GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

What is the best sill sealer option for a 8″ sill plate?

bobbyarndt | Posted in Energy Efficiency and Durability on

I am building my home with Foxblock ICF’s for the foundation wall. Because I want my exterior sheathing to line up with my foundation foam, the sill plate needs to overhang the concrete by about 2″ which forces me to size up the sill plate larger than a standard 2×6. When I go to a 2×8 sill plate, you don’t have many premium options for a sill sealer, like the epdm gasket from conservation technologies that is only manufactured to fit a 2×4 or 2×6. I am struggling to come up with a solution I feel 100% about so I was hoping to get some input and educated opinions. I have come up with 3 possible solutions but maybe there is an option I am missing?

1. Center the 6″ epdm gasket from conservation technologies on the 2×8 plate with a bead of acoustical caulk on each side

2. Buy a roll of 5/64″ thick epdm rubber and put a bead of acoustical caulk under and on top of the epdm strip. ( )

3. Use the “standard” foam sill sealer and put a very large bead of acoustical caulk above and below the foam.

I am leaning toward option #1 or #2 but would like some opinions!

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. GBA Editor
    Martin Holladay | | #1

    Sill seal is designed to seal the crack between the concrete foundation wall and the mudsill. If you are cantilevering your mudsill toward the exterior, the gap that you need to seal does not include the cantilever. All you are worried about is the 5.5 inches above the concrete.

    Use any approach you want: Ordinary sill seal (placed above the concrete, not above the rigid foam) or any fancy product from Conservation Technologies that you care to buy, with or without additional caulk or acoustical sealant.

  2. STEPHEN SHEEHY | | #3

    I'd go with the gasket from Conservative Technologies. You just want to seal the gap between the foundation and sill plate. You don't need to fill 100% of the space. It's quick to install.

    1. kjginma | | #4

      I second the recommendation for epdm seal for the cantilevered sill plate. I did that and the sill held water (ie, a small swimming pool or ice rink, depending on the temps) before I finished the framing.

      Personally, I preferred the epdm rather than using any caulking -- VOCs that can reach the interior may pose an occupant health risk.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |