Nailbase Insulation advice (unvented hot roof assembly in IECC zone 4b or CA zone 2)

This is a homeowner forum question, not news, so there's no policy or program update to report. A person building their own home — a two-story barn-style house in a wildfire-prone area of California — is asking for advice on their roof design before construction.

The builder plans a roof with no vented air gap under the metal roofing, because a gap could let embers in during a wildfire; they already lost a home to one. Instead, they want to use "nailbase" panels — rigid foam board bonded to wood sheathing — installed under a black standing-seam metal roof. Their question is what type and thickness of foam to use: rigid foam board in either graphite polystyrene (a foam insulation type) or polyiso, in a 2- to 4-inch range, laid beneath plywood sheathing. Below that, the plan already includes rockwool insulation between roof framing members and taped seams on the wall and roof sheathing for airtightness.

Since this is one homeowner's open question to a building-science community, there's no verdict yet on the right foam choice or thickness, and no broader news to report. Homeowners planning similar wildfire-resistant, unvented roof assemblies in hot, dry climates may find the discussion useful as a real-world example of how these tradeoffs — fire safety, insulation performance, and moisture control — get weighed during custom construction.