I am building a brick garage with a studio above. The roof will consist of painted metal roofing over 5/8 plywood over 2′ rigid foam insulation over 1/2 osb all attached to 2 by 8 rafters with fiberglass batts and finished with drywall. Do I need to add an air space within this insulation sandwich roof?
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Not if the dewpoint is inside the foam. Hard to predict. I would (since you asked - heheh) put the foam on the underside of your rafters, then the batts between the rafters - R-21 batts would leave enough airspace for insulation. Skip the osb.
I think it's fine the way you detail it. Add tarpaper under the steel.
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