Our home was built with a dirt crawl space that is about 2' from ground to bottom of floor joist. The floors are insulated with fiberglass held up (where it hasn't fallen down) with wire rods. The AC/heating duct runs through the crawl space and has insulation that looks worn out. There are vents all around the perimeter thru the concrete blocks.
Have had trouble for years with dampness in the crawl space. A few months ago after it had rained for about a week I was under the house and it was so damp it was dripping from some of the floor fiberglass. Also, the AC ducts always drip in the summer.
Currently I have a fan running all summer long to help keep it dry down there.
I'm considering encapsulating the crawl space thinking this should solve the moisture problem. From what I've read it should also eliminate the need to rehang all the fiberglass that keeps falling down, as well as eliminate the need to redo the duct insulation.
Would like to hear from someone who has done this before as to how it turned out.
Update, 10/3/13: Just want to thank littleturtle not only for your comprehensive info on this but your cited source. I read through it last night and am still trying to digest it. But one thing I picked up on was that you don't have to vent the crawl space to the inside of the house to exchange air. That's what seems best for me because I'm thinking that during the winter I'd have to pay to heat the crawlspace, and heating KWHs are expensive. Incidentally, I'm near Charleston, S.C. (Zone 3) where the cooling cost exceeds the heating cost and the humidity is typically high. Anyway, I'm going to keep plugging along on this topic until I get a good feeling about doing it, or not. I'm retired and have time to think this through carefully.
Update, 11/14/13: Have decided not to encapsulate for now. Why? Doesn't make sense to spend thousands of dollars when the little box fan seems to work okay. In other words, running a box fan in the crawl space all summer does an adequate job of removing the condensation, at a low cost. Keep in mind that there has never been a problem with the dampness actually rotting any of the floor joists, or sill plate. Since the condensation is the only problem, other than that the insulation keeps falling down (another story), it just doesn't seem necessary or smart to spend that kind of money. If, on the other hand, there was ample evidence that there was a payback from the reduction in the KWH used for heating and cooling, then I would do it.
Update, 4/17/14: After further research I became convinced that I should seal the vents in the crawlspace. So, that's what I did. Now I'm monitoring both the temperature and humidity down there with a handy-dandy display in my den. Something I'm also thinking about is using the cooler air down there to cool the house. It stays in the 60's (since February) down there. Depending on the temperature this summer, I may devise a method to exchange air to cool the first floor next year.