Where to put the smart vapor retarder?
We are remodeling the second floor of our 1956 Cape in Williamsburg VA, climate zone 4. I need advice about where to incorporate the MemBrain smart vapor retarder in the roof and wall insulation stack up.
Roof stackup: Asphalt shingles- Durabuild synthetic underlayment-1×8 plank roof decking- approximately 1.5 inch airspace (ProVent running soffit to ridge vet)- 3.5 inches of Roxul R15 between the 2×6 rafters- 5.5 inches of Roxul R23 between 2×6 “mooney wall” furring running perpendicular to the rafters- sheetrock.
In the roof, should the vapor retarder go between the two layers of Roxul mineral wool or on the living space side between the 2nd layer of insulation and the sheetrock?
Wall stackup: Vinyl siding- Tyvek- old wood clapboard siding- gypsum sheathing- 3.5 inches of Roxul R15 in the 2×4 wall- a second 2×4 wall must be constructed to address some existing structural issues and can be insulated with an additional layer of Roxul R15 if appropriate- sheetrock.
For the exterior walls, is there any real benefit to insulating the second wall (bringing the assembly to R30) when the roof/ceiling is only at R38? If yes, should the vapor retarder go between the two layers of Roxul mineral wool or on the living space side between the 2nd layer of insulation and the sheetrock? By putting it between the walls, it would eliminate the need to seal around electrical boxes contained in the second wall.
Relevant notes: second floor living space is 2 bedrooms & 2 bathrooms with mechanical ventilation. HVAC ductwork is inside the insulated building envelope. No combustion appliances currently installed on the second floor- possibility of a small gas clothes dryer in the future. Rigid foam above the roof decking and/or spray foam below is not an option.
Thanks for your time and advice! –Chris
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