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Can I use spray foam to insulate a mudsill sitting on shims?

Sam2112 | Posted in General Questions on

Work continues on modernizing my 1973 cape cod in southern NH. Like most houses of this age in our area, the mudsill is not made of pressure treated wood, and the foundation is poured concrete, but it isn’t sealed. 

The builder seems to have been aware of potential moisture issues because the house is literally sitting on shims placed between the mudsill and the foundation. In most spots there’s about a quarter inch gap, sometimes stuffed with fiberglass insulation, sometimes totally empty. 

Currently this gap is allowing huge amounts of air into the basement, not to mention armies of insects. There were thousands of lady bugs and a few snakes hibernating in the basement when I bought the house a few months ago. 

Now that spring is here and the critters have moved out, I want to seal this gap. I was thinking of replacing the current shims with pressure treated ones and filling the gap with canned spray foam. Can spray foam be used as a capillary break? 

There’s no way it would be worth it t jack up the house and install a proper gasket. 


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  1. Expert Member


    That sounds like a good plan. Use closed cell canned foam to stop the moisture.

    1. Sam2112 | | #2


      1. Expert Member
        MALCOLM TAYLOR | | #3


        I bet you notice a big difference once things are sealed up.

        1. Sam2112 | | #4

          That's the hope! My only worry was that some people were saying the shims the house is sitting on could degrade if surrounded by shims. My thought was that PT shims would avoid the issue.

  2. AC200 | | #5

    If the gap is only 1/4 inch, I'd consider following up the spray foam with a bead of acoustic sealant inside. But I like to belt and suspender everything.

  3. Expert Member
    PETER G ENGLE PE | | #6

    You might be able to find plastic shims to use instead of PT wood. This way, no moisture is wicked up into the mudsills. Metal would work too, but might be harder to find/make the right size. And for a final caulk seal, you wouldn't need acoustic sealant. Any urethane or other sealant specified for masonry and wood joints that wide would work. Just apply a bead of sealant and then tool it to a concave shape, with a liberal smear on both sides of the joint.

    1. Sam2112 | | #8

      That's a good point, I'll try to track down some plastic shims just to be safe.

  4. Expert Member
    Akos | | #7

    I would get a sheet of heavy duty plastic (I've used those plastic magic carpet sleds for something similar) under the new shims. This will prevent any vicking so the shim material doesn't matter.

    If your critters can chew through foam, might be worth to seal with flex caulk on the outside first before the spray foam. Thick caulk like this won't stay air tight over time without proper backer rod install though so spray foam on the inside is still a good idea. One of the smaller spray foam kits makes quick work of this, a couple of the 12 board foot kits might do it if a small area. Just make sure you have a helper that keeps an eye on the cans and keeps them vertical, you never want to run out of one part while spraying.

    1. Sam2112 | | #9

      If i can't easily find some synthetic shims, I will definitely put down the plastic!

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