I read a couple of articles regarding vapor barriers and insulation. I live in update NY, our home was constructed in 1952. We are adding a full shed dormer with 2x6 walls and 2x12 rafters. Our plan is to use closed cell in the walls and ceiling - which will have a copper standing seam roof. Code requires a Tyvek type outer wrap, but we want to consider adding 1/2" or 1" polystyrene to the inside to prevent bridging. Sheet is then applied to the polystyrene.
I do like closed cell, the excellerant is worrisome as it is a fairly potent green house gas..
thank you, Michael