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Old 07-10-2012, 05:31 PM   #1
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Location: Southeast US
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Flooring and vapor barriers

I live in the hot and humid deep south, house is on a slab. There is visqueen under the slab. I'm renovating a garage to become an office and it won't be long before I am at the flooring stage.

I'm not sure which forum this might be better posted in, as my question pertains to moisture issues.

I've considered just putting down vinyal tile, but have heard that this can cause issues, as the vinyl tile acts as a second vapor barrier. I think I am experiencing issues with this in the main part of the house. We have what I call nafco flooring (vinyl simulated wood with some sort of black adhesive on the bottom side...I believe this adhesive came on the flooring).

When it is humid outside (and A/C cool inside), a black liquid goop (adhesive) will sometimes seep through the cracks of the flooring in various places.

Again, I'm assuming this is due to the "two" moisture barriers (visqueen under slab, vinyl on top).


Thanks for any info.

Last edited by rightit; 07-10-2012 at 05:33 PM.
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Old 07-13-2012, 02:31 AM   #2
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Location: Texas
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Flooring and vapor barriers

A vapor proof layer of polyethylene, or visqueen beneath a concrete slab will prevent excessive moisture absorption from the subgrade if the subgrade moisture levels not being raised by a concentrated external water source. In your case it's more likely that excess water is being absorbed into the slab concrete. It manifests itself by dissolving the tile adhesive which is pushed up through the joints under the pressure of foot traffic. The source of the water may be a broken sewer line, water pipe, rainwater, or natural underground water. Such sources can overwhelm and circumvent the visqueen vapor barrier. If there is no vapor barrier at all under the concrete slab, even natural moisture levels can create problems with tile adhesion. Very often builders omit vapor barriers under garage slabs.

I recommend that you search for an external source of excess water. Look for leaking pipes, damp areas around the house, broken irrigation lines, improper rainwater control, or listen for running water. More sophisticated tests are to plug the sewer line in the yard and fill the sewer pipes with water. They should not bleed down over a 12 hour period. Test the domestic water by shutting off all water taps and check the water meter needle for movement. The single gallon needle should not move at all over a one hour period. In my commercial work the most common problem water sources I find are broken irrigation lines.

You can test the moisture level of the concrete slab with a $15 calcium chloride test kit (see links below). For typical flooring tile adhesives, a moisture level of less than 3 bs/1,000 sf/24 hours is recommended. Levels higher than this are a problem and manufacturers of standard adhesives will not generally warranty performance. A simple, and less accurate, test is to find a bare concrete surface in the building near the problem area (or make one by removing a few tiles and ALL the adhesive). Set a 8" plate up side down on the clean area of concrete. Remove the plate after 8 hours. If the concrete is visibly darker where covered by the plate, the moisture levels are easily over the 3 lb limit mentioned above.

Even if you find an adhesive that bonds to a damp floor, other problems will arise such as mildew and mold.


Good Luck

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Old 07-13-2012, 12:22 PM   #3
Join Date: Mar 2011
Location: Southeast US
Posts: 244

Flooring and vapor barriers

Hi Arkitexas. Sewer lines are OK here, and supply lines are leak free. There is a grade issue at one area at the SW corner of the house, which is "on the list" to correct. It will take very little effort to straighten that out, and I guess I'd be best served by moving that up on the list.

Thanks for the comprehensive response.
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