Step 43: Flooring - Install the Vapor Barrier
As a minimum, you will need to install a layer of vapor barrier between the concrete and the flooring. In fact, it's even specified in the warranty for the flooring I bought! Without vapor barrier, moisture can seep through the concrete and into the flooring, causing it to swell and grow mold. If you're using regular vapor barrier, the process is similar to what I'm about to describe, except you'll need to use vapor barrier tape to seal the seams.
While shopping at Home Depot I found a nifty 3-in-1 product made specifically for basement laminate flooring installation. It's basically a layer of foam pellets encased in vapor barrier and a vapor-permeable layer. Moisture that seeps through the floor can penetrate the bottom layer, then dissipate through the gaps in the foam pellets instead of collecting in one place. The vapor barrier prevents the moisture from reaching the flooring. The foam pellets perform triple duty: in addition to providing an air gap, they also absorb impacts to the floor and insulate against sound and heat loss.
The product I used has a built-in flap along one side with an adhesive strip. The underlayment is rolled out and laid side-by-side on the floor, with each flap overlapping the previous piece. The exception is the very first piece, which has its flap cut off because it overlaps the wall. This flap can be saved and used elsewhere for sealing gaps.
The pieces of underlayment should be laid out flat on the floor, with about two inches running up the wall so that the flooring is completely enveloped. It will later be trimmed. You can lay out the entire floor, or do one section at a time (as I did).