While water can invade many parts of a house--attic, walls, you name it--the crawlspace is the number one place where moisture might invade, because it has contact with the ground. In fact, it is the ground.
Crawlspace Moisture ProblemsIt's difficult to overstate the problems associated with crawlspace moisture. At the very least, you've got water under your house. No one likes that. If you try to sell your house, and the property inspector points this out (and he or she certainly will), your buyer will be so disturbed that they will want the problem fixed. Even if you lived for twenty years like this.
But the real problem is mold, fungi, and mildew. Remediating mold-related problems is expensive. While it's my position that mold problems have been blown all out of proportion lately, in a number of cases it's a very real problem.
Then you've got the issue of prevasive moisture on the structural elements of your house--joists, sills, posts, etc. As you're well aware, moisture will eventually break down wood. Even pressure-treated wood will eventually break down.
Vapor Barrier: Ground Plastic to Retard Ground MoistureThere are a host of ways that a crawlspace can become wet, but in this instance we're only dealing with ground moisture. We use what's called a vapor barrier. Interestingly, the U.S. Department of Energy has revised this popular term, and now calls it a "vapor diffuser barrier." It's actually more correct, since in terms of a crawlspace we're not laying not a perfectly sealed, seamless barrier. It's just a way of slowing down the process.
You'll first want to start with a crawlspace that's as dry as possible. Then lay down 6-mil polethylene plastic over every single square foot of your crawlspace. Overlap the seams by no less than 1 foot--a 2 foot overlap is sometimes recommended. Bring the plastic about 1 foot up the crawl space walls. Tape all seams down.
You don't need to really take measures to hold down the plastic, because, after all, this is a wind-free (hopefully) zone. Some wind may come through the crawlspace vents, so a few well-placed rocks on the plastic should hold it down good enough.
Crawlspace plastic is easy enough to install. If you have a little hatch, getting yourself and the plastic into the crawlspace may be the hardest part of the job.