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Community and Q&A

How to insulate and/or air seal sill plate?

Carfar96 | Posted in Green Building Techniques on

Our insulation contractor recommended that our builder insulate or seal the area where the joists meet the concrete basement wall on the exterior. We have plans to spray foam the rim joist area from the interior, but the insulation contractor said this area can be a major air leak. He doesn’t do anything with the exterior and didn’t even have any product recommendations. This area will have siding on two sides of the house (where the grade slopes). Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    I'm glad that your contractor is giving you advice on air sealing. It's true that this gap is often a source of air leaks.

    The piece of wood on top of your foundation is a mudsill, not a joist. Usually (in a North American house built after World War 2) it's a 2x6 or 2x8, often pressure-treated, installed on the flat and bolted to the concrete.

    Ideally, your builders installed sill seal (usually made of thin closed-cell foam) between the concrete and the mudsill.

    It's good to take a belt-and-suspenders approach to sealing this gap. On the exterior, you can install a high-quality caulk (like polyurethane caulk) or (even better) you can bridge the gap between the concrete and the mudsill with a European tape like Siga Wigluv.

  2. Carfar96 | | #2

    Thank you! Unfortunately, we are on a tight timeline and I can't find the European tape locally. Do you have any suggestions for a lower-voc high quality polyurethane caulk that I might be able to obtain locally? Thank you!!!!

  3. GBA Editor
    Martin Holladay | | #3

    Any Home Depot or Lowe's will stock polyurethane caulk. Here is a link to a web page with some examples:

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