Have you heard the phrase “What you can’t see matters?” There are several products and techniques I use that get “buried” inside the house, never to be seen again. The vapor barrier under a concrete slab is one of these. Its main purpose is to keep moisture in the soil under a concrete slab from migrating up into the house. (Remember that concrete is porous and soaks up water like a sponge.) This is a vital function because excess moisture in a slab can cause several big issues down the road. The worst problems occur with hardwood floors laid directly on top of a slab; excess moisture often causes movement, warping, or buckling over time.
It’s typical to see an inexpensive “poly” used for the sub-slab vapor barrier, but I think 15-mil Stego Wrap is a much better product for protecting homes from moisture issues. For starters, it's a lot tougher and holds up to the abuse of installing rebar and placing concrete, which can leave standard poly in shreds, defeating its purpose.
In the slideshow and video here, I show how I used Stego Wrap for a vapor barrier under a lakefront home with a complex pier-and-grade beam foundation. You can also find more about Stego Wrap on the manufacturer's website.
