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New construction air sealing question

jdchess | Posted in General Questions on

New construction in CZ3 (southeastern NC, warm humid).

2×6 exterior walls @ 24″ OC with ZIP sheathing.

Vented attic with gable roof using raised heel trusses.

Dense packed cellulose in the walls and blown cellulose in the attic.

Air barrier is the exterior ZIP sheathing and the ceiling drywall.

What is the best way to air seal the ceiling drywall to the top plate of the exterior wall? Air sealing the ceiling drywall to the top plate of the interior partitions? Also, does truss uplift affect this?

I found an article on Fine Homebuilding that mentions using a 3/4″ cap plate on top of the exterior and interior top plates to give the ceiling drywall a surface to seal against, but this would also involve using 3/4″ furring for the entire ceiling because of the gap created by the cap plate.

This seems like a practical solution, but would it cause issues with drywall cracking and separation due to truss uplift? Is there a better way without have to use furring for the entire ceiling?

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  1. Granular | | #1

    How about using a strip of smart membrane instead of the plywood and use beads of accoustic sealant (not construction adhesive) to adhere the membrane to the drywall and top of stud wall? The membrane isn't a rigid connection so should withstand truss movement without damaging the air seal (maybe leave a bit of slack rather than stretching taught).

  2. CMObuilds | | #2

    Spray foam
    Use 1 part if you have anything against 2 part, very effective, not complicated and it works.

  3. jdchess | | #3

    Spray foam vs caulk vs other construction adhesive?

  4. nshirai | | #4

    Consider a high quality wide tape with split backing. Siga Fentrim comes in some wide sizes and you can peel off one backing strip to adhere to the outside of the sheathing, fold it over your top plate and let it lap over until drywall goes on, at which point you peel off the other backing strip so it sticks to the drywall. This is best done before setting ceiling joists and roof rafters so that the tape is continuous. 3M 8067 also comes this way. There are others out there as well. You could take this approach with all wall top plates, exterior and interior partition, for a pretty good air seal at the ceiling level. Don't forget to hit your ceiling drywall penetrations and top plate penetrations with sealant before insulating.

  5. jberks | | #5

    Up in my parts (Canada) we have to use vapour barrier on the interior so this is usually never a problem. However my current build is an unvented flat roof so I omitted the vapour barrier at the ceiling so the roof can dry to the interior.

    This presented pretty much the same issue of air sealing the ceiling to wall intersection. I ended up using spray foam at the eave-top plate intersection. But since your roof is vented this wouldn't work.

    What granular suggested I think is best/easiest.

    Another, not as easy way to do it would be to use a good sealant like demonic 100 to seal the top plate to the drywall from the attic in between the rafters. And just to get pedantic, I'd also go the effort to seal the end of the drywall sheets to the side of the top plate before the drywall goes on the walls (not great if it's a cut end, hopefully your drywallers use factory ends at the corners to build better corners)

    Airsealing challenges are fun right?


  6. n2dirt | | #6

    Anyone build something similar to this ?

  7. rockies63 | | #7

    A couple of videos on truss uplift

    Since trusses are designed to only rest on the exterior walls (the bottom cords "float" over the interior walls - hence attaching the bottom cords to the interior wall top plates with clips instead of nailing) you could drywall the entire ceiling and then add the interior walls with the drywall going right over the interior wall top plates. Then add the clips from the attic into the tops of the interior walls (might have to add some horizontal blocking if your interior wall falls between trusses..

    This is similar to how framers can slip drywall between interior and exterior walls.

    1. jdchess | | #8

      I had thought about the idea of dry-walling the entire ceiling before raising the interior partitions, but several folks pointed out potential issues, not the least of which is the major shift in the normal construction schedule. Also, I wasn't sure how I could get around the Simpson clips and the vent stack.

      I had seen the Greg Vanden Berge videos. All of his videos are very informative. I thought it was interesting that he suggested using 3/4" material for the second interior partition top plate so the truss bottom chord would actually never be in direct contact with the top plate of the interior partitions. I understand the reasoning behind this I think, but it seems like it would create an issue for air sealing if trying to float the drywall in the corner as suggested by Joe Lstiburek here...

      Just seems like every option has it's potential drawbacks, but I suppose that's true with pretty much everything. :)

  8. Jon_Lawrence | | #9

    Skip the plywood and use a membrane, secure it to the outside of the top plate and let it flop over the interior wall a few inches (Passive House builders call these floppy bits). Once the trusses are installed, you can staple it to the bottom chord of the truss. Add a little sealant as you are installing the drywall to make an airtight connection between the two. If the hurricane clips are an issue, you can use Simpson truss screws. We wrapped all of our rim joists with Siga Majvest to make our interior air barrier continuous. This was a bit overkill since we have Zip as our exterior air barrier and an unvented attic, but the same concept should help in your situation.

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