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Optimal placement of vapor barrier under slab?

GBA Editor | Posted in Green Building Techniques on

We’re building a passive-solar home (SIPs on a monolithic slab for thermal mass). I’m wondering about the placement of the vapor barrier underneath the slab. Should the order be (from bottom up): gravel fill layer, vapor barrier, XPS, then concrete? Or can it go: vapor barrier, gravel fill, XPS, then concrete? Are there any moisture issues associated with the latter method?

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  1. Riversong | | #1

    The vapor (really a radon and ground-water) barrier under a slab should be either immediately under the concrete or under tongue & groove XPS with taped seams. Placing an impermeable membrane under the fill creates a reservoir for water (much worse with sand fill because of greater capillarity). If you use Tu-Tuff reinforced 4-mil polyethylene, you don't have to worry about foot traffic during the pour as it's extremely tear-resitant.

  2. Claire | | #2

    Hi Robert ~
    Basically, my husband (who is doing the poor), is concerned that the vapor barrier directly underneath the insulation will prevent any moisture migration through the concrete during finishing, which he was relying on for a better-looking floor. He formerly worked as a finisher, and says that if the water has nowhere to go but up, it'll create havoc in trying to achieve a good-looking floor. He says he was relying on cracks between the XPS to serve as moisture channels, which would then go into the fill, and then eventually drain to the perimeter foundation and to the drains. His one question that I've been unable to answer is: what's the probability that doing the vapor barrier-fill-insulation-concrete method that he favors will result in moisture problems in the house?


    While it is more challenging to get a slick finish on a slab with the vapor barrier directly against the bottom of the concrete it will hydrate more fully and cure harder than if the moisture is allowed to leave through the bottom. Best practice is to put the vapor barrier directly against the underside of the slab or under the foam as Robert has suggested.

    In any event the gravel fill should have a drain to remove accumulating moisture and I generally use perforated drainage pipe (4" slotted ABS drain tile) and run it under toe slab to collect radon and through the concrete (generally behind the fireplace or wood stove) and vertically in 4" galvanized dryer vent duct through the roof.

    If you pitched your sub grade to drain. which is standard practice, and provided the drainage pipe as suggested above you could probably get away with putting your foam and vapor barrier under the fill. the concrete would be less strong, and would be subject to risk of "slab Curl" where the edges of the slab rise as the concrete cures due to uneven moisture. (often seen where folks pour on sand beds) The water would leave eventually with little risk of damage so long as the poly was on top of the sub-grade and the fill was all clean washed gravel. It's not the best practice but it would probably be fine. Since the concrete would not be as strong I would increase the steel schedule to a #3 rebar every 18" each way on dobies to hold it in the center of the slab.

  4. GBA Editor
    Martin Holladay | | #4

    Another vote for putting the polyethylene directly under the slab. Here's Joe Lstiburek's take on the issue:

  5. Riversong | | #5

    Since 2001, the American Concrete Institute and the Portland Cement Association have advocated placing the vapor barrier directly under a concrete slab without an intervening "blotter" layer.

    Tell your husband he's asking for trouble in order to save a few hours of setting time before finishing. The correct mix and aggregate size and the correct water content are more important to a quality slab than an escape route for bleed water.

  6. candersun | | #6

    Hi Robert ~
    I wasn't sure how to get in touch with you outside of the GBA site, but I'm looking for someone to review our latest foundation/slab plan and make sure we're doing things right. Would you be interested? If so, can you e-mail me at claire[dot]anderson[at]homepower[dot]com?
    Thanks, Claire

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