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Community and Q&A

Clear 6-mil polyethylene vs. 10-mil proprietary sheeting

user-4435615 | Posted in Energy Efficiency and Durability on

In most of the correspondence at this site, I see clear 6-mil poly being used and recommended as an under slab vapor barrier. There is a proprietary brand of 10 mil polyolefin which makes a claim, which is that the alkali from concrete degrades “ordinary” poly over time which leads to the breakdown of the poly and hence the vapor barrier. Their pitch is of course that you only get one chance to get that sub-slab vapor barrier right and that you should use their product.
Is that advertising hyperbole or is that alkali degradation something which which we should be concerned?

I see most examples that depict the installation of sub-slab poly with the adjacent sheets of poly overlapped somewhat and TAPED.
Is there really any point in taping the overlapped adjacent sheets?
Perhaps use a couple “spots” of tape just to hold the sheets in place while the concrete is poured. According to my reading of Lstiburek literature, the vapor barrier will not be breeched if the sheets are simply overlapped; and, the concrete slab serves as the air barrier.

In a recent blog about the houses being erected at Dartmouth the PROPRIETARY poly sheeting was brought across the top of the foundation stem wall and up the outside of the Zip sheathing to be taped. If instead, “ordinary” poly were used, would that exposure of the poly between the top of the foundation and the point of taping, though covered by siding, be cause for concern of degradation. (I realize that all portions of of the ‘exposed’ poly in this area need to be taped)

Is there a difference between CLEAR and BLACK poly?

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  1. GBA Editor
    Martin Holladay | | #1

    Clearly, 10-mil polyethylene and Tu-Tuf are more robust and less likely to be punctured on a job site than 6-mil poly. If you are willing to pay for that feature, buy 10-mil poly. If you aren't willing to pay for that feature, stick with 6-mil poly.

    In my experience, poly doesn't degrade when it is in contact with concrete. (Degradation is caused by exposure to sunlight, and concrete protects the poly from exposure to sunlight.) If anyone has any examples of poly degraded by concrete, please share those stories here.

    As Lstiburek has written, if you are using poly as a vapor barrier, you don't have to tape seams or worry about a few holes. If you are using poly as an air barrier or radon barrier, you want to tape all seams, holes, and penetrations.

  2. charlie_sullivan | | #2

    Black poly is sometimes black as part of an attempt to make it more UV resistant--not an issue under a slab.

    There are actually four things you might want poly for below grade: to stop cold outside air coming in, to stop radon coming in, to stop water vapor, and to stop bulk liquid water flow. Since three of those four rely on not having holes and having it taped, I think it's worth avoiding puncturing it and taping it well. If you have confidence in your drainage system, you should never need to depend on the poly to stop water intrusion, but it's hard to have confidence in things that are buried underground and can't be directly inspected.

    As far as I can tell, the yellow color is just branding, not an indication of a different material. On this page you can buy 10-mil polyethylene in clear or yellow, and the yellow is more expensive. That does not necessarily mean it's better, unless you leave it visible and like yellow.

  3. Expert Member

    Another good reason to tape the seams of the poly is that during the pour it's very easy to get concrete between the sheets allowing one to ride up into the slab.

  4. user-4435615 | | #4

    Thank you all for your advice.

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