How bad is it to use polyiso on the deck and spray foam on the bottom?
I am having my “flat” roof EPDM rubber roof replaced, which right now is only insulated by whatever amount of polyiso foam is on top of the deck. The rafter cavities are empty and the ends, only, of those cavities is currently stuffed with fiberglass batts.
I live in St. Paul, MN, which is climate zone 6 and everything I read, indicates that I must have an R of 25 in order to keep my deck sufficiently warm to avoid moisture problems with, what will become a “hot roof” (polyiso on top of the sheathing and blown cellulose underneath with no internal vapor retarder and the rafter ends of the rafter cavities sealed with either sealed in rigid foam or with sprayed in polyurethane foam).
My dilemma is that in order to achieve an R-25 at the thinnest part of the polyiso insulation of my tapered roof (required for drainage), then I would have to have a parapet wall built on the roof in order to accommodate the requied thickness of the polyiso. Such a wall would severely degrade the appearance of my modern design house. Everything I read from Martin, suggests never to have BOTH polyiso on top of the sheathing and sprayed-in foam underneath it because, if it were ever to get wet, it would not be able to dry in any direction.
So ultimately, my question is this: Am I insane to take a calculated risk and have sprayed-in foam applied to my roof deck only where the polyiso is too thin to acheive the R-25? For the rest of the roof, I would follow the good advice of Martin such that I would not use the polyurethane under the sheathing.
Thank you in advance for your reply and thank you for your good work in providing the wonderful resouce that is GreenBuildingAdvisor.com
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