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Air sealing for unvented cathedral ceiling

Eric West | Posted in Energy Efficiency and Durability on

I am planning a remodel which will add a cathedral ceiling,and there is already an existing cathedral ceiling in another part of my home. Since much of the roof will already be removed this seems like a good time to correctly insulate and air seal.On this site (thank you Martin the outer layer of rigid insulation is taped at the seams. On the BuildingScience site (thank you Joe the unvented compact roof shows self-sealing membrane over the structural roof deck for an air barrier. Somewhere else I read the suggestion to use tape on the seams of the structural sheathing. All accomplish the same goal but differ in execution.

My question is which method is preferred and why?

(I’m in climate zone 5 (Denver) with a local snow load design requirement of 30psf. I’m planning on adding at least 2 layers of 2″ of poly-iso above the structural deck and filling the 8″ rafter bays with dense pack cellulose.)

Thank you very much for providing all the great information and answering so many questions!

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  1. Floris Keverling Buisman | | #1

    Taping the sheathing is the fastest, affordable way to airseal on the warm (correct) side of your insulation. This will keep the conditioned air where it should stay - inside. This can be done with for example (a href="">TESCON VANA that we import at <a

  2. GBA Editor
    Martin Holladay | | #2

    I also vote for taping the sheathing rather than installing peel-and-stick membrane over your entire roof.

    For more information on high-quality tapes, see Backyard Tape Test.

  3. Eric West | | #3

    The remodel is finally moving!

    We can easily tape the seams on the new construction. What about the older roof deck that has holes from the previous shingles?
    It seems like the choices are:
    a) Nothing, but that seems like a bad idea
    b) Replace the roof deck, but that seems wasteful and expensive
    c) Some sort of membrane above the structual sheathing, either peel-and-stick , or something like Siga Majcoat.
    d) An air barrier under the rafters

    The currently planned stackup looks like this:
    Asphalt shingles
    2 layers of underlayment (code requirement due to 3 in 12 roof slope)
    Nailing deck
    5" foam insulation (2" EPS, 3.5" polyiso)
    Structural plywood roof deck
    5.5" blown fiberglass

    Thanks again

  4. Expert Member
    Dana Dorsett | | #4

    To get the best performance out of the exterior rigid foam, put the polyiso next to the roof deck, with the EPS on top of that.

    A peel'n'stick membrane over the structural roof deck works fine, and presents no wintertime moisture adsorption issue with 5.5" of fiberglass (R18-R24, depending on density) and ~R28-R30 foam above the roof deck.

    In your stackup the nailer deck is essentially a sacrificial layer- when you re-roof in a few decades you'll have to replace it on sections where it might have leaked. There is no drying path for the nailer deck layer, and there's no way to make one, unless you use furring to hold down the foam, and mount the nailer to the furring. With 2x furring and soffit-to ridge venting you might end up with snowmelt dripping in at the ridge from time to time, given your low roof pitch. If you segment the furring with 1' gaps every 3' or so, with just soffit venting (and no ridge vents) wind currents would cross ventilate, which would work pretty well in Denver's climate, somewhat less well in snowier or wetter locations.

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