Parnell, IA, US
I believe, if you used foil faced foam, it would act like a vapor barrier where you want exterior drying. That is why I chose to use 7/16 OSB vs foil faced foam. I used a similar detail as this article for my home, and if I had to do it again, I would sheath the entire roof (minus the overhangs with taped zip sheathing as an air barrier use 2x material on top of the roof to create an air channel and re-roof with the actual roof sheathing. You are using the same amount of sheathing and you don't have to cut it to fit between the rafters.
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