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Insulation for Cathedral Ceiling

rubee1205 | Posted in General Questions on

I want to raise the ceiling to cathedral and is thinking of using rigid EPS board as insulation. But what is the actual R value for such application in zone 4 (northern NJ), since the chart gives different numbers for the wall, attic, etc? The rafters are 2×6. The 2-inch board gives R-10, and perhaps I can use two pieces of them?  By the way, when using the EPS board, is ventilation space no longer an issue compared to the traditional insulation? 
Thank you

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Rubee1205,

    Read this article: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling.

    You also may want to confirm your climate zone by going here: https://www.energy.gov/sites/prod/files/2015/10/f27/ba_climate_region_guide_7.3.pdf.

    The target for a roof in Zone 4 is typically R-49 (but local code requirements vary and it's possible to meet code without R-49). See here for details: https://www.greenbuildingadvisor.com/article/three-code-approved-tricks-for-reducing-insulation-thickness

    Are you considering a new roof (shingles)? If so and you have simple roof, you might consider combining inside and outside insulation. See https://www.greenbuildingadvisor.com/article/three-code-approved-tricks-for-reducing-insulation-thickness for details.

    If you are planning recessed lights for that new cathedral ceiling, your assembly will be riskier. Low-profile LEDs would be much safer.

    1. rubee1205 | | #4

      Thank you for the reading references.
      By the way, I saw the insulation at home depot for R-38 is something like 14 inches thick! I doubt this would be working for a cathedral ceiling unless the rafter will be modified. A guy at home depot told me R-13 would be enough

  2. Expert Member
    Zephyr7 | | #2

    If this is going to be an UNvented cathedral ceiling, and you're planning on putting the rigid foam INSIDE, under the sheathing and between the rafters, then that is what we call a "cut and cobble" installation. This is NOT recommended in unvented roof assembles due to moisture risk. Basically the seal between the rigid foam panels and the rafters can't be trusted to last in the long term, so you're likely to get leaks that will let mositure get to the underside of the sheathing where it will condesne and cause problems.

    Bill

    1. rubee1205 | | #3

      Bill, would it work if I leave a space between the panel and the sheathing?

      1. Expert Member
        Zephyr7 | | #5

        You’d have to build the entire thing as a vented roof assembly, you can’t just leave a gap.

        Filling the cavities between rafters with spray foam is about the only way to hit code minimum R value without furring out on the interior to get extra depth for more insulation, or putting rigid foam on top of the sheathing. I would probably go the spray foam route myself (and did in my own house in a similar situation).

        R13 is not enough in a roof.

        Bill

        1. rubee1205 | | #6

          Bill, I always wonder if the traditional insulations can be really airtight because they are porous.
          I checked the ridge board today. I did not see space for ventilation to outside.

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