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Community and Q&A

Flooring and a new slab on grade

ranson | Posted in Green Building Techniques on

I have a few questions about installing flooring on a good, new slab on grade. The planned slab construction is 6″ of washed crushed stone with drainage pipe, 4″ of EPS, 10 mil poly, and then 4″ of concrete. The slab is for a planned house near Rochester NY, zone 5A, at the top of a small hill in a field of silty loam. The drainage is naturally away from the homesite, and will be further improved as necessary.

1. When installing wood flooring over a slab like this, should I install a second vapor barrier above the slab? I would expect this slab to be room temperature and relatively dry (once cured). I’m concerned that a topside vapor barrier could cause a mold issue if it ends ups trapping moisture from construction or an accident.

2. My wife would like soft, squishy-underfoot carpet in the bedrooms. Can I achieve this without inviting mold and air quality issues?

3. What’s the best way to eliminate the height transition between floor materials of different heights? Put a step in the concrete, or build up the floor with some other material? The extra concrete’s not that expensive, but it’s not a very green material.



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  1. GBA Editor
    Martin Holladay | | #1

    As long as you remember to include vertical rigid foam at the perimeter of your slab, the slab will be warm and dry. On a slab like that, additional polyethylene above the slab is unnecessary but harmless (as long as you have waited for the concrete to cure before installing the flooring -- a step which is required in any case). You can safely install carpeting above such a slab.

    To ensure that your flooring heights are co-planar from room to room when using different types of flooring, you would need to install additional layers of plywood or OSB subflooring, or sleepers and plywood / OSB, as required.

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