Can roof sheathing dry with flash-and-batt?
I have read a lot on this site about flash-and-batt, but I can’t seem to find anything definitive about the potential danger of sandwiching the roof sheathing between impermeable spray foam below and roofing underlayment above (especially peel and stick). Isn’t it safe to assume that inevitably the underlayment may leak and some point and let moisture into the sheathing? If/when this happens, does the sheathing have any chance of drying?
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Eventually, all roofing fails. When that happens, you end up with a roof leak.
The solution, of course, is to replace the roofing. No one should expect inward drying to be a mechanism to handle roof leaks.
Some homeowners decide to replace perfectly good 15-year-old roofing just because it is getting old. That's one way to address the fact that all roofing ages. In most cases, though, homeowners wait for evidence of their first roof leak, and then they replace the roofing.
Of course all roofing leaks eventually, and as you say inward drying can't be expected to deal with roof leaks, but it seems like the interior spray foam seal could mask a leak by trapping it in the sheathing and rotting it out without any indication of a problem. So I suppose it's more of an issue of not knowing a roof is leaking or may need replacement when the underside is flashed. I am installing 75-year shingles which will far out live the underlayment, so it won't be obvious when the roof needs replacement by visual inspection.
I don't think your underlayment will fail before the roofing (since it is protected from UV exposure). Asphalt felt lasts an amazingly long time if it doesn't see sunlight, and I doubt if the new synthetic underlayments will crumble.
That said, underlayment doesn't stop leaks.
There is much speculation that a closed-cell spray foam is worse than open-cell spray foam when it comes to detecting roof leaks, but precious little evidence to back up the speculation. Joe Lstiburek discounts the speculation, and so do I. If you have a roof leak, you have a roof leak, and the roofing needs to be repaired. A large number of factors affect how quickly roof leaks become apparent, but 100% of roof leaks eventually become apparent. Roofers can fix leaks.
The best type of roof for detecting and pin-pointing roof leaks is a roof over a vented unconditioned attic, and that type of roof is my favorite, in part because it's easy to inspect the underside of the roof sheathing. Considering your concerns, that's the best type of roof for you, too.
If you choose to create an unvented roof assembly using spray foam, you lose the opportunity to easily inspect the underside of the roof sheathing. So if that bothers you, don't build that type of roof.
Also think about repair-ability - mostly getting it back together with equivalent air tightness. And small (hard to detect) leaks caused by rare weather events (say very high wind and rain or snow).
> Of course all roofing leaks eventually
Or maybe "all roofing leaks to some extent under some conditions"?
> does the sheathing have any chance of drying?
Low perms aren't zero perms and some drying can occur in both directions. For example, see below, where permeable underlayment outperformed impermeable - even with low perm asphalt shingles. The % difference would have been far higher with foam underneath.