Insulation alternatives for Cathedral ceiling hip roof unvented
maine854
| Posted in Green Building Techniques on
Trying to think outside the box for insulating to R-50 a cathedral ceiling-ed, un-vented, hipped roof over conditioned space in Massachusetts. Would applying poly wall or a self adhering membrane like blueskin on the underside of Zip sheathing be a substitute for flashing the roof sheathing and roof joists with closed cell spray foam before filling joist cavities with cellulose?
Both for reasons of any yet undiscovered bad chemical side reactions which might contaminate spaces containing CCSF, and for ease of recycling at end of building lifespan I’d prefer to avoid spray foam if possible.
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I'll tell you what I'm doing. I'm in climate zone 6B and need a cold roof due to snow and sun during long winters. (Doing an unvented roof with a cold roof above is what Joe Lstiburek calls a "vented unvented roof for snow country.") Since we are going with a hip roof design, and I also don't want spray foam, I'm air-sealing the roof sheathing, using Rockwood insulation above the sheathing, then building the hip structure above that and venting it, and that will act as the cold roof. (I'm still working out the exact membrane/air-ice shield details. Our is low slope 2.5/12)
Thanks, qofmiwok. The setup you describe would have been my preference, but wIth shingles and roof trim already installed, I’m basically forced into an under-the-sheathing solution. My understanding from the articles at GBA suggest ccsf is the only way to go to protect any interior condensate moisture from ever reaching the underside of sheathing. Was curious if any other non permeable membrane could be a substitute as long as joist bays were filled with R-50 of cellulose.