What is the best method to avoid moisture problems. Basement is unfinished, poured concrete floor and dry. Putting in a workout room and want to use bamboo or engineered hardwood. Have been told by reputable floor operation that all I need to do is put down 6" plastic sheeting and underlayment then put down the floating floor. My initial thought is that will trap water under the plastic with no chance of evaporation. What is the best approach? I have read about DriCORE and Tyroc etc, but the all seem to have drawbacks. Thank you

Your concern is that the concrete will get wet and not be able to dry? It certainly won't hurt the concrete.

If you don't have underslab insulation, now might be a good time to add some (above slab). It will also reduce the temperature and moisture differential from one side of the flooring to the other.

In many areas one unseen and unsuspected sources of unwanted moisture entering a house is through the concrete walls and floors. Sealing those by insulating and adding a layer of poly is necessary in new homes (although not always done). In an older home it cannot hurt and can greatly improve the livability of the house. The key is that you don't simply lay down a sheet of poly; it needs to be taped and sealed around the edges and at the joints.

Posted By Bob I on 02 Nov 2013 11:29 AM
In many areas one unseen and unsuspected sources of unwanted moisture entering a house is through the concrete walls and floors. Sealing those by insulating and adding a layer of poly is necessary in new homes (although not always done). In an older home it cannot hurt and can greatly improve the livability of the house. The key is that you don't simply lay down a sheet of poly; it needs to be taped and sealed around the edges and at the joints.

The insulation part is important too, with R1-2 of subflooring + finish flooring the bottom of the subfloor will trend toward the soil temperature, which may be below the dew point of the interior air. Put a sofa or stack of boxes above that and it'll run even colder. In summer when the dew point of the air is higher, the subfloor can adsorb moisture from the room air to mold-inducing levels. (The colder and more humid the climate, the greater the risk.)

Adding an inch of EPS under the subfloor is good enough to mitigate this risk in all but the coldest climates, but 2" (R8) would still have a long term economic rationale on heating cost reductions anywhere the deep-soil temperatures are under 55F. (Mostly US climate zone 5 or colder.)

EPS is preferable to XPS on a couple of grounds- the higher R/inch of XPS dissipates as it loses blowing agent over the first 50 years or so to 85% of it's rated R, whereas the R-value of EPS is stable. Furthermore the blowing agent itself is a powerful greenhouse gas ~1400 x that of CO2, whereas the blowing agent used for EPS is only ~7x CO2, and is already dissipated 99%+ by the time it hits the distributor's lot if cut into 1-2" panels.

To limit warp/wave/curl on the subflooring, use tounge & groove, and tap-con it to the slab through the foam & poly sheeting 24" on center. Don't butt the subflooring up super-tight either give it at least a 1/16" of play at the seams to allow for seasonal expansion/contraction with normal humidity changes, and stagger the seams of the subflooring & foam panels by a foot or so to avoid the potential compression-rockering from developing over the decades. It takes a lot of pounding to do that, but it's theoretically possible since the mechanical weakness of the foam seam gives it a somewhat lower localized load rating unless there is overspanning structural wood to spread the load.