6-mil vapor barriers are most commonly installed when a home is built, they are lightweight, similar to a standard trash bag, they tear easily, and are simply laid out on the ground. As soon as someone or something moves through the crawl space, the liner is displaced, torn, and bunched up, offering very little protection to the home from the earth's moisture.
Our homeowner opted to replace this 6mil with a CleanSpace 20-mil vapor barrier. This liner is similar to a pool liner, it's durable enough for service persons to crawl and work under the home, is overlapped and sealed at all seams, wrapped and sealed around all piers, and attached and sealed to the foundation walls about 6 inches above outside grade. It completely seals the crawl space from the earth's moisture, achieving the job vapor barriers are intended to do.