I suppose it matters where in the country you are located, but in the crawl spaces I have traveled, the majority of the problem crawls are lacking moisture barriers and are the most important item in the crawl to add. I find where there are partial barriers or improperly ones installed, the deterioration is always where the barrier is missing. The only crawls I have seen where the moisture barrier is present and there are moisture problems, is where the builder has buried the plastic under 4-6 inches of gravel which turns it into a liability because then it does trap the water above it. With proper overlap and carrying it up the sides of foundation walls at least 6 inches, the only way moisture can get above it is to flood the area which indicates more water control measures are needed, or a leak in the plumbing system which it would identify by the wet plastic in that area. I give my clients the link to this website for additional info on water control and the concept from Jos L's building science website.
http://www.buildingscience.com/resou...s_moisture.pdf
If the concept of water movement is to be understood to go from hot to cold and less to more, then why do they make crawl vents that open up in the summer when the humidity is trying to get into the cool crawl space.
Inquiring minds would like to know...Jeff G
Here is a picture of a crawl without a moisture barrier left to grow...