GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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 Bottom of Exterior Wall to Foundation

sb1616ne | Posted in General Questions on

Anyone have any tips on air sealing the bottom edge of the exterior wall to the foundation? What I am running into is am older home with a mix of ply wood and wood sheathing boards. They framed the walls such that the sheathing extends about 3/8” beyond the foundation wall face.  Then  house wrap or tar paper as the WRB then cedar clapboards(no rain screen). Inside the box sills are insulated with closer cell foam.

Walls are cellulose and what I am dealing with is air infiltration from
the outside. I know best practice is to not calk this area as water needs a way to drain. Canned spray foam could work, but it needs to be permeable. Some calkings and liquid flashings have a perm rating as well but it will be low after I apply a large bead to this area.

Any thoughts?  Thanks!

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. Expert Member
    BILL WICHERS | | #1

    Usually spray foam applied to the inside of the rim joist area provides the air seal too -- that's one of the reasones spray foam is often recommended for this application. Can you provide a drawing or picture of the specific area you're still trying to air seal?


  2. Danan_S | | #2

    > Then house wrap or tar paper as the WRB

    I had a similar situation with an odd joint between existing sheathing and the foundation, but
    I caulked the gap and Siga Wigluv taped after the caulk set. Then I ensured that my WRB (I have 2, Blueskin and Tyvek) overlapped that, so that bulk water shed property.

    In general, I don't see a way to allow drainage from the sill plate while also making the area airtight.

  3. JeffWilliams56 | | #3

    I also had problems doing this

  4. Expert Member


    The only situation where you incorporate a drainage plane in an exterior wall is when you have a rain-screen cavity - and that drainage plane is always outside the WRB. Walls without rain-screens do not make provisions for drainage at their base, as the assumption is there should never be sufficient bulk water intrusion to need it.

    I you have enough water getting Into the walls to require drainage, you need to replace the WRB and cladding, and preferably incorporate a rain-screen. Otherwise, just concentrate on air-sealing at the bottom plate.

  5. richmass62 | | #5

    There is a product for this. In fact I bought the product a couple of weeks ago on Amazon. It is recommended by Matt Risinger (Build show on Youtube) and it is designed to air seal.

    First you need a "sausage gun" to apply the product:

    Then you need the product. I bought two tubes of Prosoco Seam Filler:

    The product is better than spray foam because spray foam will fall apart over time and allow mice to still come in. The product can be smoothed out like joint compound (use a 2 or 3 inch blade) after you apply it to the seam. Then it can be covered with Zip Tape.

    Generally the source of moisture in that area is from snow and rain that hit the ground, not from your wall cavity. If you can keep the bottom 2 inches of your sheathing protected from most of the wet below you are likely better off. The moisture in the sill plate is best controlled by replacing some of the sill with pressure treated new lumber, if you have that issue. Moisture should not be going that deep into your wall in the first place.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |