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Community and Q&A

Air sealing Thermax to a basement floor

user-6828388 | Posted in Green Building Techniques on

What is the best material to use form an air tight seal between Thermax foam board insulation and the floor of my basement? I know that gaskets and tapes are preferred for air sealing. Any product suggestions? I’m going to use venture 1520 tape to seal between the gaps between Thermax sheets, but I don’t think this will adhere to concrete.

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  1. GBA Editor
    Martin Holladay | | #1

    The simplest solution is a high-quality caulk. If you want to use tape, the tape to choose would be Siga Fentrim tape.

  2. walta100 | | #2

    There are a few things we all know about concrete.
    1 All concrete walls will crack somewhere at some time.
    2 Concrete is full of water when we place it and it likes to be wet.

    Given the realities and the fact that you likely want a dry basement without a musty smell. I think it is worth the small energy penalty to hold the Thermax off the floor a few inches so any water that may collect behind the foam has an easy way out and will dry quickly. Due to a shipping snafu my Thermax is 7 inches off the floor the plan was 3 inches.

    I say save your air tight efforts for the highest point in your home where the stack affect will push the air out.


  3. Expert Member
    Dana Dorsett | | #3

    Thermax (like all polyisocyanurate) will wick moisture if the slab is damp. It's better to stop the polyiso above the high tide mark of any prior flooding and continue downward with polystyrene. If it's small gap, say an inch or so, fill it with 1.5-2lb spray polyurethane (FrothPak is fine), which will air seal and stick to the concrete, assuming the concrete is both dry, and at the recommended surface temperature range specified by the spray foam vendor.

  4. user-6828388 | | #4

    The worse water ever is during the spring or during heavy rains. It came in between the the gap between the slab and the footing at a single corner of the house maybe 1/16 to 1/8 of an inch. I caulked this with a concrete polyurethane and it stopped the leak. Thanks for the advice about keeping the polyiso off the ground. On this side of the house I was planning on installing ground gutters next summer to help with bulk water removal.

    The only problem that I have is that I was planning on using an adhesive to secure the insulation. Now I'll have to lay lumber under the insulation temporarily to hold the sheets up or I could use polystyrene which I have leftover from a my attic insulation retrofit. The polystyrene would be cheaper than spray foam to insulate this bottom inch or so, but I guess I would still have to seal between the polystyrene and the polyiso to prevent air leaks and condensation issues between the polyiso and the concrete wall.

    Now I wonder should I used closed cell foam/polystyrene for this bottom inch or should I just leave it bare especially in this questionable corner of my house where water did enter before? This would allow me to monitor the condition of the caulking to prevent future water intrusion, but would be a gap in the thermal boundary. It is the lowest point at least 5 feet underground so it should be a fairly stable temp.

  5. user-6828388 | | #5

    I've started cutting 1 inch risers out of for my polystyrene to elevate the Thermax above the slab. What should I do for air sealing details? Should I air seal the bottom of the Thermax to the wall using the foam adhesive I have and simply leave the 1 inch polystyrene as a friction fit for removal or should I also caulk the polystyrene to the concrete slab and the gap between the Thermas and polystyrene?

  6. GBA Editor
    Martin Holladay | | #6

    I think you are overthinking this. Whether you air seal this area won't matter much when it comes to thermal performance.

  7. Expert Member
    Michael Maines | | #7

    @Walter, I absolutely disagree with this comment: "1 All concrete walls will crack somewhere at some time." Properly designed, placed and cured concrete walls do not crack. If you are a construction or design professional and your foundations are cracking, you should educate yourself on how to do it properly. I hate seeing this myth propagated, as if people have no choice but to be stuck with crappy construction techniques.

  8. walta100 | | #8


    Before you dismiss my statement as a myth please read the website

    From “When placed properly and in the right application it will last a long, long time. However, (don’t you just love it when a “however” follows an opening positive statement) nothing good lasts forever and concrete is no exception. It will crack; it is just a matter of when. Take a look at the sidewalk, the driveway, even the floor in stores where the concrete is not covered with tile. They have cracks.”

    I will say a crack wider than a quarter and outside of a control joint is an indication of poor workmanship.


    Please note I deleted the part of my post that was clearly wrong

  9. Expert Member


    Among other things Michael is a structural engineer.

    Edit: Walter, even with your edit we are still taking apples and oranges. Slabs on grade are hard to reinforce because they thin, essentially receiving loading from both sides, and rely on the integrity of the substrate for support. Foundation walls shouldn't crack.

  10. Expert Member
    Michael Maines | | #10

    Malcolm, thanks for the defense. I'm not a professional engineer but I do have a BS in engineering through the civil engineering department of the university I attended. It was a few classes short of what those who went on to become PE's took, because my degree also included most of an architectural history degree. (If I recall correctly I was short one class each on thermodynamics, advanced steel structures and advanced concrete structures, but it's been a long time.) I've tested a lot of concrete cores, built concrete canoes (our team came in #2 in the nationals!) and designed and/or built quite a few homes and renovations. My dad was a licensed PE and designed concrete interstate highways; he taught me quite a bit as well. His section of highway, in Maine, did not crack for the 30+ years it was in service, while the next section did, and he liked to explain why.

    Walter, I'll admit that it's true you cannot absolutely guarantee that concrete will not crack, as there are many factors that all affect each other. For example, it's very hard to prevent some shrinkage cracking at inside corners on slabs, where shrinkage stress concentrates. But if you put rebar diagonally across that area, use a mix stronger than code minimum, and allow it to cure slowly for full hydration, there is a very good chance it will not crack, and if it does crack it will be well-contained.

    For residential walls, it's very common to use a code-minimum mix, with little or no reinforcing, and to strip the forms as quickly as possible. It makes sense; forms that are left on a few days are hard to pull. But when you pull the forms early, the water in the concrete evaporates before full hydration occurs, so you end up with concrete that can hold itself up but that's about it. There are many ways around this problem, but saying "it's just going to crack anyway" is the laziest.

    This is a good resource on how to design and place concrete to minimize the chance of cracking: The IRC building code has good info on proper substrate prep, drainage, backfilling, mix design and rebar schedule. As with most things code related, going above code usually provides superior results.

  11. Expert Member
    MALCOLM TAYLOR | | #11


    My mistake. I have trouble keeping your various roles straight. Cabinet maker, designer, builder and (almost) PE. That about right?

  12. Expert Member
    Michael Maines | | #12

    Malcolm, yeah, I have trouble sticking with one profession I guess. I'm also a serious hobby farmer (my wife uses the herbs we grow for her business, making skin care products and herbal teas, so I guess I'm semi-pro?) and occasional author...

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