Unvented low-slope roof with exposed beams
I am designing a 2nd-story addition to a home with a simple 2/12 pitch, shed roof to the east in Colorado, climate zone 5. The clients really want an exposed beam look so we have designed the roof with exposed glulam beams at 4′ centers. Above the beams we currently plan for 2x t&g decking, 4″ polyiso, and another 4″ nailbase insulation with polyiso, unvented. Roofing material will be Sarnafil thermoplastic vinyl membrane roof.
According to my research, we should include an air barrier between the decking and the rigid insulation as the t&g will not be air tight. The glulams are designed to protrude out beyond the wall to carry the 3′ overhang of the roof.
1. What type of air barrier should we use between the t&g and the insulation?
2. It strikes me the the 7 glulams protruding out on both the west and east sides will make it difficult to air seal the joints created in the walls. I have been trying to think of a way around this. If we eliminate the glulam extensions, then we would need to add more framing above the decking to carry the over hang. This framing would seem to complicate the insulation of the built up roof. One way I thought of would be to eliminate the t@g, and frame a structural over roof with 2×4’s to create the overhangs, spray foam in between, then add sheathing and 4″ nailbase insulation above. Then drywall between the beams (which would be acceptable). Any suggestions on this? or are the protruding glulams not a problem to seal.
Thanks for your input!
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