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Where is the building envelope?

Bbrown3381 | Posted in GBA Pro Help on

Doing several additions with new gable roofs overlaying the existing roof. New and existing roofs will have foam the underside of the roof sheathing. The question centers on where the envelope should be on the triangular overlay portion of the new/old roofs and should that triangle be vented in anyway.

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  1. GBA Editor
    Martin Holladay | | #1

    I think you should probably post a sketch so we understand what you're building.

    It sounds as if you are building a new gable roof over the existing flat (or low-slope) roof. Is that correct?

    You also wrote, "New and existing roofs will have foam the underside of the roof sheathing." It is unusual to have two layers of roof insulation separated by an attic, and it really isn't a code-approved approach. It sounds like your new attic will be big enough to stand up in. It must be, or the spray foam contractors wouldn't be able to get in there to do their work.

    If you are installing spray foam insulation on the underside of the roof sheathing of the upper roof, then you have no choice but to also insulate the triangular walls (the gables) of your new attic.

    Ideally, the spray foam layer that you will install to insulate the upper roof will meet minimum code requirements, but I suspect that your spray foam contractor has told you that isn't necessary. (They usually say that.)

  2. Bbrown3381 | | #2

    Thanks Martin...the existing roof is a 4:12 and the overlay is also 4:12. The gable overlay projects 90 deg out from the existing roof ....I've always called them a California overlay. As far as the spray foam
    ...this would move the existing attic area into the envelope...same with the gable addition. Cathedral ceiling comes to mind. The question is centered on the new roof triangle that is laid on the existing 4:12 roof. Thanks

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