Hello, I’m building an ICF home with a large flat roof in Minnesota zone 5, ICF foundations to roof. The roof trusses sit on the ICF wall.

Im insulating the roof above deck with polyiso 8” with vapor baried under the foam and TPO above the foam.

I have couple question I still need to sort out.

My trusses are 16OC so only have 12.5” between them.

how do I insulate the rims and how do I provide a vapor barrier for the Rims. I was thinking Rockwool Or fiberglass. I would want to avoid spray foam, the quotes are outrageously expensive these days.

Since there is no combustible insulation, and vapor barrier is above deck, do I need a firewall between the living space  and the roof deck, I was going to use WPC panels for the ceiling in the living room and kitchen and would want to skip drywall the ceilings there.

if k do spray foam the rims, how do I provide a firewall between the foam and living space provided that there won’t be a drywall and the WPC material is class 3 only.

Lorant