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.)
Best Answer
For Future Viewers:
No-one's forcing you get to R-anything, yet & it's coming people, but it's great that you want to. I'd call the Roofer back & get the leak fixed first. Whether you can get to the leak & don't fix it or cover it & never see the leak doesn't stop the leak or building rot.
However, you must (& I mean must) have joists below those rafters & they can even be built-up. Your idea is right, it's just not in the right location. Unless, your overall plans are to finish the attic.
If that's the case, then you don't need anything more than R-19 for that new segregated room's roof if you already have an R-38 done on the floor for the rest of the home's cap. Sure, more is better but not at the cost of Roof, Gable &/or Ridge Vents.
The roof's R-19 for a finished attic there would be best as just Polyiso boards with as much air space as you can give the roof to breathe. Literally board from the eave's venting all the way up to the Gables or Ridge to actually make a dedicated & drastically reduced venting highway for improved everything.