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I am planning to install an engineered hardwood floor over a slab-on-grade subfloor. The slab is bone dry but I understand it's wise to install a moisture barrier anyway.

My plan right now is to cover the slab with 6 mil poly overlapped 12" and taped at the seams, then put some type of foam flooring underlayment over that, then install the boards as a floating floor.

Does this sound sane? Or is there a better/easier method?

If my approach is okay, does the 6 mil poly moisture barrier need to go a few inches up the wall, presumably hidden behind a baseboard? Or is it okay to have it end at the slab edge?

One thing I'm not clear about regarding the moisture barrier is how any moisture that finds its way under or over it is actually going to dissipate. Wouldn't it just stay there?

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Concrete is effectively a sponge, water will pass through from a wet side to the dry side. So any moisture trapped under a vapor barrier will eventually pass back into the concrete when the ground dries out, unless the ground doesn't dry out in which case you'll eventually have a flood in your basement. This is why gutters and grading away from your house are important. – BMitch Dec 17 '13 at 20:43
    
I think applying a penetrating sealer on the slab is a good idea. In an old TOH episode Tom Silva recommended putting roofing tar paper between the slab and 6 mil poly sheeting. – OrganicLawnDIY Dec 17 '13 at 21:07
up vote 3 down vote accepted

For what it's worth, what I wound up doing is using the Floor Muffler underlayment, which is its own moisture barrier. As per the directions, I applied it over the subfloor and about 2" up the walls.

enter image description here

It's hard to see, but I have indeed left an appropriate half inch expansion gap between the wood and the drywall. And this is engineered hardwood so I don't expect it to move all that much.

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It does not sound sane: You never want to create a space between two moisture barriers. That will, somehow, sometime, fill up with water and never dry out.

I used a tile underlayment product to create channels for small amounts of air to circulate into the adjacent walls and thus out.

Shulter-DITRA Tile Underlayment used for hardwood floor

Drywall, wood, the floor all will transpire moisture, equalizing eventually the humidity. Your vapor barrier creates two different humidity zones, something that must be done carefully.

Your choice on the opposite end of the spectrum is to cover the concrete with the most open and permeable carpet and permeable underlayment. Building assemblies need to breathe.

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So how would you install a wood floor over a concrete slab then? Would you use one of those underlayment products with air channels? – iLikeDirt Dec 17 '13 at 23:57
    
Tile mat taped & sealed but open to vent at the edges, plywood, then a non-vapor barrier underlayment, and the floating floor. – Bryce Dec 18 '13 at 8:25
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Also, what's the space between two moisture barriers? The 6 mil poly would be one... are you counting the foam underlayment as the second? What if the underlayment was made out of a material that didn't make it a moisture barrier, like cork? – iLikeDirt Dec 19 '13 at 14:44
    
Foam, especially with foil counts as a moisture barrier. Cork is far more permeable. – Bryce Dec 19 '13 at 17:34
    
So how about just a moisture barrier type underlayment without the 6 mil poly? – iLikeDirt Dec 19 '13 at 21:22

I have used the panels that Home Depot have in stock as a base for a hardwood floor install with much success on to large floors. It ain't cheap, but it does the job. No need for nothing else, though the 6 Mil is a viable option, yes it should roll up the wall a little before drywall. That will be tough to protect, but it needs to vent to the dead space behind the wall.Dricore Dricore is this product name, Subflor, with one "o" was another brand I used. I glued the T&G, although it says it did not need it. I wanted to insure it would not separate.

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Test your floor for moisture, to be sure it is a dry as you think it is. Take a piece of poly about 1-2 ft. square and tape it securely to the floor with a strong tape. if it stays dry after a few days, it truly is dry, if it has condensate under it, do consider what I have above. – Jack Dec 15 '13 at 14:28
    
I used these with no glue, no nails, no screws and no problems :) – OrganicLawnDIY Dec 17 '13 at 20:58

Taping down a piece of plastic does NOT work! The condensation you do or don't see comes from the ambient air temperature compared to the temperature of the concrete slab. It tells you nothing about the moisture content of the concrete. Why does a glass of ice water sweat when it's placed in a warm setting and a glass of room temperature water doesn't? The same glass of ice water will not sweat when placed in a cold environment "think refrigerator". This test only sets you up for failure.

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Nah, no failure a year later. Everything's bone dry. And no, I did not "throw down a piece of plastic"; I used an approved foam underlayment product. – iLikeDirt Dec 2 '15 at 1:25
    
Since the concrete is porous, the correct metaphor is placing a wet sponge inside a plastic bag, and that does prevent the wood floor from getting wet. If your home is so humid and floor is so cold that it's condensing water, then the windows and doors will likely be covered long before the floor. – BMitch Dec 2 '15 at 1:56

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