insulation strategy for an unvented cathedral ceiling
I recently bought an old summer home in Freedom, NH, climate zone 6a. It is about 100 years old and currently has no insulation. The rafters are 2″x7″ (mostly, they vary a bit in dimension) 24″ on center. I would like to create an unvented cathedral ceiling in the attic space. My goal is around R50 without building down the ceiling too much. The plan I have come up with so far is this:
3″ of XPS (in 2 layers) against the underside of the roof sheathing, edges sealed with foam
4″ of closed cell SPF to fill out the rest of the bay
2″ of XPS over the inside face of the rafters
then strapping and drywall
This approach seams like it should work to me but just wanted to put it out there and see if anyone sees anything wrong with it. I have been reading a bunch on the subject of unvented cathedral ceilings and think I understand it but basically I’m just looking for a little confidence in my strategy.
Any thoughts or suggestions would be greatly appreciated,