Can I use 2" rigid foam between rafters (with an air gap above) and 1/2" rigid applied to the underside of the rafters?

Question

I'm insulating 2x4 rafters in an old house above what will be finished and conditioned space. My roofer is retrofitting in a continuous ridge vent as well as continuous intake vents across the bottom of each side of the roof. Is it ok to use 2" polyiso rigid foam insulation between my 2x4 rafters, flush with the lower edge of the rafters (which would leave a 1.5" air gap between it and the roof sheathing) then also install 1/2" insulation across the underside of the rafters? There is not much head room in the space and this is the best R value (R16) I can get without furring the rafters which would eat up valuable headspace. This house is in the pacific northwest. It's not possible to install rigid on the exterior of the roof.

Answer 1

Check with your local inspectors office or other authority heading jurisdiction. You may not be able to use foam insulation indoors without a fire blocking layer, typically plasterboard but in my case I was told that half inch plywood would also have been acceptable.