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Yet another unvented cathedral over roof question

ReallyNotARobot | Posted in Green Building Techniques on

Looking for confirmation and recommendation for an unvented cathedral ceiling retrofit. 

 
1970 construction, 4/12 pitch, climate zone 5, currently foil faced fiberglass bats in 2×8 rafter space. Unvented, Plywood roof deck is solid. Interior drywall at peak joint is leaking air causing drywall moisture decay. Cooling load far exceeds what manual j predicted.
 
Based on reading GBA articles my proposed fix is to tear off the asphalt roof down to plywood and remove drywall. Redo with the following stack up from inside to out. 
Drywall, 2×8 rafter with 7″ rockwool, existing 1/2 plywood, some soft of peal and stick membrane, 2 3″ layers of foam(6″ total) strapping or plywood, roofing underlayment, standing seam metal roofing, empty wallet.
 
Questions I have that I couldn’t find answers to;
For airsealing the current plywood deck before laying out foam.
Does it need to be vapor open like blueskin or would grace ice and water work? (Would prefer a peel and stick product)
Even with cold weather performance of polyiso being lesser than xps does the higher service temp rating polyiso has make it the better choice? Does the facing material matter (foil or fibermat)
Is it better to hold down the foam with diagonal stapping (1×4 or 2×4’s) or standard plywood / zip sheeting?
Not sure if metal roofing needs under venting and most manufacturers seem to prefer solid decking?
 
Any advice or product recommendation are greatly appreciated. 
Or if a better solution exists I’m all ears.
Thank you
Jay

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Replies

  1. Expert Member
    Akos | | #1

    There is not much point for going above the required minimum of exterior rigid for condensation control. Roofs with rigid insulation are pretty expensive and there is no ROI in going overboard. Stick to the minimum you need (~3.5" of polyiso).

    The permeability of your roof deck doesn't matter. You can use either vapor open or vapor closed peel and stick. Sometimes it is simpler to tape the seams of the roof deck to air seal followed by the rigid, depends on what your installer prefers.

    Make sure to air seal from your wall top plate to the roof deck, this is usually best done with one of the smaller two part spray foam kits from the inside after the ceiling is pulled down.

    For unvented roofs, usually the lowest cost rigid is roofing polyiso. This is typically fiber faced and works great for this. In this type of roof, the permeability of the rigid doesn't matter, you can use any facer that is available.

    The roof deck can be either CDX/OSB or horizontal 1x4. Check with your roofing manufacturer what they require as not all roof panels can be installed over strapping. I generally go for 1x4 as it is lower cost. Make sure there is a synthetic underlayment as well (over the new deck if CDX/OSB or over the rigid but under the 1x4 strapping).

    The edge of the foam needs to be protected from critters. Usually best to picture frame the whole roof with 2x on edge the same thickness as your foam. This 2x can also serve dual duty to extend your facia boards.

    1. ReallyNotARobot | | #2

      Thank you for the reply.
      Have run into a snag.
      Polyiso is near impossible to source. My local suppliers wont even take my money to order as they dont have firm dates when it will be available. One place said March as a best case.
      Looking for alternatives.
      Could get the closed cell foam guys out and hit the underside of the joists for 3" and then rockwool the rest and call it a day.
      Build ventilation shoots and go to town with the closed cell foam.
      Build ventilation shoots and fill with rockwool ( would be roughly r23, still better then what I have now). Could build down the ceiling for additional r value.

      I believe the only reason the roof deck hasn't failed yet with the unvented fluffy insulation it has now is due to the house being shut down in the winter. But I could be wrong.
      Opinions would be appreciated.
      Thank you.

      1. Expert Member
        Akos | | #3

        Check commercial roofing suppliers, any place that deals with flat roofs would stock it. Roofing polyiso and EPS is pretty standard and relatively cheap.

        Any of the other options you propose will work, generally the full SPF is not worth the cost as you don't improve the assembly R value all that much after you take into the thermal bridging of the rafters.

        The lowest cost and simplest is the vented roof with batts, as long as you have clear path from soffit to ridge and install the proper size vents on both ends, it should work great.

        Over here we like to talk about R60+ roofs but the reality is that you get most of your energy savings in zone 5 with an R25 wall/roof. Going much above that using expensive insulation (ie SPF or rigid) is only worth it if you are off grid or trying to hit some energy efficiency target like passive house.

        The important part, no matter which assembly you choose is to air seal. Leaky ceilings can easily loose way more heat than a lack of insulation.

        It you don't mind a bit of extra DIY work, you can bump up the assembly R value by a fair bit with something like this for not much extra cost:

        https://www.finehomebuilding.com/project-guides/insulation/breaking-the-thermal-bridge

        I would use ripped strips of R6 or R9 Zip R or another nail base though instead of the foam+wood.

        I think you are correct of why your roof held up. Condensation in unvented cathedral ceilings is mostly a wintertime problem.

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