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Air Sealing Between Double Top and Bottom Plates

tallpinescabin | Posted in General Questions on

I have a small cabin build in process on an insulated slab on grade foundation.
The exterior 2×6 24 oc framing is up, and the sheathing, roof trusses and roof are on. The bottom plates overhang beyond the slab edges to cover the slab insulation. Per Wisconsin code, I doubled the bottom plates to get enough overhang.

So, I’m wondering a bit about the best way to seal BETWEEN the 2 bottom plates and the 2 top plates now that they’re already in place. The top plates are doubled SPF 2x6s. On the bottom plates, the sill plate is LP Solid Guard treated LSL, so, much like OSB. The 2nd bottom plate (upper) is SPF.

How would you seal between 2 SPF plates and between an SPF plate and an LSL plate? Would you use tape or caulk?

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  1. Expert Member
    BILL WICHERS | | #1

    Ideally, you want to put a bead of sealant down before joining the plates together. In your case, I would use polyurethane sealant (caulk :-) in the gap formed by the radiused corners between the two plates. Use a piece of scrap to press/smear the bead into the gap for a good seal as you go. It should go pretty quickly.


  2. plumb_bob | | #2

    Your air sealing system should be spanning these joints making sealing between the plates irrelevant. For example, if you are using a poly sheet in the interior it should be sealed continuously from the floor to the ceiling, effectively keeping any air that gets between the plates out of the house. Make sure that when the poly is caulked to the floor the bead is where the bottom plate meets the floor, this is a commonly missed detail.

  3. tallpinescabin | | #3

    Thanks Bill, I'll give the poly sealant a try. No real radius on the LP LSL Bottom plate, but the radius of the 2nd bottom plate shoudl provide a nice gap.

    Bob, my primary air seal will be the sheathing plane, but since sealant didn't get put in on many of the pieces of sheathing before nailing, I'll have to seal between plates to pass the "red line test." I'll probably have "relatively" airtight drywall as well, but that's not set in stone yet. And I won't necessarily have poly over the interior either, otherwise, you're right, this would be easier to not do!

    1. Expert Member
      BILL WICHERS | | #4

      It's relatively easy to install drywall airtight. I always do it that way on exterior walls as an extra layer of protection.

      Another option is to add a smart vapor retarder like MemBrain or Intello. You can then seal that to the top of the top plate and the bottom of the bottom plate, which will cover the gaps between plates. That's what I did here in my own home as I renovated the exterior walls. What I do is to put a bead of polyurethane sealant down, then attach the vapor retarder with staples through the bead of sealant. The end result is the sealant seals the staple holes and the vapor retarder to the framing. It goes pretty quickly.


  4. tjanson | | #5

    Your sheathing doesn't overlap the joint between plates? I would be taping the sheathing to the plates and taping all the sheathing joints.

    1. tallpinescabin | | #6

      Tim, it does overlap. But taping the sheathing to the plates at this point is tough, no real access. Air could conceivably come up between the sheathing and bottom plate (where it's already overhanging off the slab) and come inside between the plates. Same on the top plates...and the sheathing on the top goes beyond the plates into the truss heels a bit.

      Weather was working against me at the framing stage, so I didn't take the time then to do some of these details or even have them completely worked fixing what I can now.

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