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Community and Q&A

Advice on air sealing sheathing to foundation

cstarcher | Posted in General Questions on

I am building a house in climate zone 4B. The sheathing is Zip and the foundation is slab on grade. I am trying to determine the best solution for sealing the sheathing to the foundation on the exterior. I have read through many posts discussing tapes and liquid flashing, but I still have some questions.

1. Is a silyl-terminated polyether (e.g. Zip Liquid Flash, Prosoco Joint and Seam Filler) better than a regular polyurethane sealant (e.g. Masterseal NP1, Dymonic 100) for this application?

2. Are tapes (e.g. Siga Fentrim, Contega Solido Exo) better than liquid flashing (e.g. Zip Liquid Flash, Prosoco Joint and Seam Filler) for this application?

3. My foundation has mudsill anchors. Several of these have caused the concrete to crack on the vertical surface as you can see from the pictures. Because of this, would it be best to use liquid flashing instead of tape? If I were to use tape, do I need to repair these cracks before I apply the tape? If so, what is the best way to repair them so that the tape will adhere correctly?

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  1. George_7224612 | | #1

    I can't claim any qualifications to answer your questions, but that doesn't seem to be a requirement for rendering opinions nowadays.

    I watched a modular house being set on a mud sill that was tied down with those wrap-around anchors. They complicate air sealing because they create gaps under the sill and up the sides where the sheathing will be nailed on. So you're faced with not only sealing the gap between the mud sill and foundation, but also the gap between the bottom of the sheathing and the sill especially where the sheathing bumps over those anchors. My opinion is that fluid applied flashing would be easier to deal with and more effective. I think that tape would not work well with the irregularities in the concrete.

    Another concern I'd have is with what kind of nails are used to nail into that treated sill. I'd be concerned if they aren't made to resist corrosion from the sill treatment.

    I'm sure someone will come along who can render a professional opinion.

    1. cstarcher | | #2

      Thanks, George. I was thinking that fluid applied was going to probably be the better option as well. Otherwise, to use tape, I figured I would need to repair the concrete at each of these locations.

      I hadn't thought about these anchors creating a gap between sheathing and the framing. Would that need to be sealed separately?

      I do not know what type of nails were used, but I will inquire about it.

      1. George_7224612 | | #6

        Eric Whetzel has a blog about building his house. It can be found in the Guest Blogs portion of the GBA site. One of his postings about air sealing is here:
        It's well worth reading, as well as the rest of his blog.

        1. cstarcher | | #14

          Thanks, George. I'll check it out.

      2. KauaiBound | | #16

        Regardless of how you seal the foundation, you need to patch the concrete spalls to protect the holdowns. A decent drypatch will do the trick. Add some bonding agent/Elmer's Glue (yes, it works) to help bond the patch. Then give it a few days to cure before covering.

        I don't trust exposed-to-UV tapes regardless of manufacturer claims. As such, my vote is for liquid flashing with appropriate backer rod to provide proper hour-glass 2-surface joint. Do not just slather liquid flashing in that space.

        1. cstarcher | | #18

          Thanks again for the response, Kauai. I spoke with representatives from Sakrete and Quikrete, and they both told me to use a polymer modified concrete repair. I ended up using the Quikrete since it was available locally.

          I'm curious about your suggestion to use backer rod in this location. I have not seen anyone do this. I'm guessing you would need to adhere the backer rod before you applied the liquid flashing, is that correct?

  2. user-6184358 | | #3

    The mud sill anchors look like a how not to install photo. The bad anchors should be replaced with epoxy anchor bolts.

    1. cstarcher | | #9

      Thanks, Tim. I had a feeling there was a degree of installation error.

  3. Dayton | | #4

    I'm with George, not an expert, but I have been using both the Prosoco product line and Siga tapes to air seal my house energy retrofit and remodel. Both products are great, just pricey. The Joint and seam has a limit to what gap it can seal, so be sure to follow those guidelines. I think Prosoco does sell a rubber style membrane called surespan, ( to bridge large gaps by embedding the membrane in joint and seam sealer. I think that would be better. Siga tapes also bond to concrete with a pre-treatment primer ( I haven't used either method yet for my foundation-sill joint, but I will.
    Also agree you should epoxy anchor bolts to fix the poor foundation pour. Very easy to do, rent a drill and bit for masonry if you don't have one, and use simpson strongtie or Hilti epoxy made for that purpose.

    1. cstarcher | | #7

      Thanks, David. I have been leaning toward Siga Fentrim for a taped application. My understanding is that it does not require primer. However, in my case, it seems it would require a suitable surface for adhesion which I don't presently have.

  4. Expert Member


    The mud-anchors haven't caused the foundation to crack, the pour just wasn't vibrated enough at the top and ended up with honey-combing. it's not ideal, but depending on where you are (high seismic or wind area) may not matter much. I'd be inclined to grout the outside surface at each problematic anchor and go from there.

    1. cstarcher | | #8

      Thanks Malcolm. This house is being built near Lubbock, TX. We do not have high seismic activity, but we do have some wind. Should this be a concern?

      When you say grout, do you mean a non-shrink grout like this?

      Also, once this is done, would it matter if I used fluid applied or tape on the new surface?

      1. Expert Member
        MALCOLM TAYLOR | | #13


        It's hard to tell how compromised the anchors are. For what it's worth, my gut feeling from thousands of miles away is they aren't a big issue.

        Once the outside is smooth either tape of fluid applied flashing should be fine.

  5. CMObuilds | | #10

    Those pics look terrible. You could use Ardex to fix it up and make it look better.

    What is between the concrete and pressure treated bottom plate?

    1. cstarcher | | #11

      I believe it is Grip Rite subfloor adhesive.

      I know now that this is not ideal, but I didn't know in time to specify a different sill seal beforehand. At this point, I'm trying to do the best I can to mitigate the situation and create a proper air barrier.

  6. bennettg | | #12

    I'd think that if the voids around the anchors would impact sealing the sheathing to the foundation, it would be as simple as troweling mortar into them (and any other voids).

    1. cstarcher | | #15

      Thanks, Bennett. This seems to be the consensus. I believe this is the path I will take.

  7. PAUL KUENN | | #17

    I've just taken a 1.5" multi tool wood blade and shaved behind the hold downs and then hammered them flush. Works pretty fast and you can seal directly over them.

    1. cstarcher | | #19

      Paul, are you saying to do this to reduce the gap between the mudsill and the sheathing?

  8. PAUL KUENN | | #20

    Yes, that makes it all flush so the sheathing nails flat to the bottom plates and sill.

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