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Seeking advice on insulation and membranes for an exposed wood structural roof

guy_dbrd | Posted in General Questions on

I am working on a little house extension located in Toronto Canada. We are shooting for an exposed wood structure, with the insulation (required r-value 31) outboard of the roof’s diaphragm which will either be tongue and groove or a very thick plywood.

I am very unclear on where my vapour barrier should be located – I was taught that the membrane needs to go on the warm side of your wood studs, in a conventionally framed wood house. In this scenario there is nothing below the warm side of my structure at the ceiling – there is analogy in the condition I am working on.

I am just wondering if anyone has opinions on the matter. What I thought we would do was deck on the wood beams, above which we would put the vapour barrier membrane. Then two layers of polyiso board to get our R31 followed by another layer of plywood and then our air vapour barrier. Then our metal roof and its attachments.

Would really appreciate feedback

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Guy,
    If you install two layers of polyiso rigid foam on top of your roof sheathing, these two layers of foam will create an adequate vapor retarder. So you won't have any vapor diffusion worries.

    This type of roof needs a warm-side air barrier, however. That means you need an air barrier under the rigid foam. Options include taped plywood roof sheathing, taped OSB roof sheathing (perhaps Zip sheathing), a peel-and-stick membrane like Ice & Water Shield, or one of the fancy air barrier membranes from Europe that are sold by 475 High Performance Building.

    For more information on the type of roof you will be building, see this article: How to Install Rigid Foam On Top of Roof Sheathing.

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