Roof assembly with rockwool: need advice
Remodel underway in CZ03C, zip 95060. My design roof assembly is shown in the attachment (unvented cathedral ceiling, inside out: 1/2″ sheetrock, 2×6 rafter cavity w blown-in fiberglass or open cell spray foam, 1/2″ plywood roof sheeting, vapor permeable peel and stick air/water barrier [or maybe just taped plywood seams], 3″ Rockwool Toprock DD, 1/2″ plywood roof deck, Delta Trella vapor permeable metal roof underlay, standing seam metal roof).
I’m having a hard time sourcing the 3″ Rockwool Toprock DD. I was told today that they could get 2″ Toprock DD. If I reduce to 2″ on hte south roof, it makes more space for my south facing clearstory windows (see detail, cross section looking from the east). On the north roof, I could use 2″ or double stack it for 4″ total.
Based on the minimum thickness required for my mild climate zone, I think I would be fine with 2″ of above deck insulation. Toprock is rated at R-4.3/in, so I’d get R8.6 above the deck.
– If I reduce the south roof to 2″,does it make any sense to increase the north roof to 4″? My minimum nominal R- per code is R-30, but I’d love to get higher (hence starting with 3″, which was the most I could get and keep the existing clerestory windows).
– What if I flash 2″ of closed cell spray foam under the deck, and put a mineral wool batt in the remaining cavity? I’m trying to avoid foam as much as I can, but recognize that this new layer would add significant R-value to the assembly. But it would make it so the roof has to dry away from that foam, right? I also like the mineral wool for the fire resistance, but if there is foam in between, does it even help? How else would this affect my assembly?
-Do I skip the whole roof Rockwool idea and go for Polyiso? I’m turned off by the foam aspect, by the insect aspect (termites and ants here), but I bet it would be a bit cheaper (fwiw, I was quoted about $1,200 for the Toprock at 3″, but they added $2,000 in freight).
Any other comments on this assembly are welcome, thank you
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