Vapor Barrier/Underlayment for Wood on Concrete

Old 08-06-04, 07:32 AM
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Vapor Barrier/Underlayment for Wood on Concrete

We have an older house that has a garage remodeled as a bedroom. The floor is currently carpeted, but needs replaced and we want to install 3/4 in t/g fir strips. The carpet is laid over a plywood subfloor on concrete, and the subfloor will require replacement as well. I have seen varying opinions on whether or not a vapor barrier should be installed under the plywood subfloor. Should a vapor barrier such as 6 mil plastic be installed, or should I lay a grid of 1x3 treated wood directly on the concrete, and attach the plywood subfloor to the grid? Also wondering about the need/value of priming the concrete with dry-lock first? I suspect dampness may have contributed to the demise of the current floor although there is no known mold/mildew problem. Thanks for any guidance.
Old 08-06-04, 03:00 PM
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You definitely need a vapor barrier. NOFMA says 6 mil poly is preffered.

First you have to test your slab. Here are the test methods -

The dry-loc wouldn't hurt but I would bet that the effectiveness does not last long. Others things you can do to keep moisture at bay and get a better install-

Use 2 layers of exterior grade plywood and stagger the seams. Glue (construction adhesive) and screw the layers together.

Use asphalt felt or rosin paper under hardwood.

Put a poly finish on the underside of your hardwood strips.

I believe NOFMA even had a method of applying adhesive to the slab, lay down a poly film and then apply more adhesive for glue downs but this was in the days of mastics and the current adhesives are not approved for this method.

There is an issue with slab on screed regarding condensation in the air space but I forgot what the accepted solution is. The poly film would prevent moisture from coming in but it would also trap condensation so if you use this method you would have to research this. This method is mostly used for sports floors where they want some give in the floor.

I'm not sure how effective any of these methods really are since some moisture will inevitably get in from the edges if your slab has moisture. Do you live in an area with a lot of rain and poor drainage?

And one other thing - especially if your plywood is floating (ie. not attached to the slab with nails, screws or adhesives) do not attach your hardwood or transistions that are securely fastened to your hardwood to the slab at any point. The whole thing will be like a floating floor to some extent.

Last edited by AlexH; 08-06-04 at 03:15 PM.
Old 08-07-04, 07:30 PM
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Whoa there Alex...

The specs still call for an asphalt mastic to be applied to the concrete before a poly sheet is laid into it, Unless your installing a floating subfloor that isn't fastened to the slab in any way. Puncturing the poly with fasteners... The asphalt mastic seals around the fastener. Without it, you might as well have not spent the time to roll out the poly at all. Not to mention the flooring you will be tearing out, because it cupped and buckled.

IMHO, I personally will never install a solid wood over concrete. The risks are too high for my pocketbook. As a NWFA Certified Wood Flooring Inspector, 90% of the cupping failures I get asked to analyze, are solid wood, with rosin paper, over a " plywood, with a poly moisture retarder, over concrete.
That line is too fine for my reputation. Another 5% are the thin solids glued directly to the concrete slab. The next 5% are acclimation issues. All installation related.

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