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Community and Q&A

Rigid foam under a vaulted ceiling

dave6736 | Posted in GBA Pro Help on

Is it possible or even a good idea to use rigid foam ,(xps or poly iso ) under the drywall on a vaulted ceiling.The house is in zone 5 with sissor trusses in this area. Above the ceiling is R38 fiberglass with venting at the soffits and ridge.The ceiling would then be refinished with drywall or wood paneling. This area is not accessible from above ,however the rest of the house has flat cord trusses with attic area and we are considering using blown in cellulose for this area.I would like to do as much air sealing as possible, so what should be done about electrical boxes and recessed lighting.In the future we would like like to replace the windows and siding and add exterior foam, is there anything to think about with the ceiling project that will help with the siding project.

Thanks for any help

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Replies

  1. davidmeiland | | #1

    Sounds like a good plan. If you're going to install wood paneling on the ceiling, drywall or better yet foam insulation should be installed first--the paneling on its own will leak a lot of air. Can lights in the ceiling should be eliminated if at all possible, and surface mounted fixtures should be installed using boxes designed for airtight drywall. There isn't much I'd do to the ceiling to prepare for the future work, but I would make sure the ceiling insulation extends fully over the top plate of the wall. Sometimes it's necessary to insulate between the trusses with rigid foam near the top plates due to low heel height, and of course you need to remove drywall to do that.

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