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Community and Q&A

Air-Sealing Gaps in Old Board Sheathing

steve41 | Posted in General Questions on

I’m re-building an addition to an old house. The exterior wall of the addition T’s off the existing structure as in the attached sketch.

A couple of challenges:
1. The existing structure has board sheathing with varying gaps up to 1/2″.
2. The wall will connect at a location that is between studs. (has to be in this location)

My plan to deal with these challenges are to:
1. Add a ripped strip of 3/4″x 12 Advantech  over the sheathing boards where the new wall will connect.  This will all0w for additional screwing area to the board sheathing. And help with easier wall raising.
2. Caulk the Advantech to new 2×6 wall joint.  Raise wall. Attach.
3. Be diligent with flashing/sealing the exterior corner.

I have considered taping or somehow sealing the sheathing board gaps before connecting the addition walls but I’m not convinced that it will really do anything for me.

Am I headed in the right direction with this?  Anyone have experience with the new to old connections in board-sheathed older houses?

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  1. Expert Member


    I'm not sure what the strip of Advantech brings here. It means you have to do twice the fastening, and will have to fur out the rest of the wall in the addition. I also don't see how it helps air-sealing the joint.

    Typically in this situation I just screw the stud to the sheathing. If you don't think the board sheathing is sufficient to tie the framing together, I would remove several pieces in that stud bay and add a 2"x4" ladder for attachment.

  2. steve41 | | #2

    Thanks Malcolm, much appreciated.

    I was planning to keep the interior edge of the Advantech flush with the face of the studs so I wouldn't need to fur it out. Or I may bring it out ~1/2" to make it flush with the sheetrock.

    I may be overthinking this. I was looking to keep the new wall assembly sealed from what I thought might be cold air infiltration making it's way into the new wall assembly via the sheathing board gaps.

    I was also thinking it would enable me to pre-plumb/level with the Advantech before the new wall is raised.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      Sorry - I was going off your drawing so didn't realize you intended to inset the Advantech.

      That will work, but there is no backing to attach it to on one side. If you removed all the old sheathing on that one stud bay and replaced it with Advantech, that would both seal and provide a substrate to attach the new wall to. Once up, a bead of sealant or tape between it and the new wall will keep things tight.

      A quick way to plumb the new wall, especially if you are working alone, is to screw a block to the sheathing near the top on the old one you can rest the wall against once you lift it.

      1. steve41 | | #4


        Thanks, great to have your input.

        I'll definitely plan for a stop block on the existing wall when lifting the new wall.

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