Unvented roof underlayment and rafter bay insulation
Hi guys…first time poster, love the site its been a huge help. I’ve just finished framing my home in Montana and am ready to move on to the roof. It consists of multiple single pitch decks ranging from 1.5″ – 2″ in 12. Framing is 2×12 rafters.
Originally I set out to do rigid foam above the roof deck, but realized late in the game it would take 6″ to do it safely. Its too cost/time prohibitive for me to go that route. I’m now leaning filling the rafter bays with rigid foam directly under the roof deck and air sealing with spray foam. Basically flash and batt but with rigid instead of spray so I don’t go broke!
The next question is underlayment beneath the metal standing seam roof. I get the sense that as long as the conditioned space is air tight, keeping rain out is the main focus, not as much allowing the OSB to breath to the exterior. I interpret many building science articles to say that if you air seal the bottom face of the OSB, underlayment permeability doesn’t matter and in fact they recommend ice and water full adhering membrane.
Personally I don’t see how a roof deck could breath being under a sealed mechanical lock roof, but the thought of sandwiching osb between ice and water membrane and closed cell rigid foam is somewhat scary.
Any thoughts would help tremendously. I don’t want to get in a vented/unvented argument, I just want feedback from people that see the benefit of both methods. My worst case option would be to to a vented space in the rafters assuming I could fit enough insulation in the 11.25″.
Thanks in advance….
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