Non-Condensing Roof Insulation

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We need a total minimum R-value of R-49. Of those, at least R-15 must consist of air-impermeable rigid insulation.

That means 2 layers of 2" [1]

Technically, we're in zone 5A, but right on the border of zone 4A, so I'm using 4A values: https://codes.iccsafe.org/content/IRC2021P3/chapter-8-roof-ceiling-construction#IRC2021P3_Pt03_Ch08_SecR806.5