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Under slab vapor barrier at two levels

user-228058 | Posted in Green Building Techniques on

I am retro fitting a existing slab on grade barn into a home , in order to accommodate plumbing, the run of the stairs and other , I am raising the floor. 2 to 4 inches of sand will be added followed by 3 inches of reclaimed XPS , then a 4 inch stained concrete finished floor. Typically I would top the foam with a vapor barrier, however there is a vapor barrier under the slab now. So I wonder if having 2 vapor barriers separated by the original concrete , sand , and foam creates a problem. Basically the sand and any moisture in lower concrete never dries out.. So being that I will have no flooring on the top of the finished layer of concrete would it make the most sense to skip the top vapor barrier?

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  1. GBA Editor
    Martin Holladay | | #1

    If I were you, I would include polyethylene under the new slab. There are at least two reasons for this: (1) You never know whether the old polyethylene under the barn slab was well installed; it might have defects. (2) The sand layer contains moisture, and you want to be sure to have a good vapor barrier between the damp sand and the new concrete, especially if you change your mind and install flooring above the new slab.

    The moisture in the sand can stay there forever without hurting the sand, the reclaimed XPS, the plumbing pipes, or the concrete.

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