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Community and Q&A

Sill seal

StephenInCalgary | Posted in Green Building Techniques on

Can XPS rigid foam be used atop the basement wall in company with, or instead of, a sill gasket?

What is the best way, or material, for a sill gasket?


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  1. Expert Member
    Dana Dorsett | | #1

    The foundation sill has the weight of the house bearing down on it, and is likely to exceed, the elastic compression limits of the foam.

    Purpose made EPDM sill gaskets are hard to beat for air sealing, and as a capillary break between the concrete & wood.

  2. StephenInCalgary | | #2

    ok thanks.

  3. STEPHEN SHEEHY | | #3

    Conservative Technologies makes a good epdm gasket.

  4. Expert Member
    Peter Yost | | #4

    Hi Stephen -

    Sill gaskets, in addition to being an air seal, should be a capillary break. To be a cap break, the material must be non-porous. EPS porosity is to high to be considered a cap break. XPS insulation on the other hand is non-porous and can function as air seal and cap break, as can the EPDM gaskets.


    1. Expert Member
      Dana Dorsett | | #5

      >"XPS insulation on the other hand is non-porous and can function as air seal and cap break, as can the EPDM gaskets."

      Capillary draw issues aside, XPS should not be used under a load bearing wall without doing the math. It can't be recommended as a generic solution under sill plates supporting walls & joists the way EPDM sill gaskets can.

      1. STEPHEN SHEEHY | | #6

        In addition, a gasket is applied quickly and gets stapled to the bottom of the mudsill. Cutting up xps and sticking it to the sill ( sealant?) takes more labor time.

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