Vapor barriers come in many colors and thickness but, all are used in a crawlspace to keep moisture off your sub floor. About 40% of moisture in the crawlspace comes naturally from the earth. Installing vapor barrier will help reduce this moisture. Vapor barriers come in different thicknesses depending on what you are going to accomplish in your crawlspace. A standard 6 mil is great for just a simple floor, wall to wall, job or 12 to 20 mil for encapsulations. I personally prefer a 10 Mil non-reinforced because it is a durable and easy to work with vapor barrier.