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Roofing – rigid insuation above, plus foam in roof framing bays

Micah Smith | Posted in Green Building Techniques on

Hello,
I am refering to and creating details based on this on your site https://www.greenbuildingadvisor.com/sites/default/files/Insulated%20cathedral%20ceiling.jpg
My question relates to the note that calls for breathable insulation on the interior. I want to use non-breathable closed cell foam in the bays so we do not have to vent the roof, which does not have an attic. Project is located in Pismo Beach, CA. Any way we can accomplish this?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Micah,
    I'm not sure why you want to use closed-cell spray foam between the rafters. The reason you give -- "I want to use non-breathable closed cell foam in the bays so we do not have to vent the roof" -- doesn't make sense, since you don't have to vent the roof if you build it the way it is shown in the drawing. (The drawing shows an unvented insulated roof assembly.)

    For an article that explains all of the different ways you can build an insulated sloped roof assembly, see How to Build an Insulated Cathedral Ceiling.

    You don't have to install rigid foam above the roof sheathing if you don't want to. But if you do, it's best to make sure that the roof sheathing can dry to the interior if it ever gets wet. A wide range of insulation materials are vapor-permeable enough to install between the rafters in this type of roof assembly; these include fiberglass batts, mineral wool batts, dense-packed cellulose, blown-in fiberglass, or open-cell spray polyurethane foam.

    If you have your heart set on installing closed-cell spray foam between the rafters -- and I don't really recommend this approach -- the best way to proceed is to omit the rigid foam above the roof sheathing.

  2. Micah Smith | | #2

    Hi Martin,
    Ok, I also just spoke with my Title 24 person who clarified that open-cell in the bays is the way to go, so disregard my question. You are right, the detail is right, I'm all set!
    Thanks!

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