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

Hi, thank you for any advice.

Roof assembly: Bottom to top

-1/2″ Sheetrock (Low Sag)

– 2″ air gap

– 2×10 purlins @ 24″ OC running perpendicular to roof slope  w/ 7.25″ rockwool insulation R30 pushed up tight to underside of roof deck

-1/2″ zip sheathing, tape all seams over to walls w/ Zip R6 wall sheathing

-Nailbase  with 19/32″ CDX laminated to what type and depth of foam?

-2-4″ GPS (Graphite Polystyrene)

-2-4″ Polyiso

-High heat underlayment (Ex. Sharkskin Ultra SA)

-Prefer no air gap due to fire ember risk (already lost a home to forest fire)

-Matt Black clip lock standing seam metal roof (striated panel)

Question:

What exterior foam thickness and type should we use for our nailbase panels? Any other roof assembly modifications you would recommend?

This is to be our forever home, two story barndominium type build, (metal frames with wood purlins and girts) 32′ x 48′ footprint. All owner build with his son on a budget.

Any help is greatly appreciated!