Unvented Roof Insulation Problem
I have recently settled on an unvented roof for my 1000 Square foot, 2/12 shed roof, cathedral ceiling, zone 6, coastal Maine.
The roof will have taped zip sheathing, EPS outsulation, ice and water shield (or equivalent) and a standing seam metal roof on strapping. The rafter span is 28ft with a mid span beam dividing the span in two..
I am hoping that the structural engineer specs 2×10’s for this span but with the snow loads, it could be 2×12’s. Either way, my problem is the same. I am using dense pack cellulose everywhere else in the building (suspended floor -along with a thin layer of rigid for air sealing- and double walls) and I want to fill my rafter bays with it as well. According to the laws of this unvented roof assembly, however, I need to maintain at least 51% of my R value on the outside of my sheathing. So, if I used 9 inches of dense pack to fill the rafter bays, I would have an R of 34. I would then have to create an R of 36 or more with my EPS. This means 10 inches of EPS for a total R value of 70.
My goal is closer to R-60 and I am not sure that I can afford that much of an upgrade. Is anyone aware of how I can dense pack my roof without filling the whole cavity? Its looking like I might have to go with batts, and I am leaning towards fiberglass (although I dislike the stuff) since its cheaper than rockwool and the embodied carbon is about the same.
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