We are planning to install hardwood floors over a concrete slab. This is in a workshop that's getting converted to living space, so unfortunately vapor barrier wasn't put down underneath before pouring the slab 7 years ago. In the summer, there are definate moisture issues--Wisconsin is wet in the summer.

My husband wants to put down vapor barrier, then use the sleeper-on-slab system, then use nail some solid hardwood flooring to the sleepers.

I don't trust this to keep the moisture out of the hardwood, since the vapor barrier would be penetrated (plus it seems like a hassle to lay). I would prefer to use the Kahrs engineered wood (the woodloc system), over foam underlayment, over vapor barrier. We recently put down a laminate in a cabin (over a concrete slab with the same issue), and while I don't want a laminate in this particular space, I sure did like the ease of laying the floating floor (and I don't mind the sound or feel of a floating floor at all).

SO: which system would be less likely to have moisture problems, warping, bending, cupping, etc? Kahrs engineered wood seems less likely to get distorted (but maybe I'm just being lazy).

Or would it be best to lay down vapor barrier over mastic, then float 2X8' pieces of plywood subfloor, then nail the flooring to the plywood subfloor? If we did this, could we use real wood, or would engineered wood be safer? Thanks.

Engineered floors were designed for slabs on grade so they will always be a better choice if your main concerns are stability. But solid over sleepers can and is successfully done all the time. What to be done is apply a concrete vapor sealer such as Bostick's MVP or Taylor's "Lockdown" over the entire slab. Then glue down pressure treated 2x4 sleepers spaced 12" oc using the same manufactureres adhesive. For Bostick's, use BEST, for Taylor 2170. I also shoot 2&1/2" concrete nails down as well, every couple of feet. Then lay an 8 mil poly vapor barrier over the sleepers. Make it clear so you can see them. Don't stretch it tight, let it drape loosely. Then lay your floor. Wide planks would be a bad idea. Choose a strip floor and plan on keeping interior climate controlled. BTW, This is alot more work than a Kahr's floor. Good Luck!

Thanks for the tips. Humidity control isn't possible in this space--in the winter it ranges from 20-40%, and in the summer, even with a dehumidifier, it's often up at 80%. I assume that engineered wood can deal with those fluctuations better than solid wood--is that a reasonable guess?

With an engineered, symptoms of moisture are found on the end joints.

The ends peak under high moisture, and gap when less moisture.