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Community and Q&A

Window flashing tape and my own “bathtub test”

DarkNova | Posted in General Questions on

I’m trying to settle on which flashing tape to use on the sill of windows. A lot of people seem to use Dow FlexWrap but I am a little concerned about the tape sticking when applied now (as the temps are below freezing). 3M 8067 All Weather Flashing tape is an acrylic which is rated to stick down to 0F so that looks like it has a lot of potential. The main negative with that that I see is since it can’t stretch like FlexWrap, doing the corners is more complicated. 3M has a video showing the installer applying a couple of layers to handle the corners — I’d be interested in hearing if anyone has an opinion on how they are doing that, if they see a flaw in doing it that way, or does that seem acceptable?

I was also curious about how waterproof the tape would be considering there would be seams. I’ve read Martin’s article about a “bathtub” test of peel-and-stick flashings, but this tape wasn’t around then so it wasn’t tested. I decided to do my own simple test for that. I took a small cardboard box and lined it with the 3M 8067 tape. I used the 2″ wide tape so there are actually 3 seams on the bottom of the box that water would sit on top of. I just pushed the tape down with my fingers and filled the bottom of the box with about 1/4″ of water and let it sit. I also put a paper towel under the cardboard box to make it more obvious if it was leaking. I left it for 72 hours (overkill I’m sure but I kind of forgot about it) and was pleased to find no indications of even a drip of a leak. The bottom was totally dry. Now, this isn’t scientific or anything, but considering the water was on 3 seams and I didn’t use a J-roller or let the tape sit for more than a minute before filling it with water, I feel pretty good about the tape’s ability to be waterproof.

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  1. DarkNova | | #1

    Here is a picture of the box

  2. GBA Editor
    Martin Holladay | | #2

    Thanks very much for sharing your findings. I hope that the backyard test movement grows!

    Your test is an interesting indication that liquid-applied flashings (all of which apparently performed worse than the 3M tape in your test) may not be ready for prime time.

  3. JC72 | | #3
  4. Adam Emter - Zone 7a | | #4

    The 8067 tape actually stretches pretty well. I have never used FlexWrap, but have used other brands of flexible flashing tapes and the 8067 is more tenacious in my opinion. The only drawback that I can see is in the thickness: the 8067 is pretty thin and more susceptible to damage than other thick flashing tapes. All in all, I really like the 8067 tape and use it quite a bit. Its vapor permeability is very low, something to keep in mind.

  5. Expert Member

    I'd make the distinction between Flashing Tapes and Peel and Stick Membranes. For sill pans I would use a thicker membrane and reserve the 3M flashing tape for sealing your WRB to the window flanges once they are installed.

  6. Brian Knight | | #6

    I agree with Malcolm there. Sill pans are exposed to abuse during window installations, especially with bigger windows. The sharp edge of a shim might be enough to pierce it with the weight of a window bearing down but its more about setting the window that brings the risk. Even the new ZIP stretch tape seems a little light on mils and is not cheap.

    Flexwrap is awesome but very expensive and usually special order. We use more affordable and commonly available tapes like Protecto wrap and Fortiflash. Here's how we deal with stretching the corners in this weatherproof window installation video.

    Me and many of the carpenters I work with are whitewater kayakers and we are always putting flashing tapes to the test with our broken or cracked boats. 8067 is definitely a top performer but like most tapes, doesn't take well to being scraped by rocks. If you can get a good seal from the inside though, it keeps out water really well.

  7. DarkNova | | #7

    Well now I've heard from a couple people that the 8067 might be too thin, so I'm seriously thinking about this. It seems waterproof but of course not if it gets torn. My main concern about the FlexWrap is applying in low temperatures. The datasheet says it works best when applied at temperatures above 25F. We would be applying right around 25F probably. Does anyone have experience applying at colder temperatures? I don't want to spend all the money on FlexWrap and go to apply it and it doesn't stick. Thanks.

  8. GBA Editor
    Martin Holladay | | #8

    "I don't want to spend all the money on FlexWrap and go to apply it and it doesn't stick."

    That's why they invented hair dryers.

  9. Expert Member

    To be fair those aren't your only options. For window sills we use Blueskin (or similar) peel & stick membranes that don't bend and place bow ties at the corners.

  10. DarkNova | | #10

    Hair dryer -- genius! You can tell I'm a novice :-)

  11. Expert Member
    MALCOLM TAYLOR | | #11

    Brian definitely wins the "backyard test" award.

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