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Insulating a cathedral ceiling

user-7286635 | Posted in Energy Efficiency and Durability on

Question in regards to cut and cobble method for an invented cathedral ceiling. 

I’ve read and think I understand that you could cut the rigid foam to fit between the joists on the ceiling. It seems that getting that sealed is important and some people suggest spray foam to seal or caulk. I’m wondering why not just add an additional layer of rigid foam directly on the ceiling joists on the inside once the bay is filled. Couldn’t this seal the bay?



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  1. kbentley57 | | #1


    Maybe you've misunderstood the purpose of the method. The rigid foam is cut to fit between the rafters, so that it can go up near the sheathing, preventing condensation from forming. Below that, non rigid insulation, or open cell spray foam can be installed to provide more insulation. It's primary purpose to to block air, and condensation, and secondary purpose is to add r value to the assembly.

  2. Expert Member
    Akos | | #2

    Cut and cobble does not work for an unvented roof. The install can never be tight enough to prevent air and moisture from getting between the foam and roof sheathing, since there is no drying path for this moisture, it will cause sheathing damage.

    In colder climates, for unvented ceilings you have two options:

    -rigid insulation above the roof deck plus batts bellow.
    -closed cell SPF applied to the underside of the roof deck.

    In some climates you can get away with open cell foam applied to the underside of the roof deck plus a variable perm vapor retarder under the drywall.

    You can read more about these options and ratios of rigid/SPF to fluffy here:

  3. Expert Member
    BILL WICHERS | | #3

    You need fully adhered insulation if you are insulating only from the underside of the roof. That means spray foam. Cut and cobble is not reliable over time, since, as Akos mentioned, you always end up with little leaks somewhere and then moisture gets in. With spray foam, there are no gaps or voids, and nothing that can crack, since the spray foam is fully adhered -- literally glued to the sheathing EVERYWHERE so there is no place for moisture to get in, and not place for it to accumulate either.

    You could use rigid foam over the sheathing, but you need a lot of rigid foam to do that, and it means replacing your roof. If you only have access to the underside of the roof, and you can't build a vented assembly, then closed cell spray foam is really your only option.


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