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Need some help with insulating cathedral ceiling

5326-DIYer | Posted in General Questions on

Hello all,
I am building a new house and the roof is scissor trusses with a cathedral ceiling. With the trusses I am having trouble figuring out what the best way to insulate is. I am building an unvented attic as described in articles on this site. My system is a standing seam metal roof, sheathing, 3.5″ Closed cell spray foam and I want to add fiberglass under the foam. My problem is with the truss construction I do not have anything to attach the Batts to in order to keep them up next to the foam. I would like to know if anyone has any suggestions for attaching the batts or would I be safe with installing the fiberglass directly above the drywall on the cathedral ceiling?
If I attach the batts above the drywall this will create about a 3′ air gap between the 2 layers of insulation(foam,Batts). Would this work and be safe?

I know the code says ‘directly below’ but does not state intimate contact.

I have asked this question on Fine Home Building but this site seems to get more traffic with a lot of experienced people.
Thanks, Terry

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  1. GBA Editor
    Martin Holladay | | #1

    Before I get around to answering your question, I'm going to address some advice to GBA readers.

    If you are building a house, the time to finalize your insulation plan is at the design stage, before construction begins. You want these insulation details nailed down early -- not after the roof is already framed. For more information on this topic, see Plan Ahead for Insulation.

    Terry: The type of insulated roof assembly that you want to create is much easier to do with I-joist rafters than with scissors trusses -- but it's too late now to change the framing.

    You have several choices:
    1. You can install open-cell spray foam on the interior side of the closed-cell spray foam, with a total insulation thickness that meets the minimum code requirement in your climate zone (usually R-49).

    2. You can spend many hours in your attic adding additional framing members to support InsulWeb fabric. You want this fabric to be installed at the proper distance from the cured closed-cell spray foam, so that once the space between the fabric and the InsulWeb is filled with dense-packed cellulose or blown-in fiberglass, you'll have your R-49 (or whatever).

    3. You can install the fiberglass batts as best you can, and try to keep them in place with lots of steel wire, woven into a mesh. I don't recommend this approach.

    Of these three options, Option 1 makes the most sense. Next time, plan ahead.

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