The mil of a plastic is simply the thickness of the plastic. Here is a quick guide on the uses of sheet plastic by ‘Mil’…
2 mil poly (light duty): paint protection, as a dust barrier, or a drop cloth
4 mil poly (medium duty): vapor barrier in between insulation and drywall. Can also be used as a drop cloth or a temporary cover
6 mil clear or black poly (heavy duty): used to cover crawl spaces and as a temporary cover for equipment and supplies. Also commonly used as a vapor barrier in between walls, under flooring, and under concrete slabs.
12 mil fiber-reinforced, virgin resin poly: crawl space encapsulation
20 mil fiber-reinforced, virgin resin poly: crawl space encapsulation
Most non-crawl space companies that claim to offer vapor barrier installation cut corners and install a short term solution. Many will try to install the vapor barrier on top of an existing vapor barrier and debris. Also, many amatuers attempting to install vapor barriers fail to secure the poly to the ground leaving the plastic to pull away and leave exposed earth releasing moisture. We install the vapor barrier with great results for years to come!
When we install our vapor barrier, we always clean out all existing plastic and debris on the ground. Next, we roll out and tightly fit our 6-mil heavy duty plastic around and under obstacles and penetrations. Lastly, we secure the 6-mil poly to the ground using stainless steel 6 inch square stakes as well as a patented vapor barrier button that will not allow the plastic to tear before being pulled away.