Exterior roof insulation: top plywood?
I’m in climate zone 5 insulating a low slope (1:12) unvented assembly.
The slope can’t be increased, unfortunately, due to site restrictions.
Insulation will be 9.25″ mineral wool batts in rafters, then 6″ of rockwool toprock DD above the roof sheathing (5/8″ plywood).
Roofing is standing seam metal with solar panels attached (S5 clamps).
I am having difficulty deciding on whether to include a second layer of plywood as a roof deck. Examples I could readily find details for were
1) The Fine Homebuilding California 2018 house where the stack was Plywood -> underlayment -> Rockwool -> underlayment 2 -> standing seam. They used long fasteners to attach the panels to the framing below, through the Toprock DD.
2) Matt Risinger’s house in production, with Zip -> Polyiso -> Zip -> Standing seam.
I am wondering if anyone has experience or comments with one or the other of these approaches.
Currently my stackup is
Intello Plus -> batt MW -> plywood -> Solitex Mento -> Toprock -> Panels, with no secondary underlayment (the roofing manufacturer has no stipulations about the stackup)
Approach #1 is attractive, but I have concerns about shear on the fasteners. Theoretically the fasteners should allow panels to slide and the Toprock will be separately nailed down so shear shouldn’t be communicated, but I am not sure about in practice. Also it makes details with an attached veranda somewhat tricky.
Approach #2 I worry about the ability of plywood or OSB as the top roof deck to dry in this unvented roof with solar panels shading it. Is this worry unfounded?
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