Install rigid foam on bottom of rafters in unvented cathedral ceiling filled with OC foam?
The house is new construction, but OC spray foam has already been installed before I was aware of the potential moisture concerns cited on GBA (otherwise I would have gone with closed cell). So I'm wondering, to address thermal bridging from the rafters, as well as prevent interior moisture from migrating through the OC foam and rotting the roof sheathing, is it a good idea to install an inch or two of rigid foam on the bottom of the rafters directly underneath the sheetrock?
Ceiling type: Cathedral ceiling, 2x10 rafters, completely filled with open cell spray foam. Home is located in the northern part of climate zone 4A (but not too far from zone 5).
Proposed finished assembly, from outside to inside:
1) Metal standing seam roof
2) 30lb tar paper
3) 1/2" OSB
4) OC spray foam (9")
5) 1" foil faced polyiso
6) 1/2" gypsum wallboard (screwed through polyiso into rafters with long screws)
Will this help block movement of interior moisture, or am I effectively creating a sandwich where the roof will be unable to dry to the exterior due to the metal and the interior due to the foil-faced polyiso? Is there a better way to keep moisture out of the cathedral ceiling and reduce thermal bridging of the rafters?
Posted May 5, 2014 2:16 PM ET
Edited May 5, 2014 2:19 PM ET
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