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Will spray foaming my basement rot my sill plate?

McLean28 | Posted in General Questions on

Hi Martin or whoever would like to add input to my query. I live in Eastern Canada near the coast in a Zone 6/7 environment. I live in an old house with an low ceiling unfinished basement that is freezing cold all winter.

I was 100% ready to proceed with spray foaming this year however a little bit of lookin around online has me somewhat concerned. My sill plate sit directly on the concrete, a 10×10 timber, quite weathered, water damaged on the exterior where it’s is partially exposed. My basement walls a mostly subgrade with about 12 inches above grade. As I spray foam I orginally had intended to spray up the concrete directly over my sill plate right up to the subfloor.

I see elsewhere a theory that water pushes in traps itself behind the closed cell foam and through capillary action goes upward into my sill plate ( no damp proofing ) and accelerates the rot process dramatically. Around my sill plate is the draughtiest spot in my basement so I would really like to be able to spray foam all the way to the subfloor but am scared to accelerate rot in my very old sill plate.

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  1. GBA Editor
    Martin Holladay | | #1

    There is no doubt that water wicks up a foundation due to capillary action. Ideally, you need to have a capillary break (for example, foam sill seal) between the top of your wall and your sill.

    Currently, your sill can dry in both directions. When you encapsulate the interior side of the sill with closed-cell foam, you are reducing or eliminating one drying path. That means that your sill won't dry as fast -- it can only dry in one direction.

    If the sill gets sun, and is well above grade, and isn't shaded by bushes, this may not be a problem.

    In some cases, however, the foam can makes things much worse.

    If you are worried, you can jack up your house 1/2 inch and slide a rubberized membrane between the foundation wall and the sill. It's not as hard as it sounds. Here is an article that talks about the process:
    Rubble Foundations.

  2. McLean28 | | #2

    Thanks Martin for the very quick response. Confirmed what I had more or less figuired to be a real concern. Jacking the house seems daunting for an advanced do it yourselfer but the instructions are detailed. Thanks again, what a fantastic website this is.

  3. charlie_sullivan | | #3

    Martin says it might or might not be a problem, but it sounds to me that you've already confirmed that you have a problem, and I think it's certain that the problem will be at least a little worse with spray foam. I'm thrilled to learn that it's actually feasible to put in a membrane. That sounds ideal for this situation. It's also possible to reduce the moisture in the concrete--steps that help that would be insulating the exterior of it (being sure to use vapor permeable inslulation for the above grade part), and if you've dug up the outside to install that insulation, waterproofing the below-grade concrete and adding a perimeter drain if you don't find a working one there.

    Note that spray foam emits gasses with a radically high global warming impact. You might consider EPS boards instead if you are interested in your global warming impact.

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