I am building a PGH and I have a 32′ run of unvented, low-slope roof that we will start framing out shortly. The interior stack-up is gypsum, 2×3 service cavity, Siga Majrex air and vapor control layer (let’s vapor out of the cavity, but not in), 2×12 joists, dense pack cellulose between joists, 3/4″ Advantech seams taped with Wigluv. I was planning on using a unvented nailbase product that has 4.5″ of polyiso with a layer 1/2″ osb attached to the top. I am in Zone 4 and this will provide more than the required .31% of exterior to total r-value even if I de-rate the polyiso r-value to 5. I am trying to decide where to create the pitch, above the nailbase or below it. If I put it below, would I have to add a layer of foam that is tapered to match the pitch of my ripped 2×10’s used to create the pitch or can I leave the unvented air space? I have found a local EPS manufacturer who can provide me with the tapered EPS. Alternatively I could place the pitch above the nail base and then the issue becomes trying to secure my 2×10’s to the structure below, although this is somewhat mitigated by the fact that the ceiling joists below are doubled-up every other bay so I have a wider area to try and hit with a screw.
Btw, the purpose of this low-sloped roof is to provide a place for solar panels that cannot be seen from the street, which is why it was engineered with the extra joists.
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