In a flash & batt roof, should the foam encapsulate the rafters? – GreenBuildingAdvisor

In a flash & batt approach to a flat roof deck or unventilated attic, should the closed cell foam encapsulate the rafters?

This is for an inhabited attic, with a sloped roof, in new construction but in working on the problem I realized it would also apply to a low-slope roof over living space. So I’m asking it for both.

My first thought is that I could simply detail the CCSPF (closed cell polyurethane foam) to be of the required minimum depth, between the rafters, and then make up the rest of the insulation with batts, also between the rafters. However, I’m questioning whether the rafters themselves, only providing a much reduced R value, will be inclined to act as condensing surfaces.

I could have the foam coat the sides of the rafters to their full depth, an inch or so thick. This would act as a vapor barrier, and then I could still use batts in between. The downside is that it will be much harder to install the batts because they will be too wide for the left-over space. And of course it would use more foam.

I work in several different climate zones so for purposes of discussion lets assume I’m in Zone 5 and I can work it out from there.

And also, yes, I know, there are other solutions that don’t involve spray foams. No need to bring that up. It won’t help me get an answer to my specific question. Thanks.

I’ve attached a photo that shows a flashed attic before the batt goes in.