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Help with vapor barrier – On-grade engineered wood

Ram777 | Posted in General Questions on

Hello All. Thanks for a great site. I have been reading quite a few Q&A regarding this topic but seems to be still confused. So I appreciate any assistance.

The home is located in the Dallas area (zone 3a I guess) and was built in 2010 with a plastic vapor barrier under the slab. There’s also gutters and grading all around.

I’m trying to install engineered wood [flooring] on the first floor (on grade) and based on the consensus here is to put another vapor barrier on top of the slab as a safety measure in the long run. This is where I get confused.

I want to use a floating double 3/8 or double 1/2 CDX stacked perpendicular or 45 angle but I don’t know what I should put in between the plywood and slab. Some suggests 6-10 mil poly plastic, some suggest DMX Airflow or Delta FL. But regardless, here’s my question:

1. Won’t the plastic you put on top of the slab trap moisture if there are any?

2. What happens to that moisture?

3. I can’t see any difference between what I’m trying to do and what others do with putting laminate under a plastic underlayment, is there?

Thanks for any input.

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  1. Expert Member
    Dana Dorsett | | #1

    Vapor barriers on top of the slab create a moisture trap for the concrete slab, but so what? Concrete can take it, and has a long & successful history of working well in applications even where it is completely saturated, such as bridge foundations. A vapor barrier between the concrete and your subfloor is cheap insurance against any slab-moisture ending up in the wood subfloor, which (unlike concrete) is susceptible to moisture damage.

    Use a more vapor permeable underlayment between the engineered wood & subfloor.

    In Zone 3A there is a comfort & long term financial rationale for an inch or two of EPS under the subfloor, if that's possible. See Table 2 of this document:

    A bit of insulation under the subfloor also keeps he temperature on the bottom of the subfloor above the average summertime outdoor dew point as well, keeping it from accumulating summertime moisture even if you're not air-conditioning (as if anybody in Dallas can live without AC these days :-) ).

  2. GBA Editor
    Martin Holladay | | #2

    Dana gave you the right answer.

    Q. "What happens to that moisture?"

    A. It stays right where it is. As long as it's not in your flooring, you should be a happy homeowner.

  3. Ram777 | | #3

    Thank you for the responses. I think the biggest concern we have is if there's any moisture trapped between the slab and 6-10 mil plastic film, won't that promote mold or mildew? Or will it dry quickly once the concrete dries? Are there things you can do to prevent any formation of mold or mildew?

    Thanks again.

  4. Expert Member
    Dana Dorsett | | #4

    You don't really care what mold or mildew grows in the concrete (likely none), any more than you care what mold or mildew grows in the soil underneath the vapor barrier under the slab (probably all sorts of stuff).

    The only mold/mildew you need to care about is what's in the subfloor and anything above it. The subfloor was never going to dry into the slab, and the slab doesn't need to dry (ever).

  5. Ram777 | | #5

    OK, thanks. Now, most of the 'Professionals' do they use the 6-10 poly or something else? What is the consensus on products like Delta FL where there's dimples in the membrane to create airflow? Does that really work or just marketing?

  6. GBA Editor
    Martin Holladay | | #6

    Delta FL doesn't create air flow. It provides a small air gap between the concrete and the layer above the concrete, which reduces wicking -- sometimes a good thing.

    If you have a recently built house with polyethylene under the slab and no history of wet-slab problems, don't worry. I think everything will be fine.

    One way that slabs sometimes get wet is due to the enthusiastic use of lawn sprinklers. You don't want your lawn sprinkler to soak the edge of your slab for hours at a time. So you want to avoid that error.

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