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When adding foam insulation to a roof assembly, is there any reason to bring it significantly past the exterior building line?

The assembly I am asking about is:
I joist rafters filled with dense pack cellulose/sheathing/4" polyiso/sheathing/waterproofing/metal roofing.
Is it ok to stop the foam layer at the building/insulation line to save material on the gable and eave overhangs?
Thanks

Asked by stefan straka
Posted May 17, 2014 5:22 PM ET

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6 Answers

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1.

Wouldn't that depend on how you intend to tie all that to the
wall assembly? The insulation layer should be as contiguous
as you can manage, I would assume. The typical roof/wall junction
is a squeeze area subject to a lot of thermal bridging, probably
not an area to skimp on.

You might want to delete your duplicate post [if that's
possible..] so answers accumulate in one place.

_H*

Answered by Hobbit _
Posted May 17, 2014 5:36 PM ET

2.

Hobbit,
There shouldn't be much thermal bridging-the dense pack will cover the top plate and so at the exterior wall the insulation line would be a nearly continuos stack: wall insulation(cavity fill and exterior foam) then dense pack over the plate then foam over the first layer of roof sheathing.

You are getting at the heart of my question though. Is there any benefit (to mitigating thermal bridging or possible areas of condensation) to having insulation outside the building line that is not insulating the conditioned space directly

I'll try to delete the duplicate post, thanks.

Answered by stefan straka
Posted May 17, 2014 6:57 PM ET
Edited May 17, 2014 7:17 PM ET.

3.

Stefan,
In some climates, solar radiation on the south side of the building can heat the south-facing siding, and cause a pocket of warm air to form under the soffit on sunny days. This pocket of warm air can encourage snow on the roof to melt at the eaves, leading to ice damming. If you continue your rigid foam all the way to the eaves, the added insulation can reduce ice damming under these circumstances.

Of course, if you live in a climate where ice damming isn't an issue, this concern doesn't apply.

If you omit the rigid foam at the eaves and rakes, you still have to install shims to support the top layer of sheathing and roofing. It's usually simpler to just extend the rigid foam to the edges of the roof.

Answered by Martin Holladay, GBA Advisor
Posted May 18, 2014 6:43 AM ET
Edited May 18, 2014 6:45 AM ET.

4.

Thanks Martin.
I live in a Zone 4 climate north of Seattle, with little/no snow accumulation. The gable and eave overhangs are a bit more than 3', equaling about 35 extra sheets of foam at 2 layers. Financially/material use wise it makes sense for me to leave these out. So other than the ice dam concern, are there any other reasons to insulate this area?

Answered by stefan straka
Posted May 18, 2014 7:35 AM ET

5.

Stefan,
Q. "So other than the ice dam concern, are there any other reasons to insulate this area?"

A. No.

Answered by Martin Holladay, GBA Advisor
Posted May 18, 2014 10:00 AM ET

6.

Thanks for your help.

Answered by stefan straka
Posted May 18, 2014 10:31 AM ET

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