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Community and Q&A

Under-Slab Vapor Barrier

severaltypesofnerd | Posted in Building Code Questions on

I’m looking at City of Oakland California bulletin B19-001, which applies to conversion of non-habitable space (read basements, garages).

I’m trying to understand the fundamental reasons here.  In lieu of the 6 mill polyethene that was never installed under the slab, they want a 4-mill poly on top, or an ASTM E96 topical vapor barrier of .06 perms or less.

In the past I’ve used a vented insulated floor system (like DriCore R+) without any special attention to a vapor barrier.  In fact DriCore prohibits adding a second vapor barrier. DriCore R is rated at .79 perms, far below the .06 perm requirement.

Could you help me understand why the City wants a tight vapor barrier, and what ways that might be better or worse than a vented system between permeable slab and subfloor?

The bulletin is at


Vapor barrier BELOW the slab means a dry slab most of the year in our climate.

Vapor barrier ABOVE the slab means a wet slab basically all of the year.

Is that a good thing?

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  1. Jon_R | | #1

    > In fact DriCore prohibits adding a second vapor barrier

    I don't see "prohibit", only "We do not recommend the installation of a vapour barrier beneath our Subfloor panels as doing so could lead to trapped moisture." But trapped slab moisture is only a potential problem when the above slab vapor barrier isn't fully adhered (so safest to avoid poly).

    Also of interest is "The installation of a continuous vapour barrier on top of DRICORE® is permitted as long as it is not sealed to the perimeter wall". So they are counting on air movement to remove moisture faster than it comes from below - leaving the OSB acceptably dry. Clearly, reducing the moisture from below will improve the chances of this being true. Sufficiently dry basement/room air would also be a requirement.

    1. severaltypesofnerd | | #4

      Dricore replied to me endorsing essentially the above. They'd be cool with 6 mill poly above the slab, below the dricore foam layer. It would still vent out the sides, so there would be some drying of the trapped space at the foam level. And some drying up through the finish floor for the OSB layer.

      Note my application is a grade level slab level garage conversion to habitable space.

  2. Expert Member
    PETER G ENGLE PE | | #2

    A search for Dricore on this site might turn up additional threads. Many of the posters here (me included) are somewhat uncomfortable with Dricore's concept. A proper vapor barrier for a concrete slab, whether on top or underneath, stops moisture vapor from coming up into your living space. As a side benefit, it also reduce other soil gases (radon, insecticides, buried pollutants, etc) from coming up into your living space. Dricore, OTOH, assumes that these gases WILL come up through the slab, and then hopes that they will vent around the edges - into your living space! That doesn't seem like the best option. Yes, the OSB layer might stay acceptably dry, but that's not a guarantee that the rest of your basement will. Also, the airspace between the slab and the Dricore will be constantly near 100% humidity. This will allow mold to grow on any organic materials in this space, so unless you clean every piece of sawdust or other debris off of the floor and the underside of the Dricore, you'll be getting mold growth, and the Mold VOC's will also vent around the sides of the Dricore - into your living space. The approach recommended by your jurisdiction would allow application of insulation and flooring directly on top of the slab or VB, and they would always be warm and dry. The slab would always be wet, but concrete really doesn't care about that. It'll be just fine.

    1. severaltypesofnerd | | #3

      If not using the Dricore venting, then I might as well put down poly, 1 inch of foam, and the subfloor of my choice.

      In my observations over the years with inspections wet concrete definitely deteriorates faster than dry concrete. But it's a slow process. The wet situations typically involve efflorescence (moisture is transported through the concrete demineralizing it). I'm not sure what would happen with a vapor barrier keeping the concrete wet, but I'd love to read a building science article on the subject :-).

      Note I'm in a low radon, low soil gas, low problem area. There's no radon to block. An interesting variation though would be to add positive ventilation to dricore, constantly drawing off soil gas and moisture using the dricore channels.

      Too bad Dricore has not done real research. Probably they don't want to know.

      1. Expert Member
        MALCOLM TAYLOR | | #5

        It brings up an interesting dilemma. Martin has repeatedly made the point that not following manufacturer's installation instructions is a code violation - but many of them are wrong or self-serving.

        I use two metal roofing panel suppliers who sell virtually identical products, but have some meaningful differences in their installation manuals. I wonder what obligation I have alter my practices based on which one I happen to buy from?

        1. severaltypesofnerd | | #6

          Add that to the laments over code inspectors who care only to see the relevant UL/whatever stamps, but don't seem to care if they're installed badly....

    2. severaltypesofnerd | | #7

      Thank you. @Pete
      And indeed, the DriCore install manual specifies the subfloor must be completely clean of organics, apparently that's come up.

      This particular application is a ground floor garage to ADU conversion.

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