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Unvented Cathedral Ceiling with Spray Foam – Roof Deck Gonna Rot?

okayrene | Posted in General Questions on

Hey folks,

I’ve been renovating a rural house in Vermont (zone 5b) that was once teeming with squirrels — yuck.  Had to rip lots of drywall and insulation, then decide how to re-insulate my cathedral ceiling in the living room.  The roof in question sits on 4×11 rafters, and I started with closed-cell spray foam, then used mineral wool, then foil-backed mineral wool, which I taped pretty nicely (but not perfectly around the lights), then finished with poly-coated t&g pine.

So 1.5″ R-6 Foil-backed Roxul, then 3.5″ R-15 Roxul, then ~3″ handifoam spray foam 3″ R-19.5.  In total, 8″ of insulation for about R-40 between the rafters.  I have grace ice + water 2/3 of the way up the roof and finished with Tiger Paw.  

Now I’m thinking I should put 2 layers of rigid foam (probably xps) for another R-20 above the roof deck.   After that I imagine putting down 2x4s and another layer of roof deck, (more tiger paw?) then shingling. 

I’m worried about the foam-sandwich or permeability issues, I’ve been reading all the articles I can find, but there seems to be some exceptions to the old it’s gotta be able to dry out in at least one direction advice and if I don’t add the foam on top, then my total R-value is pretty low. 

Is that assembly gonna rot my roof deck? 


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