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Rigid foam + spray foam in unvented roof

quantumgirl | Posted in General Questions on

I have an unvented cathedral ceiling (2×6 rafters) with open cell spray foam as cavity insulation. The 5.5” of foam only give me an R value of about 20. We are about to reroof the house and I would like to add rigid foam on the exterior side of the roof sheathing to increase the R value and also act as a thermal break. I’m in climate zone 2. R38 is required for ceilings.
My question is is this setup gonna work? I know you should not put rigid foam on the exterior when closed cell foam is used on the inside. I can’t find any info regarding open cell foam though. Am I correct in assuming that open cell foam is vapor permeable and I am therefore NOT creating a moisture trap?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Quantum Girl,

    You can add rigid foam insulation above your roof deck. The R-value you need to hit with the exterior insulation is not about reaching the code minimum R-value for a roof in your area, though. It's about having enough R-value on top of the deck to keep the sheathing warm and prevent condensation from occurring inside the assembly. I recommend that you read this article. It should answer your questions: How to Install Rigid Foam on Top of Roof Sheathing

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