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Community and Q&A

Air seal sheathing to foundation

BurkeW | Posted in General Questions on

We are in the process of building a new house in Des Moines, IA (climate zone 5).   I’m looking for recommendations for products to use to air seal the Zip sheathing to the foundation.  Our foundation, along with most others around here, have a brick pattern presses into them from the forms  

I’ve been looking at the Prosoco  R-guard Joint & Seam Filler. Any experience using this product, or is there another you’d recommend?

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  1. Expert Member
    NICK KEENAN | | #1

    Normally the air seal at the foundation is a gasket under the mudsill. The rim joist goes over that and the interior of the rim joist is sealed. I guess you could seal between the sheathing and the rim joist before putting the sheathing on.

  2. Expert Member
    Michael Maines | | #2

    I've had good experiences using Siga Fentrim tape to seal the sheathing to the concrete foundation. It would take extra effort to conform to the indentations representing mortar joints but it should work. I don't have first hand experience with R-guard but I imagine it would work as well. As DCContrarian notes, standard practice is a foam sill gasket, but when the top of foundation wall isn't perfect it's not reliably airtight. If you plan to spray-foam the interior of the rim joist, the insulator can seal to the concrete with foam. But taping the sheathing to the foundation is the most reliable approach that I've found.

    1. BurkeW | | #3

      We will be using the usual sill seal (and potentially using some acoustical sealant on the top and bottom of it), and will be spray foaming the interior side of the rim joist.

      I’ve thought about tape, but for some reason I just don’t trust that I can properly roll it to the rough foundation.

      I did just find an article on FHB suggesting the Zip liquid flash, so that might be a good route.

      Thanks for the responses!

      1. Expert Member
        Michael Maines | | #5

        Burke, from what I understand, Zip liquid flash is made by Prosoco and is very similar to R-guard.

        1. BurkeW | | #12

          Good to know, thanks!

  3. ERIC WHETZEL | | #4

    I used the European-style tapes on our build, along with Prosoco's R-Guard series of products.

    Towards the middle/end of this post I detail my experience with both kinds of products:

    They both work well, although, as others have reported, there can be some 'origami' involved when using the tapes. It can be challenging to avoid 'fish mouths', or wrinkles, while using the tapes if you don't take your time to get it to lay perfectly flat as you go.

    If you're comfortable using a standard caulk gun, then the liquid membranes are fairly straightforward, although they can get messy, too, if you're not careful.

    In this Matt Risinger video, he shows how to use it while keeping a clean, neat finish:

    It's definitely worth the time to set-up the painter's tape (or something similar) in order to maintain a clean surface when done. Others have suggested doing the same in other articles/posts here on GBA.

    Even if you're not building to Passive House (or even, it's a good idea to air seal your house as well as you can (comfort/mold/energy demand), so, if you're interested, I've got an inventory of possible penetrations worth thinking about at the end of this post (in addition to your sheathing/foundation connection):

    I purchased the Prosoco products both from Amazon and this site:

    On a side note, if you're thinking about indoor air quality for your new home, a good resource in your area would be

    I've had good luck anytime I've ordered from them. They have a nice range of products.

    Good luck with your new home!

  4. AJ__ | | #6

    If your concrete is nice and level I would go with tape like Siga Fentrim. It doesn't need primer. My concrete was a bit up and down, plus some rushed cuts by the framer meant I wasn't working with straight lines so tape seemd like it would wrinkle and be difficult to apply. I used Zip liquid flash and personally I found it's really time consuming to get good coverage. The self spacing lip on the zip panels makes it trickier than working with just a square edge. The nice square sections I had I used tape and it's much faster. Fentrim has a fleece backing on it and I found the liquid flash works really with it if you wanted to mix the two.

  5. Expert Member
    NICK KEENAN | | #7

    Convince me I'm wrong -- the whole point of a discussion forum is to learn things -- but I don't see why the Zip should be sealed to the foundation at all. My thinking: the Zip serves as three layers of the building envelope, the weather barrier, the air barrier and the vapor barrier. At the foundation however, the weather barrier is on the outside but the air and vapor barriers should be on the inside. Concrete wicks moisture, which is why the IRC requires that their be a capillary break between the foundation and the mudsill and that the mudsill be pressure-treated. So the path for the air barrier and the vapor barrier is down the zip, across the mudsill, and then to the interior sealing.

    The path for the weather sealing is down the zip and then down the foundation. However, where the zip meets the foundation I would argue that any sealing doesn't keep rain out, but instead prevents water that gets in from draining to the outside, forcing it to the inside.

    1. creativedestruction | | #8

      Only the outside coated face of the Zip sheathing serves the three barrier functions you named for walls. Without a robust sealing method from that outside face of sheathing to the face of foundation, it is incredibly difficult to otherwise achieve an airtight transition to the foundation control layers. The sill gasket is the vapor control at the top of foundation, but it's insufficient as an air barrier transition if it doesn't fully interface with the outside face of sheathing. Blower door and infrared experience tells us this.

      Drainage of bulk water occurs at that outside face. The minimal water that gets behind that should be manageable by diffusion. If it's not, we have bigger issues...

      1. BurkeW | | #11

        I think Jason S said it well. My understanding as to why this detail is needed is that the air barrier needs to be continuous. There is no real way to “tie” the exterior face of the zip to the sill seal, so instead you seal the zip to the foundation. I also think stack effect implies that the most vulnerable spots for air leakage is going to be bottom and top of the well, which incidentally is where materials transition.

    2. maine_tyler | | #14

      DC, I've had concerns similar to yours after seeing sheathing that was tight to the foundation show mold/rot.

      It seems to me that a VB should be fully between the concrete and any wood parts, which seems to preclude the discussed detail as I understand it.

      Of course the detail itself isn't precluded—one would just have to add a step before the final seal coat (such as applying R-Guard, tape, etc. to the concrete BEFORE placing the sheathing, then again after to make the seal. But I don't think that would be typical, and would certainly be more expensive.

      Is this simply not a concern?

      What I have done (perhaps a finicky detail) was to use edpm gaskets to seal the sheathing to the sill plate, and sill plate to the foundation (along with some poly draped over and acoustical type sealant to completely isolate the concrete from the sill plate), thereby eliminating the need to seal the sheathing to foundation itself. In the end, it was a continuous air barrier with a VB between the concrete and all wood components. But not as simple as running tape/sealant along the sheathing-concrete interface.

  6. Trying2save | | #9

    I'm actually excited that their is an active thread about the situation im having.
    Objective: air seal for an unfinished detached garage.
    Location: Central TX (rural)(hot summers but windy winter)
    I have a few tunes of dap alex Jones plus from a previous project and want to know if that is recommend to use to help seal some of the area where a there is liner houses between the wall studs and sheathing before I install insulation bats.
    I have like 5 tubes of alex plus will it keep a seel? I have also have some 1/2 in poly board that I think would be good to couple with the dap Alex plus. What should I do?

  7. Trying2save | | #10


  8. user-2310254 | | #13


    I'm not sure the latex caulk with maintain a seal over time as the framing moves. (This also is an argument for not using the cut and cobble method you are considering.) If I didn't have access to the assembly from the outside, I might consider taping the building paper and using Prosoco (or a similar product) to caulk the exterior of the framing. Let's see if the expert think those steps are worth the effort.

    I suggest reading Eric's blog posts (links in #4) and following his advice.

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