Polyethylene barriers below slab floors serve two purposes - to prevent moisture from wicking into concrete and disbursing it into homes, also to create a radon barrier. Radon gas occurs naturally in the soil, a result of decomposing uranium in the ground. So it's everywhere, but more intense in some locations than others.
A passive radon stack are also a prerequisite under the LEED for Homes program. Radon gas is the second greatest cause of lung cancer after smoking.
Rather than using the typical 6 mil polyethylene as a sub-slab vapour barrier, we used a 10 mil barrier and a radon evacuation stack to lower radon gas levels in homes, research shows (see below) that thicker membranes offer more effective protection.
In this video we are installing a 10 mil Perminator vapour barrier from WR Meadows. The thicker barrier is not only better for mitigating soil gas, but it is far more durable during the construction phase than the 6 mil poly.
More on radon protection: