Get a relative humidity meter from Radio Shack or Wal-mart and check the relative humidity in your basement. The relative humidity should be less than 50% RH all the time. This may require removing +5-8 gallons of water a day during the humid times of the year. This is more than the typical residential dehumidifier is capable of. After maintaining less than 50% for a couple weeks, check for condensation. If moisture is present behing the vapor retarder, then remove the retarder.