GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Air sealing the cap of a double stud wall

Chris Roche | Posted in Green Building Techniques on

I am currently building a double stud home in NH and am in the process of air sealing all exterior cracks with siga tapes. Since my builder decided to build the interior portion of the double wall after the exterior, we were left with a 4 inch gap between the two walls at the cap. We will be filling the 12 inch cavity with dense pack cellulose, so this gap at the ceiling needs to be blocked off. We decided to fill the gap with strips of plywood which has resulted in cracks on either side of the plywood filler that I’m worried may leak air. All other sides of the wall will be air sealed with tape/membrane on the exterior and interior. My question is do I need to tape over these cracks? Or is it not important to air seal at the top of the double stud wall? I have included several photos

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Chris Roche | | #1

    Thank you for any help! Trying to decide if I need to order several hundred feet of tape to air seal this.

  2. GBA Editor
    Martin Holladay | | #2

    This is either a single-story house or the upper floor of a two-story house -- right? I think those are roof trusses above this wall.

    You definitely need a good air barrier at the top of this wall. You also need a good air barrier at your ceiling. What's your plan for a ceiling air barrier? And will you be insulating the attic floor, or the along the roof slope?

  3. Chris Roche | | #3

    Hi Martin,

    You are right, it's a single story ranch. I bought Siga Majrex as a barrier for the ceiling and walls. We plan to insulate the attic floor with r-60 cellulose. Should I just tape this interior of the wall cavity cap on either side of the plywood filler? This website has been instrumental in all parts relating to building this house so I have to thank you for everything.

  4. MarkM3 | | #4

    Chris, I wonder if you might instead prefer what I would call a "super caulk" - FastFlash, from Prosoco? Seems to me that would be easier than tape. Best of luck on the build, Mark.

  5. GBA Editor
    Martin Holladay | | #5

    Before I provide you with advice, I'm going to indulge in an aside to GBA readers.

    Readers: You need an air barrier plan before you begin building the house. You can't make up these details on the fly. Your builder needs to know how the ceiling air barrier connects to the wall air barrier, and the manner in which this happens needs to be clearly marked on the plans.

    For an example of a good method of connecting the ceiling air barrier to the wall air barrier , see this article: A Practical Air-Sealing Sequence. The article suggests installing a cap of 3/4-inch plywood above the top plates of the walls, and suggests ways to seal this cap to the wall air barrier and the ceiling drywall. While the illustration (see below) depicts a single stud wall, the same technique works for double-stud walls, as long as the plywood is ripped to the correct width (leaving an interior lip).

    OK -- I'm done with my aside.

    At this point, Chris, your options are (a) tape, (b) FastFlash (although I've never used this product), or (c) Two-component spray foam installed from above.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |