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Should vapor barrier in slab be under 4 in. of gravel or on top of gravel?

user-982027 | Posted in General Questions on

2-part question:

1) I’m building a 2-car garage with upstairs apartment in zone 4, 26′ x 28′. getting different opinions on whether the vapor barrier -6ml plastic- should be under the 4” of gravel or on top of the gravel directly under slab? Some are saying it is not good to pour concrete right on plastic, although this seems to be the most common way to do it. any input would be appreciated. carl

2) I’m putting in radon mitigation in slab — 3 in. PVC — will this make a difference where plastic should be?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should vapor barrier in slab be under 4 in. of gravel or on top of gravel?"

    A. On top of the crushed stone, directly under the slab.

    Q. "I'm putting in radon mitigation in slab — 3 in. PVC — will this make a difference where plastic should be?"

    A. No.

  2. user-577475 | | #2

    While this is generally correct, unless you use a reinforced vapor barrier (Stego Wrap or similar), it is very likely that it will get punctured during concrete placement if not before. It could easily get punctured even if reinforced. If that happens, much of the reason for putting in a vapor barrier is lost. You might consider the more expensive vapor barrier material and/or put a layer of sand on top to provide some protection.

  3. user-577475 | | #3

    Additional comments are that some do not like to put concrete directly on a vapor barrier as it can cause curling of the slab due to the more rapid drying of the top surface. This can be minimized with proper curing. I also assume you are putting the 3" PVC below the slab (in the gravel) for radon mitigation as you would need a pretty thick slab if you put the system in slab as noted.

  4. homedesign | | #4

    According to this Lstiburek podcast... a punctured vapor barrier below a slab is not a problem.

  5. jbmoyer | | #5


    You are wrong. The vapor barrier should go directly underneath of the concrete slab. A sand layer should NOT be installed between the VB and the concrete.

    Millions of cubic yards of concrete are poured directly on top of fluted steel (an obvious vapor impermeable material) floor decks every year. I actually don't know the exact number of cubic yards but I think it's safe to say it's a !@#$ load.

    But don't take my word for it. Heres what Dr Joe says about sand layers beneath concrete slabs and curing issues:

  6. GBA Editor
    Martin Holladay | | #6

    Puncturing the vapor barrier won't hurt its effectiveness as a vapor barrier; it will still stop vapor diffusion. The concrete slab is your air barrier.

  7. Christopher Briley | | #7

    I second Brett's comment. Proper curing will prevent curling. Long after the first critical 28 days of curing, a layer of sand between the VB and concrete can act as a layer for water to hang out. After a long humid week, you may have a really nice dry day, and the water (in the form of vapor) that collected in the sand will drive through the concrete. This can sometimes cause problems with any finishes directly applied to the slab.

    Chris Briley, architect

  8. davidmeiland | | #8

    The best thing I've found for preventing slab curl is to pour a low-water mix. Water reducer doesn't cost much, and you can use a lot less water in the mix while improving the short term workability. It is a whole lot easier to get the mud on the ground when it flows like a 7-inch slump for the first hour, then suddenly turns into a 3" slump. I'll typically mist a new slab for several days after the pour. I definitely think it's a mistake to put a "blotter layer" of sand directly under concrete.

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