I am in Upstate NY -- primarily heating climate, we don't intend to install AC at all at first and hope not to need it at all. We are building a home with a vaulted/cathedral ceiling. I am sourcing reclaimed foam board insulation for the roof, which will be a built up roof. I already have 5" thick sheets of EPS foam, which will be doubled up to make 10". I am looking at either 1.5" of polyiso, black paper faced, or 2" of foil faced polyiso, for the remaining layer. I will be using the polyiso on the warm, interior, side of the house to maximize its R value. My question is - do I need the foil faced insulation so that I get a vapor barrier? Or is 1.5" of paper-faced polyiso considered a vapor barrier? I am not sure how the 10" of EPS foam factors in but, with SIPs, there is no vapor barrier used, so maybe I'm good either way?? I will mention that I plan to leave an air space between the foam and the roof sheathing. I am worried that the metal roof may trap moisture and if that happens, I would like an air space to help dry out the roof sheathing. I'm not sure if this would factor in at all.

First, code in NY does not require or need a true vapor barrier (under 0.1 US perms) anywhere in the assembly, but for an unvented roof it depends...

The Canadian code definition for vapour barrier is very close to one US perm.

At 5" any standard density of roofing EPS is well under 1 perm. Most of it is Type-II (1.5lbs per cubic foot nominal density), which is under 3 perms @ 1", and would then be under 0.6 perms @ 5".

Most asphalted paper facers on 2lb density roofing polyiso brings it under 1 perm at any foam thickness.

Dry half inch OSB is also less than 1 perm, but is above 1-perm when it's moisture content is in the mold-susceptible range. It's a "smart" vapor retarder.

If all of the foam is on the exterior of the roof, the vapor retardency of the foam doesn't matter. The moisture susceptible roof deck will be at warmer temperature, and thus drier. But putting foam on the underside of the roof deck complicates it by impeding drying toward the interior. In US climate zone 6, long as the above-deck foam is at least 50% of the total R, it's fine to use fiber insulation between the rafters. You could use 2x10 rafters with R38HD batts, and 9" of EPS above the roof deck and call it a day.

The structural roof deck should be under the foam, not between the foam and roof deck. You can use furring/purlins thorugh screwed to the structural roof deck to mount the metal roofing. Using a self-healing peel'n'stick membrane (eg Grace Ice & Water Shield) between the foam and roof deck ensures that any seepage stays out of the roof deck. If your metal roofing needs to be mounted to flat sheathing, mount a OSB deck on top of the furring. Half-inch OSB is usually good enough for the outer nailer deck, along with whatever the manufacturer's slip-surface underlayment stackup is, but verify the minimum fastener penetration requirements for the metal roofing system.

OK, thanks for your help, you are always answering my questions Dana!

So in short - my stack up is perfectly fine. Thanks so much!