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Air-Sealing at Exposed Beams

jms_designbuild | Posted in Energy Efficiency and Durability on

I’m currently under construction on a project and wanted to run by my air sealing strategy at some exposed beams by some other minds.

The project is a double-stud wall house in climate zone 6B. About 1/2 the house has a typical attic, the other half has a cathedral ceiling.

For the cathedral ceiling, the structure is parallel chord trusses that are 24″ deep. They will get a continuous ventilation channel from eave to ridge that will be sealed to the truss chords. The remainder of the cavity will be blown full with fiberglass. The trusses have 2 lines of bearing where they are supported by exposed beams (and we have added 2 more decorative beams to balance the design of the ceiling plane.) I am doing airtight drywall on the ceilings and walls in the project and am trying to find the best way to get a continuous air barrier at these beams. The ceiling finish is going to be 5/8″ drywall. I have been planning to put ice and water shield along each beam (adhesive side down) centered on each beam so that during drywall install the rest of the backer can be pulled off and the drywall on each side can be adhered to the ice and water shield. Does anyone have a better way to keep a continuous air barrier at each beam/drywall transition?


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  1. GBA Editor
    Martin Holladay | | #1

    This question has been discussed before on the Q&A forum, but right now I can't find the old thread. Are any other GBA readers capable of coming up with a link?

  2. GBA Editor
    Martin Holladay | | #2

    Your method will probably work, but there are other ways to do it.

    Instead of a peel-and-stick product, you could use an air-barrier membrane like Intello Plus or MemBrain. These materials could be draped over the tops of the beams, and the seams could later be taped to rectangles of the same product that you could install over the rest of the ceiling.

  3. jms_designbuild | | #3

    So you are suggesting that it might be simpler to just put a smart vapor retarder on this part of the ceiling of the project. I had not intended to use a vapor barrier or smart retarder in the rest of the project and just use airtight drywall details. Are these membranes any less tear resistant than installing standard vapor barrier?

    It also seems it would be best to wait to install this until the insulation install (blown-in fiberglass) and inspections are complete to eliminate any chances rips/tears.

  4. GBA Editor
    Martin Holladay | | #4

    I don't know what you mean by a "standard vapor barrier" -- but you probably mean 6-mil polyethylene. I strongly advise you not to use a polyethylene vapor barrier.

    The main purpose of the membranes I listed is to act as an air barrier, not a vapor retarder. Of course, care must be taken with these membranes to avoid rips or damage. However, any rips or damage can be repaired with tape.

  5. jms_designbuild | | #5

    Thanks Martin, the MemBrain looks like it will be a simpler solution.

    And yes, I meant 6-mil poly. And no, I definitely will not be using poly (ever).

  6. Expert Member

    Another alternative to consider is to strap the cathedral ceilings with 1"x4's. At the beams you can use several inches wider strips of 3/4" plywood which can be gasketted or sealed.
    You might also consider substituting 1/2" ceiling drywall for the 5/8". It is easier to work with and just as strong.

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