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Vapor Barrier for Cathedral Ceiling

artisanfarms | Posted in General Questions on

In my pretty good house renovation I have cathedral ceilings in two rooms.  Starting at the roof, the layers are as follows:

1) Steel roofing material
2) Furring Strips
3) moisture barrier
4)Plywood/OSB deck
5) Roof framing
6) ~16″ air gap
7) 7″ Polyiso foam
8) OSB
9) R23 Mineral Wool batt style insulation
10) vapor barrier???
11) planned knotty pine T&G ceiling

My question is what is the most appropriate material to place between the T&G ceiling and the mineral wool, given that the T&G will be very permeable.

The house is located in Central NY, Zone 5 and sees summer temperatures in the 90’s and winters down to 0 F.

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  1. Expert Member
    Michael Maines | | #1

    That's an unusual assembly so it's hard to say... any chance you could share a sketch of what you're planning? Variable-permeance membranes are usually safe and beneficial, but with 7" of polyiso it may not be necessary.

  2. PAUL KUENN | | #2

    I'd like to see that as well!

  3. Expert Member
    BILL WICHERS | | #3

    I'd try to leave out that 1/16" air gap at location #6. Air gaps in assemblies like this make me nervous, because they are a place moisture can accumulate over time an potentially cause problems. Your "roof framing" looks to be ABOVE the polyiso too, which is unusual with so much polyiso. I agree with the other posters that a drawing would be very helpful!

    If the framing is part of a vented roof assembly, then you probably won't have any issues execept for fastening the OSB through all that polyiso. That's going to be tricky. If you put the polyiso ABOVE the framing, things will probably be easier, and that polyiso is about R45 so it should prevent moisture issues on the interior side of the OSB. I'd probably still put one of the cheaper smart vapor retarders (like MemBrain) on the interior for some extra protection though, although it's probably not really necassary with your R value ratios here.

    Drywall is commonly used behind T+G as an air barrier. I've used 1/2" polyiso before too, with the seams taped, in which case you also end up with a vapor barrier if you use foil faced polyiso. I would be reluctant to put an interior side vapor BARRIER in your assembly though.


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