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Air sealing with a sill gasket with a cantilevered sill

ranson | Posted in Green Building Techniques on

The 2×6 sill of my house will probably overhang the edge of the slab by an inch or so. Will an EPDM sill gasket from Conservation Technology still provide an effective air seal, since only one of the thickened edges will make contact with the concrete?


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  1. GBA Editor
    Martin Holladay | | #1

    A 2x6 is 5.5 inches wide. If your 2x6 sill is cantilevered 1 inch, that means that about 4.5 inches of the sill will be in contact with the concrete.

    So you should buy the BG63 gaskets from Conservation Technology -- the 3.5-inch-wide gaskets designed for 2x4 plates -- rather than the BG65 gaskets designed for 2x6 plates. See the illustration below from the Conservation Technology web site.


  2. ranson | | #2

    Let me extend this question. My walls will be similar to this:

    If I'm using EPDM for the outer wall, how should I consider providing a capillary break for the rest of the wall, including the space between the plates.


  3. GBA Editor
    Martin Holladay | | #3

    If the slab is detailed as shown in the illustration you linked to, it is likely to stay dry. So I wouldn't worry about a capillary break.

    It never hurts, however, to have redundant approaches to air sealing if you can afford them. For that reason, you might want to consider a method of sealing the gap between the bottom plate and the slab to reduce air leakage. Options include another EPDM gasket, ordinary foam sill-seal, or caulk.

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