CZ4 foamed attic, is R-20 enough?
I’m on the southern edge of CZ4. My spray foam contractor recommended R20 of open cell. That’s what is currently sprayed on my roof deck. The code inspector came by today and is insisting on R38, as if it was a conventional vented attic with loose blown fibers.
My current ACH50 score is 1.7.
1) Is R20 enough? All four foam contractors I received bids from were quoting R20 for my house. My walls are R25 (Zip-R6). It’s a bit strange to have walls that are better insulated than roof. Where is the realistic point of diminishing returns? I know the encapsulation itself has benefits that go beyond just raw R value, even though the inspector doesn’t quite get this, but perhaps I do need more foam thickness before the diminishing returns kick in.
2) I’m worried about thermal bridging through roof rafters. The 5.5” of ocspf isn’t enough to fully cover them. Is this reason enough to increase thickness?