Hey All, 
I read all the roof insulation articles but want to make sure i’m understanding correctly and that my plan works:
I have an old cabin that I moved that does not meet code in zone 6b. The current roof deck is – t&g, roofing felt, then a weird cross cross of 1x with 4.5″ rigid foam stacked inbetween (with some sections having fiberglass insulation inbetween). There is no rot so this roof assembly worked for 30+ years (I assume because there was no insulation on the interior?). I am thinking of ripping the current roof off down to the T&G, applying a peel and stick membrane, and then stacking the entire r-value of the roof with rigid foam, 2×4 on flat (1.5″), then metal roofing. The reason i’m thinking of this path is the second floor ceiling height is already fairly low and I don’t want so sacrifice headroom + I like the t&g look. Problems: It looks like I need 12″ of rigid foam to meet the r-49 requirement which seems crazy thick. It would mean new fascia boards (or I can potentially span the gap with really long L flashing to cover the foam?). I’d like to re-use the fascia as its in nice shape and matches the aged look of the cabin. I am also a bit confused on the alternative u-factor calculation that could potentially reduce my insulation thickness. I’m on a tight budget as the roof is just the first of an entire restoration. Since I won’t have any framing, and the rigid foam will extend to the end of the eves past the walls, can I get away with going thinner on the insulation? the only connection I will have is the screws going all the way through so the total assembly insulation value should be better than with other methods right? Thanks for your help.