Vapor retarder in exposed structure roof?
i am designing an exposed rafter roof assembly that will have exposed 2x t&g decking as the finished ceiling. Structural is requiring a layer of plywood or osb over the decking for diaphragm. The remainder of the roofing assembly would be 6+” of polyiso, 2x vent furring space, 1/2″ osb roof nailing surface, building felt and the asphalt shingles. My question is regarding the space between the structural plywood and the insulation. I was planning on making the plywood an air barrier by taping the joints. If the plywood acts as a vapor retarder and air barrier, would any other ‘barrier’ be advised between insulation and plywood?
The project is in Portland Oregon
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