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Knotty pine T&G for Cathedral Ceiling

artisanfarms | Posted in General Questions on

I am rehabbing a ranch house and considering a knotty pine T&G cathedral ceiling for the living room.  The insulation above this room will be either fiberglass or mineral wool batts in the rafters and 6″ of polyiso screwed onto the deck.  There is a second set of rafters above the polyiso insulation that will be vented from the soffits to the ridge and steel roofing is being used as the roof material.   

Should I put a layer of polyethylene sheeting between the batts and the knotty pine, or just install the knotty pine underneath the batts?

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Replies

  1. charlie_sullivan | | #1

    What the issues are also depends on your climate. Additional questions:

    1. Depth of rafters?

    2. How good is your air sealing at the roof deck, or your plans to upgrade it?

    I wouldn't use polyethylene, but I might use a permeable air barrier material, depending on the answers to those questions.

    1. artisanfarms | | #4

      The deck under the polyiso is OSB. I will be using a ZipTape or equivalent to seal the seams on both the OSB and the polyiso. The vented roof deck above the polyiso, will have a membrane underneath the steel.

  2. user-2310254 | | #2

    Andy,

    I think you need an air barrier. GBA often recommends drywall (installed in an airtight manner) or one of the specialty membranes.

    I'm also wondering if the 6 inches of polyiso will be sufficient since it looses R-value below 40 degrees F. (See here: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing.) Let's see if an expert will provide some advice.

    1. artisanfarms | | #3

      I'm in climate zone 5. 4" of polyiso is the recommended minimum, so with 6" I should be OK. I'm trying to avoid installation of drywall above the T&G if I can because it will be a PITA and also a fairly significant time and $$ suck. So, if some other, quicker to install and effective barrier is available, that will be my first choice.

  3. user-2310254 | | #5

    Andy,

    You could look into MemBrain, but I've read that it can be a bit fragile. One of the Intello products would probably be more robust. Maybe a few posters with hands-on experience will chime in.

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