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Dealing with something similar in my "new" 22 year old house. I'm certainly no expert, but have done a bunch of research, and done a bunch of stuff that seems to be working. Mike's suggestions are similar to what I'd recommend:
1. Look to see if water is "seeping" up through the soil, or running in via cracks in the foundation, around access doors, pipe holes, etc... Seal this stuff up to prevent "mass entry" of water. I used a couple bags of quick set concrete to patch small and not so small holes.
2. Clean, Check and fix your gutters. Install pipes to direct water away from the house. My gutters needed complete replacement, as they were galvanized and had rusted through in a number of places.
3. Allow water that enters a good way to exit. My house had drain tile added along the lower foundation wall 5 years ago (inside the crawlspace), with a sump pump in the lowest corner to pump out the water that entered. When I bought the house, there were still moisture problems, so I had the seller pay for addition of drain tile around the rest of the foundation walls as well. A "good-ole-boy" quoted $1k for this work using standard drain tile. We used Regional Waterproofing who charged $1500 using the latest and greatest drainage mat stuff. I wouldn't have payed for the more expensive stuff myself, but I wasn't paying.
4. All that should be left now is moisture entering from the vents, and evaporating from the soil:
a) install heavy plastic over the ground, covering most of the surface.
b) do a) plus install some sort of fan venting. Be careful though, as bringing in outside "moist" air into a cool crawlspace can sometimes make the problem worse becuase the warm moist air will condense on cool pipes, wood, etc.... I didn't do this.
c) If the problem is not TOO severe (as mine was, and this came recommended from an installer of the more expensive "professional" options), do a) plus close and seal the vents and any other air leaks to the outside and into the house. Install a dehumidifier in the crawlspace, draining outside and away from the foundation. Whirlpool "xxxxx50" came recommended for this from somebody who professionally. I got the xxx70 (from Lowes) becuase it is somewhat more efficient. This method is a reasonable fix, so long as the dehumidifier only has to run some of the time (they cost a bit to run), which is of course govererned by the amount of moisture evaporating. I took it as a $250 experiment. One other note on this option: some soils may excrete toxic Radon gas. Sealing the crawlspace will not allow this to escape easily. This is not a severe issue in this area, but I will be testing for Radon on occasion, just in case.
d) for more severe cases, have a professional company come out and install a sealed crawlspace system. Assuming you do not have new construction, these systems will seal the floor and lower 90% of the walls from the air in the crawlspace. In most cases, they will install a dehumidifier as well, and chemical spray to kill any molds that currently exist. These systems also provides a Radon vent, so that's not an issue with this system. This is a thorough proceedure and carries a warranty, but comes at a price. My crawlspace, which is probably <1400sf quoted at $7000 for this option, and that's with part of the drainage already installed. This is why I'm experimenting with the dehumidifier.
My results so far:
starting humidity in the crawlspace, during the "dry" weather was 80% RH. Within 2 days of running the dehumidifier, this dropped to 61%, and didn't increase with the humid weather. Since installing the new gutters, my sump pump has not run, indicating that water is not penetrating up through the soil. I no longer have standing water. I still haven't completed sealing all the gaps and holes, and have only closed, not sealed, the vents. I plan to do this in the next couple days. My goal, and I'm pretty confident it is possible with my current setup, is to maintain very close to 50% RH with the dehumidifier running <25% duty cycle. It will take a few weeks to dry the top soil and wood in the crawlspace to get stable. At this rate, this solution will cost ~$15/mo to maintain, as opposed to $7k up front (which still has a dehumidifier to run, but it would run much less often as the soil would be completely sealed out). That $15 will be somewhat negated by savings in heating and cooling costs as well, as my crawlspace is no longer exchanging hot air in the summer and cold air in the winter.
Good luck in whatever you decide to do. FYI, there are a bunch of resources on the web and some recent research into the controversial issue of sealed crawlspaces. If you need some links, let me know and I'll try to see if I can find what I used to make some of my decisions.
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Dustin Fredrickson
-- I'm a nobody --
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That's pretty annoying since it's okay to just wash your hands under the spigot outside your house (and thus the drain water hits the ground), but if you were to set a small sink under it you are in violation. Sort of off topic, but something to think about. I'm sure inspectors vary from county to county on how they handle this, too.
