Zone 3 – Unvented roof assembly
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I’m currently re-roofing our 50’s Ranch, we have an addition with a cathedral ceiling and have currently wrapped the entire roof deck with Grace Ice and Water Shield, which also serves as an Air and Vapor Barrier. Would it be OK to install R30+ Mineral Wool batts in the unvented assembly? Or should we go with closed cell spray foam? Can you foresee there being an issue with moisture build-up with both materials because of the Ice and Water Shield? Keep in mind, we’re in Zone 3, so the bare minimum for insulation is R30 — we have about a 7″ cavity to work with, the roof is also asphalt.
Best,
Casey
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Replies
Casey,
Q. "Would it be OK to install R-30+ mineral wool batts in the unvented assembly?"
A. No.
Q. "Should we go with closed cell spray foam?"
A. That would be better from a moisture perspective.
Q. "Can you foresee there being an issue with moisture build-up with both materials because of the Ice and Water Shield?"
A. An unvented assembly with closed-cell spray foam will work, but an unvented assembly with mineral wool batts is asking for trouble. For more information, see this article : "How to Build an Insulated Cathedral Ceiling."