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Gaps in exterior foam - how to insulate underside of roof?

We're finally getting ready to insulate our roof, & after many hours of research have been unable to come to a consensus on what type of insulation to use. We are in climate zone 3 (central AL). This is an unvented cathedral ceiling, 12/12 pitch, & once finished will have sheetrock installed on the underside of the rafters (no can lights, etc). Currently the roof consists of:

2"x10" rafters on 24" centers,
1/2" OSB,
30lb roofing felt, with Ice & Water Shield on the ridge & in valleys,
1.5" foil-faced polyiso,
metal roofing (3' wide sheets with ribs running vertically on 9" centers), screwed down through the polyiso into the OSB.

When the roofing was installed, we were unaware of the potential condensation problems that could form on the underside of the roof deck. According to http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulat..., it looks like we would be ok to insulate with fiberglass batts on the interior side of the roof, since we have a layer of exterior foam?
However, the roof was done last year after lots of tornado damage in the area & our overworked roofers didn't live up to their references. We observed gaps between some of the polyiso sheets, especially in the valleys. It's been about a year & a half by this point since the roof was installed and we're at least confident that it doesn't leak. Given these gaps, would it no longer be acceptable to use the fiberglass batts in the rafter bays? Or would the flash & batt method or spray foam be preferable, & if so, should it be closed or open cell?

Thank you for any advice you can give.

Asked by Jessica Akau
Posted Fri, 11/23/2012 - 20:30

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Jessica,
For more information on all of the different ways to build an unvented cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.

Ideally, your roofers would have filled the gaps between the sheets of rigid polyisocyanurate with canned spray foam, and then (after the foam had cured) they would have taped the seams with a high quality tape. It's too bad they didn't do that.

However, your roof will probably perform well. If the quality of the polyiso installation really bothers you, and you won't sleep well at night, you could always install spray polyurethane foam on the underside of the roof sheathing instead of fiberglass batts.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 11/24/2012 - 05:48

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