In SC there is a very high humidity rate, especially in the coastal areas. I have found many homes having moisture exposure due to many factors ( humidity, leaks, lack of ventilation, lack of moisture barrier,etc). More often than not, the problem stems from either a lack of ventilation ( because they wanted to save money on construction or they install cheap vents that do not work after several years). I install ventilation in a home's crawlspace when I locate moisture content in the high 20% range and mold/fungi( since at 28% it becomes wood decaying fungi. I have seen a good many homes having the sills and joists destroyed due to fungi simnply because the homeowner didn't have enough vents installed in the crawl( also due to the configuration of the home, where cross-ventilation could not occur.) Another tool that can be used is a power vent. This should pull the air out of the crawl ( not blow it in) and should be placed where the moisture problem exisits. Automatic vents are excellent for controlling ventilation and saving on the utility bills. In SC,typically, if you do not have a moisture barrier you will eventually have moisture problems. An ounce of prevention is worth a pound of cure!
Many contractors will tell you, no vents are better than having them, or they will tell you to keep them closed. Why then do they install them in the crawlspace foundation? Interesting,uh?
Do yourself a favor and have your crawlspace checked out. Many companies do this for free.
NOTE: Do not cover more than 70-80% of the crawl! If you do the wood will dry out and start cracking and that is another problem! Leave 2-3 feet around the inner edge of the crawl uncovered.