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Community and Q&A

Poly under the sill plate

Alex P| Posted inGeneral Questionson

I’ve seen a few projects use poly under the sill plate with traditional sill seal on top. Steve Baczek’s passive house build featured on this website comes to mind.

When does this become a good idea? I’m soon to begin framing and undecided on whether to include this or not. What do I lose by not including it?

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Replies

  1. Expert Member
    Malcolm Taylor||#1

    Alex,

    I'm struggling to think of what purpose the poly would have that the sill-seal doesn't fulfill. What's your air-sealing strategy at the bottom of your walls? Tape? Sealant?

    1. Alex P||#2

      Liquid flash from sill to framing and maybe another bead of sealant between the concrete and sill seal, or on the interior where framing meets the concrete.

      1. Expert Member
        Malcolm Taylor||#3

        Maybe someone else has an insight into why Steve Bazek used it?

  2. Expert Member
    Michael Maines||#4

    Sill sealers are almost never airtight. The poly is the air control layer and is connected to the wall's air control layer. At least that's how I often detail it. There are other ways to ensure a continuous air control layer.

    1. Expert Member
      Malcolm Taylor||#5

      Thanks Mike,

      So Alex is fine omitting it as his air control layer is the sheathing and he is sealing it to the stem-wall.

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