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Community and Q&A

Sealing bottom plate to slab

hughsdb | Posted in Green Building Techniques on

My framer says that Sill Seal does not do a good job. I am inclined to agree with him. He is recommending a bead of liquid nails. I have been looking for real information. Certainly by now there have been some test run to measure the efficacy of various products and methods. And a new home slab is not a laboratory. How well does this or another product stick to a slab that has been wet and muddy?

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  1. GBA Editor
    Martin Holladay | | #1

    Foam sill seal does an OK but not perfect job. Many builders back up their sill seal with a bead of silicone caulk on the interior side of the mudsill.

    Passivhaus builders go a step further, and use an expensive European tape like Siga Wigluv to seal the crack on the exterior side. Wigluv is a good tape.

    A third option is to use a product from Protecto Wrap called Triple Guard. It isn't cheap, but it works very well.

  2. hughsdb | | #2

    Thanks, Martin. I was thinking about using the same material that we use to seal around windows as an additional outside barrier. Triple Guard seems like it was formulated to stick to concrete. I can imagine that this seals well. I wonder how well it will have held in 10 years. Any sense of how well the liquid nails would work over time?

  3. GBA Editor
    Martin Holladay | | #3

    I don't really know. But I wouldn't depend on Liquid Nails as a long-term caulk. Construction adhesives like Liquid Nails tend to get stiff and brittle as they cure.

  4. bobhol | | #4

    Google...EDPM sill gaskets ...they look real good and may solve your problem...regards,Bob

  5. hughsdb | | #5

    Yes, I guess I am going to bite the bullet, annoy the framer and go with the EPDM gaskets. Maybe I will set up a test. The test could be misleading as how well it holds over time is a large issue that is not going to show up under test conditions. I wonder why this has not be done. Is leakage under the plate that negligible?

  6. davidmeiland | | #6

    Sill seal does not do a good job of what? Providing an air seal? Providing a capillary break between the concrete and the wood?

    I use sill seal for the latter reason, and do more air-sealing with sealants, generally after the wall is standing up. You can do yet more after the drywall is up.

  7. hughsdb | | #7

    David, my concern is air sealing. We also do treatment after the wall is stood. My fear is that this is veneer and will be compromised more easily and quickly.

  8. Expert Member
    Dana Dorsett | | #8

    The foamy sill gaskets are both fairly vapor-permeable and don't nearly as good a capillary break as EPDM, but are still way better than nothing.

    If you had a high moisture content in the concrete most foamy sill gaskets still transfer about 0.05- 0.1 oz of water per square foot of surface area from the concrete to the wood on vapor diffusion alone, as opposed to about zero with EPDM.

    A bead of low-expansion can-foam under the foundation sill does a pretty good job of air sealing, with or without a sill gasket, but it isn't much of a capillary break.

  9. Expert Member

    Hugh, we use acoustical sealant. It adheres well to most surfaces and does not harden or crack.

  10. user-1072251 | | #10

    we've been using the EDPM gaskets and are very pleased with them. they are relatively inexpensive, easy to install and most important, they work, as evidenced by the very low infiltration numbers we've achieved. Sold by

  11. Gairnealair | | #11

    Location: Milwaukee, Wi
    Using approximately 10" of a small diameter hose, I created a flexible extension for my caulk gun, allowing me to apply 100% silicone under the interior side of the mudsill. Chose silicone for it's bond & flexibility. Previous owner did use spray foam on the interior of the rim joists, however, without caulking to seal gaps, condensation does seem to be present on the interior of the rim joists causing the spray foam to pull away.

    I am also oiling the floor joists and exposed underside of the subfloor, and perhaps the rim joists too. This way I can monitor their condition, Come spring, seal the exterior mudsill the same way. Like old cars, I'll call this kind of work Tuning Stage 0.

  12. wjrobinson | | #12

    Construction adhesive like liquid nails does indeed harden and get brittle and let go too.

    Silicone... many think it is one of the best caulks but, in my travels it does not stick well to some items, not great on roofs, does get quite inflexible with time, does go bad in wet showers no matter what the manufacturer says so squeegee your shower if that matters to you... I love silicone but it has it's place, I love it with glass for sure.

  13. Richard Beyer | | #13

    Only one adhesive bonds to wet foundations... It's not silicone or liquid nail.
    Polyurethane sealant, meaning polyurethane caulk will remain flexible and it will stick to everything! Be sure to wear the proper PPE when using this product.

  14. [email protected] | | #14

    I like Silyl Terminated Polyether sealants over silicone or polyurathane.

  15. Expert Member
    MALCOLM TAYLOR | | #15

    Acoustical sealant?

  16. [email protected] | | #16

    Malcolm, the Tremco rep said acoustical sealant wasn't designed for sealing sill plate to concrete foundation. What does that really mean? Not sure as I didn't get much additional information. I'm not ready to accept that it will have problems. Moisture in the wood or concrete may be a problem??

  17. Expert Member
    MALCOLM TAYLOR | | #17

    Debra, We use acoustical sealant at the base of our walls at the slab and press the poly air/vapour barrier into it. We use a foam sill seal too but its role is to stop moisture migrating up into the framing. Tremco is probably right in that a bead under a bottom plate wouldn't be very effective, but at the interior intersection, where we use it, it works fine.
    We use it in a similar way when the sheathing is the air barrier, keeping the plywood up off the slab 1/4" and running a bead along the joint.

  18. binovc | | #18

    "13. Only one adhesive bonds to wet foundations... It's not silicone or liquid nail.
    Polyurethane sealant, meaning polyurethane caulk will remain flexible and it will stick to everything!

    Liquid Nails has many products. Liquid Nails (LN-950) is polyurethane sealant.
    Reason I'm replying is that I'm searching the web (and found this year-old thread) to find a way to seal a new wall that I'm building between my house and new back porch. My main dilemma is that the porch slab is the same level as the interior floor. I want to find a positive way to insure no rain leakage occurs. Maybe the gaskets that were referred to is the best answer for me...

  19. Chaubenee | | #19

    How about an elastomeric caulk? Also as a capillary break, has anyone tried using Grace Iceandwater type Membrain on the bottom of the wooden mudsill, and then used flashing tape on the outside to seal against air filtration?

  20. Rob Myers | | #20

    I can vouch for the EDPM gasket. I had to install the sill plates early on a frost protected concrete slab because I was raising a timber frame and wanted to make sure that timber bents/scaffolding did not accidentally slide over the edge of the slab. For the next few months, with every rainfall the slab became a swimming pool and I finally had to cut the sill plates in a few areas to provide drainage. There was absolutely zero leakage through the gasket.

  21. MeriMeri | | #21

    Our mudsill / sill plates project 1 1/4" outboard of the face of the foundation wall. As the external insulation is already installed to the top of wall we will be unable to use an air-sealing tape to connect the WRB/Air barrier to the foundation. (the insulation is in the way).

    We are considering using either Henry Air Bloc LF or ProClima Contega HF. The approach is to lay down a bead of the material at the outside edge of the concrete and simply set the mudsill on top and fasten. There will be a sill-seal product as well. Contega HF is an exterior caulk. I'm not sure about using the Air Bloc LF in this way....

  22. kjginma | | #22

    Many folks seem to like the EPDM gaskets however I am concerned that any water that get trapped in between the EPDM and the sill will not "air out", as might be the case with the normal open celled foam sealers. One way this could happen is a flooding event, such as what happened to Rob Myers (#20, above). Couldn't this lead to mold growth or sill warpage due to heavy wicking from below, instead of airing out from all sides? I *want* to believe EPDM is undeniably good because I do not want to have VOCs from caulk pollute the living space, BUT can someone offer reassurance that this is not only 'common practice' but that it is actually 'good practice' as well? Thanks.

  23. GBA Editor
    Martin Holladay | | #23

    Your worries are groundless. If your house gets flooded after construction is complete, you'll have a lot more important things to worry about than the moisture content of your bottom plates. (You'll be spending all day at the insurance office, filling out forms.)

    If you are worried about rain during construction, relax. Your bottom plates are hygroscopic, which means that they can dry out from all directions. Once the wall sheathing and the roofing are installed, everything will dry out.

    Installing a gasket between the foundation and the mudsill is, indeed (a) common practice and (b) good practice, as you put it. I would say more: it's best practice.

  24. Stockwell | | #24

    Is there any preference between EPDM gaskets or Denarco SureSeal?

    I seem to recall Dana(I think?) mentioning a cheaper alternative to the EPDM gaskets that Conservation Technologies sells, but I failed to write that down. Anyone know what that might be?

  25. Stockwell | | #25

    Has anyone used the products from ? They seem to be similar to Denarco's product.

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