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Community and Q&A

Insulating a Cathedral Ceiling

mykey | Posted in General Questions on

Trying to figure out the best way to insulate a cathedral ceiling:
climate zone 6 – northern NY
Roof is constructed as follows-
2×12 rafters>1/2” plywood sheathing>3” polyiso (2 layers)> grace peel & stick membrane (lapped over ridge and onto zip Rsheathing walls)> 2×4 on flat screwed through to rafters> 5/8” plywood sheathing> 15# felt> standing seam metal roof.

The question is how to insulate inside the rafter bays? I know the polyiso is on the thin side for thus climate. This is a studio/shop building and not a full time living space. It will be heated though.

I believe filling the bays with fluffy insul can get me to minimum requirements, but it will throw the balance of interior/exterior r value off and there is risk of the sheathing condensing.

Would an interior membrane solve my issues? Something like Intello? Or Membrain?



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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Michael,
    I am going to give your question a bump. In the meantime, if you haven’t already, you might want to read these two articles by Martin Holladay: How to Build an Insulated Cathedral Ceiling and Five Cathedral Ceilings That Work.

    1. mykey | | #2

      Thanks. Yeah I have read those articles.
      They don’t specifically deal with my situation since my above sheathing foam is not of adequate thickness. Hoping someone can shed some light on some other options.


  2. Jon_R | | #3

    If your foam is too thin, you can a) add spray foam underneath or b) reduce the amount of air permeable insulation added to keep the R ratio implied by code.

  3. Expert Member
    ARMANDO COBO | | #4

    I’ve never worked on CZ6, and there, you need 4" R25 min. polyiso above the roof decking. You have R18-20 depending on the brand of polyiso, so if the option of adding another 1”-1.5” on top of what you have is a no-go, having 11.25" available to install 9” R32 min. with different impermeable insulations, and Intello Plus, it’s edgy, but it should work, as long as you maintain the RH40%, or less, in the winter, and RH50%, or less, in the summer.

    1. mykey | | #5

      Thanks. Yeah adding more polyiso above the roof deck is a no go. The building is already roofed. Yes, 11.25” to fill with insulation. Is it too risky to use roxul in the cavities with intello plus? Is my only option filling the bays with impermeable insulation?
      I guess as Jon R stated above I could add a layer of closed cell spray foam and then fill the remainder of the bays with fluffy insul.
      Would I still need the Intello plus if I decided to use the spray foam option? And what would be the required thickness of the spray foam layer?

  4. Expert Member
    ARMANDO COBO | | #6

    I don't believe in installing ccSPF or rigid foam on the interior side of the roof sheathing. If any water gets past the WRB, it won't have a way to dry out. Spend your efforts in controlling low humidity.

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