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Install rigid foam on bottom of rafters in unvented cathedral ceiling filled with OC foam?

Stephen McMurray | Posted in Green Building Techniques on


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, 2×10 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?



  1. Aj Builder, Upstate NY Zone 6a | | #1

    The drywall layer with paint on it is enough of an air barrier. But adding a rigid layer would add insulation that is continuous which is nice too.

    Why is expensive insulation spec'ed and then low cost OSB sheathing?

  2. Expert Member
    Dana Dorsett | | #2

    Do NOT add foil faced polyiso- the facers would make the #30 felt & foil a moisture trap- any moisture that got in (even wicking in on nails) would never leave!

    At 1.5" XPS would have a vapor retardency in the class-II range, and might work, but it would also slow the drying rates. A better bet would be to use 1-2" of unfaced Type-II (1.5lbs per cubic foot nominal density) EPS for the thermal bridging issues, with a "smart" vapor retarder like Intello Plus or Certainteed MemBrain for moisture control. At 1.5" Type-II EPS has a vapor retardency of about 2 perms, which is plenty of drying rate, but still a bit too vapor open for a zone 5A climate with an OSB roof deck, probably OK for a 4A climate, but for insurance a smart vapor retarder works. Smart vapor retarders are sub 1-perm in winter, limiting the rate of moisture accumulation, but become vapor open when the moisture levels in the foam or proximate air rise. With 1-1.5" of EPS and a smart vapor retarder you'd get much faster drying rates than accumulation rates, which is what you really want.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Dana, if an unvented roof leaks it will damage the OSB. Your roof assembly is good but the whole plan is basically expensive. High R roofs should be scissors truss with vent space.

  4. Expert Member
    Dana Dorsett | | #4

    AJ- he's between a rock and a hard place, looking for the best way to mitigate the situation:

    "The house is new construction, but OC spray foam has already been installed..."

    I'd hazard that replacing the whole roof & rafters with a scissors truss system is already off the table as an affordable solution (nicht wahr?).

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