Our mid-century modern home in Maryland/DC region has one large room that has a 3:12 pitch shed roof. It has 2×12 rafters sitting on the walls on each side (the lows side has a 2′ overhang and is vented at the soffit; the high side terminates on the wall and has venting). It is currently insulated with fiberglass insulation. It is western exposure.
I plan to have standing seam metal installed. I want to take the opportunity to add insulation, as the heat gain on sunny days above 75 is significant. I see two viable options: 1) remove existing roofing, existing plywood decking, spray foam between the rafters (closed cell?), lay new decking, and then add metal roof (making this an unvented assembly); or 2) remove existing roofing, add 2″ of foam on existing decking, add second layer of decking, and then add metal. The second option seems troublesome since the existing venting is going to be right in the middle of the two layers of insulation, causing all sorts of moisture problems.
Any problems foreseen with option 1? Any other options I’m missing? Thanks for any guidance.
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