Moisture/heat transfer issues in a rigid foam framed over roof?
I have an old house with an uninsulated attic with 2×6 rafters 24inches OC. I am replacing the roof and want to insulate the roof deck with polyiso rigid foam board. My engineer recommends that I beef up the roof framing by adding a 2×8 rafter on top of the roof deck, over and in line with the existing rafters, basically converting a 2×6 into a 2×14. But I am concerned about the implications of having the new wood members embedded in the insulation layer. Here is what I’m thinking of doing: (1) apply peel and stick I&W membrane for air sealing and WRB over the existing roof sheathing; (2) install the 2×8 rafter “extensions”. (3) infill the spaces between the 2x8s with ripped down sections of polyiso rigid foam board (4) finish with a one inch layer of polyiso over the rafters to address thermal bridging and to get an air-tight layer, tape the seams. (5) make a vent channel by applying 1x furring on top of 1 inch foam and in line with the rafters, then OSB decking over the furring, and asphalt shingle roof on top of that.
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