When a house is built and placed over the ground, whatever moisture is in the soil will be drawn upward into the higher levels of the home. This is what is called stack effect. The EPA reports that 80% of the moisture in a home comes from under the basement slab.
It is for this reason Grate Products recommends a closed basement drainage system along with a high quality vapor barrier on the foundation walls will help keep ground humidity from easily entering the basement and home.
High efficiency dehumidifiers like the Santa Fe Classic or Advance are effective in removing excessive moisture. When the relative humidity in a basement goes above 50% then this allows mold a perfect environment to grow in.