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Community and Q&A

Effective and affordable way to air seal between sill plate and slab

Myrtleboone | Posted in Green Products and Materials on

Hi. I am trying to find out more information on ways to effectively air seal the pressure treated sill onto a concrete slab? I have run across foam sill sealers, such as Pactiv Green Guard and have also read about acoustic sealant caulking. Of course, conflicting opinions exist.
Has there been any study on the longevity and effectiveness of these materials? For example, I can picture over a decade, that the acoustic sealant would become brittle and cracked, letting air infiltrate.

Any information would be appreciated. Thanks in advance.


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  1. GBA Editor
    Martin Holladay | | #1

    Foam sill seal is effective and inexpensive, so you should certainly include it -- unless, of course, you want to spring for a more expensive solution like Protecto Wrap's Triple Guard sill seal or an expensive European tape like Siga Wigluv (either of which is an excellent solution).

    Assuming you go with ordinary foam sill-seal, the next step is to caulk the crack between the slab and the pressure-treated plate with a high quality silicone or polyurethane caulk.

  2. Myrtleboone | | #2

    Thanks for the info. Now that I'm thinking about it, as my dedicated air barrier is further within the wall, should I skip the sill sealing at the outer wall and seal only the air barrier layer to the slab?

  3. GBA Editor
    Martin Holladay | | #3

    As long as you're not using fiberglass batt insulation, one air barrier is sufficient; you don't really need two air barriers. Since your slab is your floor air barrier, you need to connect the slab to your wall air barrier, wherever that is.

  4. Cjb80 | | #4

    When you say "caulk the crack", do you mean add caulking along the edge of the foam seal after the sill plate has been installed? It seems a better approach would be to put down a bead of caulk before the foam goes on the concrete, and then again on top of the foam (under the sill plate). Is that reasonable and effective?

    I noticed that you also called out a pressure treated sill plate. Do you still need a pressure treated sill plate if you use the foam?

  5. GBA Editor
    Martin Holladay | | #5

    Most building inspectors interpret the code to require both sill seal and the use of pressure-treated lumber for sill plates attached to concrete. Some builders have tried to argue in favor of using ordinary lumber at this location. Whether or not the argument would be successful depends on your local building inspector. I think it's best to specify pressure-treated lumber at this location, so I certainly wouldn't argue the point.

    There are lots of ways to seal this crack. You can use an EPDM gasket (installed before the plate is attached). You can use caulk, before or after the plate is attached. You should always include sill seal.

    Use a method that you feel comfortable with. In general, the more effective sealing products tend to be more expensive.

  6. Cjb80 | | #6

    OK, well, let me ask this: Do you think that "concrete-caulk-foam-caulk-wood" using ordinary/typical green foam would be an effective air seal?

    (Let's say "effective" means it meets PH standards)

    On an off-topic question, I wanted to confirm that interior walls do not need pressure treated wood on their sill plates.



  7. GBA Editor
    Martin Holladay | | #7

    If it is sitting on concrete, the bottom plate needs to be pressure-treated. At least that's the way we build in New England.

    If you want to meet the Passivhaus airtightness standard, I wouldn't depend on spray foam to create an air seal. I would choose an EPDM gasket or one of the European tapes if I were you.

  8. user-757117 | | #8

    I second Martin's recommendation to use an EPDM gasket like one of these:

    These gaskets conform nicely to irregularities in either surface and will retain their "memory" for a long time.
    These gaskets are also impermeable and do not require any caulking.

    I don't mean to gainsay Martin's comments, but where I live in Ontario it is quite common to use untreated lumber as a bottom plate provided there is an impermeable gasket between the bottom plate and the concrete.

  9. Myrtleboone | | #9

    I now understand the effectiveness and labor saving of Conservation Technology EPDM gaskets below the sill plate. How about on the top plate? Does the same thing apply to this area?

  10. user-757117 | | #10

    Do you mean between the top plate and rim board?
    If so, then there is a gasket listed for that application also though I've never tried it.

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