Yet another unvented cathedral roof question
Roof sheathing is air barrier (plywood with seams and all penetrations sealed with prosoco joint and seam sealer, fast flash and cat 5 weather barrier). Roof overhangs will be attached on top of rigid insulation by running 2×4’s vertically and horizontally and connecting to rafters through roof sheathing. Because of dormer window sills, I am limited to a total height of 7.5″ above roof sheathing for insulation, roofing and roof overhang framing. Because I already have (long story) almost sufficient amounts of thermacork rigid insulation (3” for R12, ship lapped edges), I would like to use the thermacork in place of rigid foam on over roof sheathing. Since I have a 3.5″ space between the 2×4’s attached to top of roof, I would like to add roxul comfort batts to up the total R value above roof to ~R27. R 30 roxul batts will be added to rafter bays. See attached diagram.
This is not a typical roof design. I think for my zone, this approach is fine, Sheathing is the air barrier and class II vapor retarder. Above the structural sheathing insulation levels control condensation in zone 4c. As I get older, I find I am wrong more times than I expect. So I am asking those who know, is there a problem with this approach?
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