Cathedral Ceiling with metal roof insulation and underlayment

This is a homeowner's question posted to an online building forum, not a news story, so there's no industry development to report. The person is building a new house in Culpeper County, Virginia, and is working out how to insulate a cathedral ceiling (a sloped ceiling that follows the roofline, with no flat attic above it) under a metal roof.

Their local building code requires an insulation value of R-60 for the ceiling. To hit that number using 2x12 rafters, they'd need about 10 inches of closed-cell spray foam, since a lighter approach that combines a thinner foam layer with fiberglass batts (sometimes called "flash and batt") can't reach R-60 in that rafter depth. They're also planning a white standing-seam metal roof and asking whether the underlayment (the protective layer installed under the metal panels) matters much, since the assembly won't rely on drying to either the inside or outside. They're considering a full peel-and-stick waterproofing membrane for extra protection and note that because the white roof reflects more sunlight, they likely won't need a high-temperature-rated membrane.

No responses had been posted yet, so there's no expert guidance to pass along. For a homeowner following similar plans, the takeaways worth noting are the specific numbers involved: R-60 ceiling insulation, 2x12 rafters, roughly 10 inches of closed-cell foam, and the choice of underlayment for a reflective metal roof.