Attic insulation upgrade — 4/12 hip roof
I am upgrading the attic insulation in my house and have two questions for you all. First let me give some background.
The Roof is a hip roof built with 2×6 24″ OC and the pitch of the roof is roughly 4/12. The Roof Joists do not line up withe ceiling joists. I am considering converting the roof assembly from a vented to an unvented attic using spayfoam directly adhered to the underneath of the roof deck. I live in Zone 4 roughly 20 south miles from the Zone 4/5 border. Specifically I live in the south east corner of Colorado.
I have recently obtained quotes and in the process of deciding whether to go with closed cell or open cell spray foam. The contractor that I am considering noted within his estimate that he can spray 8 inches of open cell for the same quoted price as 2 inches of closed cell.
Initially I was going to spray 2″ of closed cell foam and then fill the remaining 3.5″ with rockwool insulation. Then at a later date apply 3.5″ reused polyiso over the top of the roof deck apply 1/2 OSB then metal roofing. An online whole wall calculator I am using rates this assembly at R-49 not considering the metal roofing. This assembly will need to be completed in steps and make take years to come to fruition.
Therefore I am now considering filling the roof joist bay with open cell roughly 5.5 inches then spaying the remaining 2.5 inches over the roof joists in order to compensate for thermal bridging. The same online whole wall calculator I am using rates this assembly at R-45, which is close than the initial plan and quite a bit cheaper both labor wise and time wise. Given I have been obsessing about this project for quite some time I wish to be done with it as soon as possible so that I can get back to my true hobby of woodworking.
So my first question is would I be setting myself up for a failed roof deck by pursuing the cheaper option of open cell foam over the closed cell foam + Rockwool + Polyiso + Reroofing?
My second question what would be the easiest way to block between the roof joists so that I can ensure that the unvented attic seals at the top plate, given that the roof joists don’t line up with the ceiling joists, and the low pitch of my roof? Would it make sense to remove the soffit and block from the exterior in this situation?
Thank you for your help both in this specific question and many other problems that I have used your site in order to address.
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