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Community and Q&A

How to build a wood floor over a new slab

mikeyrosen | Posted in Green Building Techniques on

I am building a “healthy home” based on Building Science/ Bau-Biologie principals. I’m trying to follow them to the greatest extent possible, however I’m stuck on one problem:

We poured the slab, 18″, 9 months ago. The bottom floor (on the slab) is a walk-out basement. Since the slab is new, it still has a lot of moisture (I bought a meter and am measuring it). I have radiant heat in the slab, and have cranked it up for two months now, though there isn’t much of a difference in moisture content per the meter readings.

About half of the floor is going to be tiled over, and b/c I’m putting the tile directly over the deck mud, with no slip sheets (the mud has chicken wire in it), I am not worried about moisture from the concrete b/c it should be able breath through the grout lines between the tile.

HOWEVER, the other half of the floor is going to be a floating floor with engineered hardwood. My contractor wants to build the wood floor with (1) 2×4’s that are sitting on moist stop on top of the concrete [only where the wood hits the concrete, moist stop would not cover the whole slab floor]; (2) CDX plywood [I wanted to use dragonboard but could not get the mfg to send me any, and couldn’t find any other mold-resistant alternative that was also low VOC]; (3) Kahrs engineered hardwood flooring.

MY QUESTION IS: How do I avoid mold on the plywood or Kahrs wood?

My contractor originally wanted to put a vapor barrier between the Kahrs and the plywood but that makes no sense to me b/c it would trap moisture below it and then the plywood would definitely get moldy. So… the best idea I’ve come up with is to use no vapor barrier, but just leave the radiant heat on at a low level, which would keep a constant temperature with the slab, air, plywood, and Kahrs, and hence facilitate moisture from slowly moving through all of those, as opposed to condensing on any of them any creating mold. However, this is just my theory. I’d love to get some expert thoughts on this from this community… and/or any other suggestions.



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  1. GBA Editor
    Martin Holladay | | #1

    Q. "How do I avoid mold on the plywood or Kahrs wood? ... My contractor originally wanted to put a vapor barrier between the Kahrs and the plywood but that makes no sense to me because it would trap moisture below it."

    A. Don't worry about "trapping" moisture in the slab. The concrete will always have some moisture in it. It isn't trapped. It's just there. There is absolutely no reason to invite that moisture into your house unless you want to.

    We establish thermal barriers and moisture barriers around the perimeter of our buildings for a good reason: we want to keep the outside out. Outside is often cold and damp. Inside, we want to be warm and dry. Barriers are good.

    So, I agree with your contractor. It's OK to install a layer of polyethylene between your concrete and your 2x4 sleepers.

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