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Community and Q&A

Polyurethane caulk for bottom plate to slab sealing

user-6258531 | Posted in Green Building Techniques on

I’ve been reading the various posts on bottom plate air sealing on this site. I’d like to avoid the expense of EPDM or Denarco sill seal. I’m wondering if laying down a healthy bead of a polyurethane caulk on top of the foundation wall prior to tipping up the walls will produce an adequate seal? In addition, the bottom plate would have the standard contractor 1/4 inch foam seal. Your thoughts appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Your aim is to reduce air leakage to an absolute minimum. It's certainly possible to attain this goal with caulk, but if I were you, I would want to verify the results of your sealing efforts during the blower door test.

    One advantage of an EPDM gasket is that the gasket will perform very well under changing conditions. The wooden components of a building move: wood will expand and contract, for example, due to changes in humidity and temperature, and the mudsill will move as weight is added to the walls during the construction process (and during a snow storm, as snow accumulates on the roof). After a snow storm, the mudsill will move again as the snow melts, and weight is removed from the walls.

    An EPDM gasket accommodates these changes better than caulk. You might want to compare the cost of the EPDM gasket with the total cost of construction, to determine whether the savings you seek by not installing the EPDM gasket amount to false economy.

  2. user-6258531 | | #2

    Cost for EPDM gasket BG75 with shipping is $325.
    Cost for contractor foam SealR plus OSI SC-175 sealant $90

    A few hundred here, a few hundred there adds up to real money.

    Will acoustical seal move adequately to adjust to the changes in loading you've identified?

  3. user-6258531 | | #3

    Oops, I switched caulk on this. If I were to go with Polyurethane, the cost would probably be lower. After reviewing Martin's comment's it seems like the Acoustical might be better for long term flexing with movement.

  4. Expert Member


    If you are going with caulking or, as I'd suggest, acoustical sealant, rather and laying a bead and squashing it between your bottom plate and foundation, it is more effective to place the bead at the outside and inside intersections between the concrete and wood after the plate is installed. You can be a lot more generous there making it more forgiving of movement.

  5. user-6258531 | | #5

    I did ultimately purchase the EPDM gasket. I'll see how it looks installed and may add some sealant as well.

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