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In zone 4A, would I be OK to put 1.5″ of closed SPF under my roof deck with unfaced R-38 below that?

EHt2PWoEKw | Posted in Energy Efficiency and Durability on

I have 2×8 rafters, and they are furred down with 2×4’s and there is a 1″ gap as a thermal break between them. I want to foam the rafters entirely as well as the underside of the roof deck. I would use .5″ sheetrock as an air and thermal barrier below the furring and batt insulation. I know code calls for R-15 air-impermeable foam, but with this set-up in southern, coastal Virginia would this be OK?

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  1. GBA Editor
    Martin Holladay | | #1

    There are two ways to build a cathedral ceiling (an insulated sloped roof assembly): either as a vented or an unvented assembly.

    It sounds like you are talking about an unvented assembly, but I'm not completely sure, because of one confusing detail: you mention that "there is a 1-inch gap as a thermal break between them." I'm not sure what "them" means, so I'm not sure where this 1-inch gap is, so I'm not sure whether you intend this 1-inch gap to be a ventilation channel, or something else entirely.

    Assuming you are building an unvented assembly, and assuming that you are following the flash-and-batt method of insulation, the code is clear: the spray foam must have a minimum R-value of R-15 in climate zone 4A. (For more information on this issue, see How to Build an Insulated Cathedral Ceiling.)

    Your proposed spray foam depth (1.5 inch of closed-cell spray foam) only gives you R-10. That's not enough. So your proposal gets a "thumbs down." You need to install at least 2.5 inches of spray foam to make this assembly work.

  2. EHt2PWoEKw | | #2

    Thanks for your response! The 2x4's are held off of the bottom of the 2x8 rafters by plywood strips nailed into the sides of both. The 1" gap is between the rafters and 2x4's. And yes, it was planned to be left unvented. My only thought that was with the unfaced R-38 batts under the foam, and the drywall beneath that it would be enough that the bottom side of the foam would not condensate. Do you think it would be better to vent it and use paper faced R-38 batts? This is not a house I am building, but rather one I am inspecting. The homeowner is trying to avoid paying for 2.5" of ccSPF. Again, thank you for your response!!

  3. EHt2PWoEKw | | #3

    And this is all conditioned attic, no habitable living spaces.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Do you think it would be better to vent it and use paper faced R-38 batts?"

    A. No. In terms of thermal performance, that choice would be worse, not better.

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