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Community and Q&A

“Cheap” Alternatives for Sealing Sheathing/Foundation Interface

mrigney | Posted in General Questions on

Wasn’t sure where this question fit, so put it in general. About to break ground on our house this week (well, the 1,000 ft driveway at least). We’re building a relatively small (2100 sq ft) house on 16 acres. Shooting for a HERS score of 50-60. Most builders around here don’t typically build overly green houses (no real demand, northern edge of Climate Zone 3). Our builder is more cognizant than most (standard use of Zip system, does a lot of the air sealing stuff I asked for standard), but I”m still asking him to do some things outside their normal operations. Two things that I’ve asked (and he’s fine doing) is to use an improved sill gasket and to tape the interface between the sheathing and foundation.

I went ahead and suggested the BG65 gasket from Conservation Technology ( and the Fentrim tape from Siga ( Like I said, he has no problem doing it, but his comment was, “those are pretty expensive.” I suppose they are relative to what typically gets used, but didn’t think too much about the cost. But, with that said, would anyone have recommendations for alternative products? For the tape, was looking to stick with something that didn’t require a primer on the slab just to keep the process more in-line w/what his subs are used to doing.

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  1. creativedestruction | | #1

    This is a difficult connection to airseal well. Fluid-applied or caulking tends to bond best to concrete. I haven't seen too many tapes fare well without first priming the concrete edge.

    Search "seal concrete to wood". You'll find lots of articles and suggested products. Your builder can help navigate.

  2. Expert Member
    Michael Maines | | #2

    Siga Fentrim tape is somewhat expensive, but I have not found another product that will stick as reliably to relatively fresh concrete without needing extra steps. Previously I spec'd Protecto Triple Sill Seal, a clever design but it doesn't always bond well to the concrete. One alternative I've proposed on a few projects is to cover the top of the wall with Vycor (or similar), extending 1/2" to 1" past the exterior face. Then seal the sheathing tape to that. The Vycor doubles as a capillary break.

  3. tjanson | | #3

    There's Zip Liquid Flash. A 29 oz tube costs $40 and, according to Huber, covers about 30 ft of panel seam, so might not be any cheaper then 6" Fentrim. I would figure it have less linear coverage for a sill to foundation seal. I just bought some for my garage sill to try to rot proof the sill and bottom of sheathing, but haven't used it yet.

    1. mrigney | | #4

      Yeah, good point on the Zip Liquid Flash. I had originally suggested a tape to the builder just b/c it seemed "easier" from the perspective of less work for the subs (since this is non-standard for them). But I'll talk to him about a liquid flash. I'd prefer that all things considered, and there are other options out there like Prosoco's joint and seam filler. I think it's a little cheaper than the Zip Liquid Flash (looks like $30 for a tube that should cover about 30 linear feet). I've told the builder I'm happy to go out there and slap it on myself if need be.

      This is changing the focus here slightly, but why don't builders make sealing the sheathing/slab interface a standard practice? It seems relatively low cost/easy to me (but maybe I'm missing something) and the return on investment seems high.

      1. AJ__ | | #7

        If you can, I'd use the fentrim tape. I'm doing this detail right now and chose to use liquid flash because my concrete wasn't square, level or straight, so in some places the plates are flush with the concrete and others not. Add in the inconsistencies in how flush the sheathing is with the bottom of the plate, rough/pitted concrete finish in places and it didn't seem like using a tape was the best idea to get a good seal without fighting wrinkles etc.

        The liquid flash takes a long time to get right. My sheathing runs horizontally and the self spacing lip on the zip panels make it tricky to cover the exposed osb. It takes a lot of care and patience. It's not the prettiest either. It's also pretty humid in my area right now and the liquid flash gets tacky faster.

        So if your situation permits, a tape might well be the way to go.

      2. Patrick_OSullivan | | #8

        I use Henry AirBloc LF for a lot of these joints on my house. Like Zip Liquid Flash and Prosoco products, it's an STPE chemistry. It runs $15-16/20 oz. sausage depending on where you get it.

        (As an aside, I've had my fill of 29 oz. Zip Liquid Flash tubes. I've had multiple fail at the back plunger, making a mess of my very nice Makita 18V caulk gun. Never have this problem with the much cheaper PL Premium tubes I've run through it.)

      3. George_7224612 | | #10

        If you're thinking about Prosoco, take a look at World Class Supply
        I gooped my house with Prosoco that I bought from him. He was less expensive than anyone else that I found.

  4. user-5946022 | | #5

    If you are at all in a termite area, consider a product like PolyShield's Term barrier on the foundation - that will give you a termite barrier and a capillary break. Then use an EPDM sill seal. How well this entire thing seals depends upon how flat and level your foundation is...which the builder theoretically controls...

  5. user-6184358 | | #6

    I used 1/2" drywall edge profile over the shear panel to the sill & set that in a bed of Sikaflex. Then I had a good 90 degree corner to add a bead of Sikaflex from the metal to the concrete foundation. The peel & stick WRB was then stuck to the edge metal for a no exposed wood edge at the bottom of the wall. It was a DIY and don with parts from the big box stores.

  6. Expert Member
    Akos | | #9

    I've used WP200 with the spray primer. Pretty quick application and sticks quite well. If you also use this for waterproofing your foundation, running a couple of inches up the wall over the sheathing adds very little extra cost or labor.

  7. andy_ | | #11

    One of the best ways to get a good seal to the foundation is to get a good foundation. Make sure you've got a better than average concrete crew when it comes time to form and pour. Even then, there will be some inconsistancies, but nothing a grinder with a diamond cup wheel won't fix. A morning spent finding and knocking down high spots with a laser and grinder will pay dividends many times over from framing to finish.

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