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Community and Q&A

Interior bottom/sill plate to slab transition: seal or leave alone?

Matt McLagan | Posted in General Questions on

I’m wondering if I should seal this transition on the inside? I have a continuous exterior air barrier with Zip & a sealed horizontal transition to EPS with zip liquid flash.

Attached photo of transitions and wall system, focused on the floor transition from bottom plates, to the concrete it’s ontop of, over the EPS, onto the slab.

Floor will be finished, tiled over.

I thought to use Zip Liquid Flash over the 2 bottom plates all the way to the slab, but wonder if there’s a better, less expensive alternative, or if it’s even necessary at all on the inside.

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  1. Expert Member
    Peter Yost | | #1

    It's always a good idea to have a capillary break between any wood and any masonry. Even on interior partition bottom plates going down on a concrete slab, I use sill sealer as a capillary break.


    1. Matt McLagan | | #2


      Are you suggesting I cover the area in the highlighted red boxes with a sill plate gasket like this? :

      This was used under the PT plate already.

      The only thing that would cover this area is tile, when finished.

      If you look at my 2 pictures posted, the photo is of an interior corner, and the wall model is accurate for about 60' of basement wall.

  2. Kevin Spellman | | #3

    I used liquid flash outside, as well as tape. On the inside, you could caulk the sill plate to foundation joint and use a tape like 2" 3M 8067 on the framing-to-sill-plate junction. That is the combo I used. It's overkill and redundancy, but that's why we are here!

    What about the daylight we can see and the nails-to-nowhere?

    1. Matt McLagan | | #4

      The daylight & nails to nowhere was why that pic was originally taken. I've since taken care of that.

      I like that tape & caulk idea. Thank you. Any idea where to get the 8067 for a good price?
      Although I'm still wondering if I should bother covering the transition from |concrete wall| 2" EPS | Slab
      But maybe mortar when tiled is all thats really needed.

  3. Kevin Spellman | | #5

    Amazon has 8067 available and the 2" is very reasonable.

    Is radon a concern? I can't answer the question about mortar and tile being a good enough "cover" for the concrete/EPS/Slab junction. Not sure what your options would be. If you used a 6" tape to seal it up, would that interfere with your mortar bond to substrate?

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