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Community and Q&A

Foam over roof question

joenorm | Posted in General Questions on

I asked this the other day but never got an answer.

When you are adding foam on top of the roof deck to create a sealed assembly do you typically go all the way out the the eave/overhangs or do you stop at the buildings thermal envelope (above exterior wall)?


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  1. Expert Member
    Dana Dorsett | | #1

    With deep overhangs it's common to only run the roof foam a handful of inches beyond the exterior of the wall's top plate (and any sheathing insulation.) With shallow overhangs it's less work to just run it out to the edge of the roof, even if it's over a soffit area.

  2. GBA Editor
    Martin Holladay | | #2

    In almost all cases, you cover the entire roof -- because it's just as expensive to come up with a system of shims to cover the overhang as it is to use 100% rigid foam.

  3. Expert Member
    Peter Yost | | #3

    Hi Joe -

    If you build a "flush" box and add overhangs AFTER you complete the roof frame and insulation, you have a better opportunity to completely connect the wall and roof air and thermal control layers (you ladder frame the overhangs and attach them as an "appendage"). In this case, you don't run the rigid insulation any further than the eave wall.


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