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Community and Q&A

Missed Thermal Bridging in the Slab

utah_matt | Posted in Green Building Techniques on

We missed putting the thermal bridge in the doorway and just poured a 4″ slab through it. Is there any reasonable solution? Climate zone 5, Salt Lake City. See attached picture.

Thanks!

Matt

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Replies

  1. Expert Member
    PETER G ENGLE PE | | #1

    No great solutions, but it's not going to really cause much trouble either way. You can minimize the heat loss by installing an inch or two of edge insulation between the floor slab and whatever your stoop treatment will be. Depending on your floor coverings, even a half inch of rigid foam on top will warm up the slab at the threshold. Of course more would be better, but eventually you get a big enough step that people trip over it. It wouldn't be all that hard to cut the concrete out of the door opening and replace it with a 2" concrete (or stone) slab with 2" of XPS foam on top. That would be plenty stable for installing a threshold over it.

    The biggest issues with it as-is, are: 1) some heat loss. If you are going to super high performance, you will notice the difference. For a PGH, probably not. 2) There is some potential for warm, humid interior air to condense on top of the slab, under the threshold in winter. But in Utah, the air is pretty darn dry and in winter, the interiors are even drier. So unless you have some reason that the interior air will be particularly damp in winter, this is probably not an issue for you.

  2. utah_matt | | #2

    Condensation is not a problem in winter. My current home is slab on grade with no insulation for the concrete and it has no issues.

    Would that 8” concrete lip break off of we did a 2” saw cut and used a can of spray foam in the 2” gap we cut?

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Matt,

      Is this a garage?

      1. utah_matt | | #4

        No, it is a walkout basement. I just added a photo that shows what it was like before we added the slab. I want to fix it but maybe it is more headache than it is worth. My contractor has not got back to me with his perspective yet. I suspect he will recommend letting it go.

        1. Expert Member
          MALCOLM TAYLOR | | #5

          Matt,

          I was just confused because it is rare to see a slab extending out between the stem-walls. If you decide to fix it, it's probably easier to remove that chunk of slab (saw cut or just jack hammer), continue the insulation, then re-pour the sill with several anchor bolts to keep it in place. It may well be more of a headache than it is worth to do.

          1. utah_matt | | #6

            Thanks for the reply. There is about 200 linear ft of basement wall and this 4ft represents 2% of my wall missing insulation. I will have to see if the contractor has any ideas.

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