The ideal humidity for a basement, finished or unfinished, is at or below 50 percent relative humidity. Relative humidity is the percentage of water vapor in the air as compared to the maximum amount it can hold. At 50 percent humidity or below, mold does not propagate.
A high humidity level can be lowered by using a de-humidifier, a household appliance that removes water vapor from the air. When purchasing a de-humidifier, it is important to determine the square footage of the basement as well as its humidity level in order to choose a unit capable of doing the job while using the minimum amount of electricity. The appliance should be used to keep the relative humidity between 40 and 50 percent.