A 6-mil vapor barrier is what is traditionally installed at a home's construction and required by building code. However, it is not required to be sealed to itself or to walls or piers. The material is staked down, creating more holes and weak points for an already flimsy material to tear. Because this liner is not durable it can't be sealed to walls because it will just tear off and has an oily or slick surface that doesn't allow tape to adhere and seal the seams. This means that the 6-mil vapor barrier is allowing water vapor from the earth to rise into the crawl space contributing to its relative humidity and ultimately mold and mildew growth.