I was planning to use foil faced 2" polyiso plus an air gap, plus 5 1/2 rock wool, to get 13+3+23, or r39.
But this is all between the rafters below the roof deck, and the polyiso is impermeable. If any water gets into the roof assembly it will have no way to dry out. I was willing to say there won't be any water, until a severe thunderstorm last night. The house is 100 years old, and the wall above this roof is cedar lap siding, and when I checked after the storm there was dampness at the ledger in one spot.
So I'm reluctant to stick the impermeable foam under the roof deck. But with permeable foam, 2" foam + 5 1/2 rock wool is 10+23=33.
How should I get code required r38?
(There is some kind of insulation board on top of the roof, which is a new modified bitumen roof. But those boards were only 3/4" thick and I don't know what they were. I hired some guys to do the roof, and there were done communication problems.)