasonably easy to apply. But it's hard to be sure how it would behave on the cracks.

    1. Lindaloowho | | #9

      Thanks Charlie!
      So something like Tyvek (a WRB) could be used in the attic over a tongue and groove ceiling?

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