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Vapor retarder in exposed structure roof?

user-993901 | Posted in General Questions on

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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  1. GBA Editor
    Martin Holladay | | #1

    Your plan to tape the plywood seams is a good one. I suggest that you use 3M All Weather Flashing Tape (8067) or one of the European tapes from Small Planet Workshop or Four Seven Five. (For more information on tape selection, see Air-Sealing Tapes and Gaskets.)

    To further improve airtightness, you should consider taping the seams of the polyiso. Multiple layers of rigid foam (with staggered seams) are better than a single layer of 6-inch-thick foam.

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