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Community and Q&A

Brand of tape to seal below grade polyethylene beneath a slab?

Myrtleboone | Posted in Green Products and Materials on

Hi. I am trying to find a relatively affordable but high-performing tape (and/or caulk) to seal the seams of polyethylene that will be placed between below grade XPS and a concrete slab.

Any suggestions? Some say I should caulk and tape? Overkill?

Lastly, any recommendations on poly thickness? I was thinking 10 mil for durability to prevent tearing during installation and prior to slab pouring.

Thanks in advance.


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  1. GBA Editor
    Martin Holladay | | #1

    The main purpose of the polyethylene installed under a slab is to act as a vapor barrier. Because it is a vapor barrier, and not an air barrier -- the concrete is the air barrier -- there is no harm in having occasional rips, splits, or flaws. Even if 4 percent of the polyethylene is entirely missing, the vapor barrier will be 96 percent effective -- which is fine.

    It's always a good idea to install 3 or 4 inches of crushed stone under your rigid foam as a capillary break.

    If you still want to tape the seams of your polyethylene, the least expensive tape that performed well in my tests is Polyken Shadowlastic 627-20 tape (31 cents per foot). For more information on tapes and tape performance, see Return to the Backyard Tape Test.

    Because small rips and tears don't significantly affect performance, there is nothing wrong with using ordinary 6-mil poly in this location. If I were you, I wouldn't bother paying for thicker polyethylene.

  2. Expert Member
    Dana Dorsett | | #2

    I've read there is some evidence that 10 mil poly has a measurable improvement over 6 mil from a radon diffusion point of view, but that's probably not going to be the make/break factor for whether more radon abatement is called for.

  3. brp_nh | | #3

    Inexperienced owner/builder here with only 1 slab pour experience, but here's my two cents...

    If you get the poly in large rolls (20' x something) you may not have many seams depending on house size. But it wouldn't be a bad idea to tape seams, even just to help keep things in place during the pour.

    10 mil poly wouldn't hurt as the material will get lots of abuse prepping for and during the pour. But plan ahead to have this on hand, we couldn't find poly thicker than 6 mil in stock in our area, even at dedicated building/lumber supply places.

  4. dickrussell | | #4

    When I did mine, using 6 mil poly, I used a product listed as "Scapa 136 Polyethylene Film Tape" on I got a quantity of 2" by 36 yard rolls, with a unit cost of $7.16 (almost three years ago). That works out to 6.63 cents/foot. I found that the tape stuck very well indeed to the poly. As Martin noted, the taping isn't all that important from a vapor barrier point of view. I viewed it as insurance toward keeping the taped assembly of sheets intact during the pour. I can't say how the tape has fared over time, and I'm not inclined to jackhammer the slab to find out. I used the same tape for installing MemBrain as an interior air barrier, and the tape sticks very well to that material and also to the wide flanges of the plastic AirFoil electric boxes used on exterior walls. The poly, MemBrain, and box flanges are smooth plastic surfaces.

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