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Community and Q&A

Drywall or not?

user-3811928 | Posted in GBA Pro Help on

I am building a house with cathedral ceiling, which will be finish with some pine wooden plank. I remember reading in GBA the necessity to have plastered drywall underneath the wooden plank so it can serve as an air barrier.
In my case I will have some roxul insolation then polyisoyanurate panel then fur strip and then the wood plank, my question is do I still need to have plastered drywall under the wood plank or is the polyiso panel will also act as the air barrier therefore drywall is not necessary.

Please advise
Thank you in advance


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  1. GBA Editor
    Martin Holladay | | #1

    Taped foil-faced polyisocyanurate can be an adequate air barrier, as long as you choose a high quality tape (housewrap tape or one of the European tapes). Don't forget to seal the perimeter of the ceiling (where the polyiso meets the walls) with caulk, canned spray foam, or high quality tape.

    The only controversy concern whether or not rigid foam in general -- or polyiso in particular -- might shrink or swell, popping the tape seams. My own judgment is that the polyiso won't move much, especially if it is kept warm and dry (as it would be on the interior side of your ceiling insulation). If you are worried about foam shrinkage, you are always free to install gypsum wallboard if you want.

    Your insulation plan (mineral wool batts with polyiso on the interior side) will only work if you have a ventilation gap between the top of the mineral wool and the underside of the roof sheathing -- so I hope this is a ventilated roof assembly.

  2. user-3811928 | | #2

    Thanks Martin,
    Yes it is ventilated over the roxul, there is à 5.5 inch gap for ventilation. Is tuck tape OK or do I need something better than that?

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