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Wall to ceiling air-sealing detail

Don_Christensen | Posted in Green Building Techniques on

I have seen designs where taped wall sheathing is carried inside to the ceiling for air-barrier continuity.  This detail often includes a plywood or OSB flange, projecting inward at the top of the wall, before roof trusses are installed.  The sheathing is taped up over the top of this flange, which is in turn sealed to the ceiling (tape, flashing membrane, etc.)

While this method provides a good air seal, it unfortunately interrupts the wall sheathing at the top plate.  If you have a raised heel truss sitting directly above the wall framing, there is a significant structural advantage in spanning the wall, top plate(s), and truss heel with a single piece of sheathing.  Many sources strongly recommend this practice, especially for windy locations, to strengthen the truss connection.

To air seal this assembly, could you caulk the above-mentioned flange piece to the top plate and then also caulk the sheathing to the outside face of the top plate?  On the inside, the flange could still be taped to the ceiling.  Will this method provide a reliable long-term air-seal?

I have attached a rough sketch (not to scale) to illustrate what I am suggesting.  Thanks, Don

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Replies

  1. Expert Member
    Rick Evans | | #1

    Don,

    You can run your sheathing up to the middle of your top plate. Then tape the top plate to the sheathing. This tape should run over the top plate to the interior to connect to your ceiling air barrier.

    To tie the trusses to the wall, your sheathing will be nailed to the upper half of the top plate and to the raised heel truss vertical members for a continuous load path.

    I've attached a detail of this. In this case, I am using ntello Plus as my ceiling air barrier here. (It is red in the detail.) Tape is in blue and the sheathing is green. You can do the same if you chose the plywood flange that you described as the air ceiling/ load path detail is the same.

  2. Expert Member
    Rick Evans | | #2

    Here's a zoomed out view if helpful. (This happens to be a Corson-style detail with i joist out-riggers.)

    1. Don_Christensen | | #7

      Thanks everybody. Rick - that's an option I hadn't thought of, splitting the sheathing at the center of the top plate. If that provides adequate shear strength for the raised truss, it solves the air-sealing dilemma without relying on caulk.

      In your assembly, the strapping immediately below the Intello is fastened cross-wise to the trusses, correct? Do you just staple the Intello to the trusses first, and that is strong enough to hold up the attic insulation between the rows of strapping? Info is probably on the ProClima site somewhere. I imagine the Intello is easier to get up there than sheets of OSB or plywood, though I guess you want to avoid stepping on it. The other thing I like is that you don't need an extra flange piece above the wall. Thanks, Don

      1. Expert Member
        Rick Evans | | #8

        Hi Don,

        You are correct: Intello is stapled to trusses and then strapping is added later. I like 16' ripped 2x4s. We blew in 28 inches of cellulose above our Intello and it is holding up just fine. (Trusses and strapping are 24" OC.) Pro Clima will tell you that Intello will hold R-100+ in cellulose but you may need to "stitch -tape" the stapled areas as this adds a little rigidity. In Canada, they use 6 mil poly all the time so that would likely work too.

        I was able to install the Intello and strapping alone. It wasn't easy but I got it done in 2 days using a home-made jig, two ladders, a $20 electric stapler, and an impact driver. Sheet goods like OSB or plywood would work great but you might need a team or at least a drywall lift. Drywall laborers could bolt it up in no time, I imagine... so maybe that is the way to go. Matt Bowers built a Passive House in NY that used ZIP sheathing as their ceiling air barrier. You can read about it here: https://rochesterpassivehouse.blogspot.com/2016/01/completed-air-barrier.html

        The one benefit of this design above (OSB or Intello) is that you have shear panels on the exterior but can add your hurricane ties on the interior. The Intello just laps over the H-clips and you tape it. A service cavity also keep wires out of the attic and leaves a small space for ultra thin 'can' lights.

        Regarding splitting the sheathing on the top plate, this is very similar to two layers of sheathing meeting between floor or at blocking mid-wall. If you live in a high wind zone or are worried about tornadoes/hurricanes, then you can consider using a single 4x4 as your top plate and crown plate. That way, the two layers of sheathing are meeting on a larger piece of wood and is less likely to split in half than a 2x top plate. You can tie the plates together using advanced framing techniques.

  3. Expert Member
    Rick Evans | | #3

    And I found an old 3d version. This detail uses sheathing as ceiling air barrier, as you described.

  4. AlexPoi | | #4

    I think you could use hurricane straps instead of running the sheathing past the top plate no? This what is done here most of the time.

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Alex,

      The sheathing and straps do quite different things. The straps resist uplift and the sheathing creates a small shear-wall to resist lateral movement. If the heels on the trusses are too high, engineers in seismic zones will often detail them with not only sheathing, but small stud-walls between each truss.

      1. alexqc | | #6

        Didn't know that thanks! I would have to ask a structural engineer about it though. The way I understood it is that the sheating like you said acts as a shear wall and need to be there. But you can either run the sheating past the top plate or use hurricane straps to connect the trusses to the walls as long as the trusses heels are sheated.

        I know in Europe some people put the sheating on the inside so I'm wondering how they do it. They may not be in a seismic zone though.

  5. kyeser | | #9

    I have tried running sheathing pieces over all the top plates and then caulking the sheetrock to the top plate and sheathing. It was just more work so now we just frame as usual then sheetrock just the ceilings and then go through and caulk the ceiling sheetrock to the top plates. Then caulk and tape all wire and plumbing penetrations.

    Much simpler, won't get you to Passive House levels but I can get below 1 ach/50.

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