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Membrane in unvented roof

haroto | Posted in General Questions on

I am installing 1×6 T&G boards above my 2×8 rafers in an addition cathedral ceiling project. I plan to follow with a layer of 5/8″ plywood (for roof structure), and 6 inches of polyiso above that.
I am wondering if it’s OK to install the air sealing membrane above the tongue and groove, which would allow me to wrap it down out to the walls (T&G stops a the walls, but the plywood layer continues to the rake and eaves). I am concerned about that plywood layer trapped between the membrane and the insulation. Is this a risky configuration? Should I put the membrane above the plywood instead and air sealing at the exterior walls with foam insulation?
I am in Austin, Tx. I appreciate any feedback.

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  1. GBA Editor
    Martin Holladay | | #1

    It's perfectly OK -- in fact it's a good idea -- to install an air sealing membrane above your board roof sheathing, as long as you have 6 inches of rigid foam on the exterior side of the membrane. You can use a peel-and-stick membrane like Ice & Water Shield, or a European air barrier membrane (sold by 475 High Performance Building Products) developed specifically for roofs.

    You shouldn't install a peel-and-stick membrane on your wall sheathing, however, unless you also plan to install most of your wall insulation on the exterior side of your membrane.

    For more information on these issues, see:

    Getting Insulation Out of Your Walls and Ceilings

    How to Install Rigid Foam On Top of Roof Sheathing

  2. haroto | | #2

    Thanks Martin, this helps a lot.

  3. jackofalltrades777 | | #3

    SIGA Majcoat is the Swiss-made breathable roof membrane that stops wind and rain from entering past it but it also is permeable so any moisture trapped underneath can dry through the membrane. Tested by the Berlin Institute.

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