Good idea for R-38 under a 2×4 shake roof?
I want to expand a top-floor bedroom to include an uninsulated 16’x9’ corner of my 1920s craftsman (zone 5). Its southwest exposure, 7/12 roof is cedar shingles on skip sheathing, across 2×4 rafters which bird-mouth onto the exterior wall exposing 2” of the backside of the frieze boards; exposed rafters outside, no vents.
If I fur out the rafters with 2x6s, then filled between them (top down) with a 1” gap, 2″ double foil sided polyiso board R-12.9, 5½” fiberglass batting R-21, then nailed to underside of 2×6’s with 1” XPS R-10, then ½” sheetrock. Then add vents to the frieze boards. Can you think of issues created by this plan i.e. condensation, ice dams…?
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