We are in the process of finishing our 1000 sf basement. All the walls are up, plumbing is in, and all that is left is a bit of drywall sanding, painting and laying of flooring (I plan to finish the drywall & painting this spring). Got a good deal on some engineered wood flooring last year, so that's been acclimating in the basement for about 3 months, and the Shaw Selitac should be here next week. Question: Should we still lay the 6 mm vapor barrier under the Selitac? Any reasons NOT to? Background: We live in East Tennessee, and there is some amount of humidity that comes up from the slab. The Selitac is supposed to take care of that (and we will, of course, tape the seams with the polyethylene duct tape). When we didn't run the dehumidifier in the basement last year and turned off our A/C during a poorly timed vacation, there was a small amount of mold on the baseboards. Understandable, since the humidity in the basement was almost 80% when we got back from vacation due to record-setting rainfall in July--it rained almost every day for 3 weeks while we were gone. Of course, hindsight is 20-20, and we realize we should have left the A/C and dehumidifier on. Took care of the mold issue with a buckets of stuff from Home Depot that turned the concrete and baseboards white, and the humidity has been at or below 40% since last August thanks to the dehumidifier and decrease in precipitation. Anyway, the point is that I want to make sure my basement floor is as impervious to moisture as possible since I and my daughters all have mold allergies and I am laying the floor myself this summer in my "spare time." Thoughts? Suggestions?

Not needed if the space above the slab is unconditioned and you do not care about moisture levels. Sidewalks are directly on grade with no barrier. I would put down the barrier to make the garage dryer and more comfortable.

So what are you having as the FINAL floor? It sounds like you are installing 1/2" cork FLOORING TILES....but then a little further on you are talking about wood over cork. This tells me you are talking about cork UNDERLAYMENT underneath hardwood. OK. Now we are on to something. A vapour barrier is required if you are AT GROUND LEVEL or BELOW. Your installer can glue down the cork to the SUSPENDED SLAB (aka....10th floor concrete slab) without worrying about a vapour barrier. A sheet vapour barrier is NEVER USED with glue down anything. The glue must act as the vapour barrier. Glue down hardwood over glue down cork underlayment is a MAGNIFICENT floor. And it is one of the MOST EXPENSIVE installation formats for hardwood that we have (yep....it is more expensive than you can imagine). If your engineered wood can be FLOATED then you will save HUGE amounts of money (could be as much as $7 - $10/sf worth of savings). The cork can be floated (it just sits there....no glue) and then the wood is floated over that (glued at the edges and tapped together). It is a MUCH faster installation (can be 2 days faster). But first: what floor do you live on?

It is a requirement. You still need it and here is the reason why: We ALWAYS ASSUME that the "underslab" vapour barrier HAS BEEN or WILL BE compromised at SOME POINT in its life. We don't know when, we don't know how badly, but it will happen. Heck, it can happen WHILE they are POURING the concrete. Or when they are spreading concrete. Or when tree roots grow underneath the concrete, etc. So, the moral of the story is: always use vapour barrier (6mil poly sheeting = $0.11/sf at HD) over slab on or below grade when installing rigid flooring. It is the CHEAPEST FORM of "warranty saver" you will ever have. I promise you, that vapour barrier will come in REALLY handy at some point in the house's life span.