Basements are naturally cooler than the rest of the house, which makes them subject to high humidity. Because the basement floors and walls are in contact with the soil, and soil temperatures several feet below the surface remain at a constant temperature of 50°F – 60°F or less, basement floors and walls tend to remain cool. Also, basements generally do not have many windows, if any, so there is little or no solar heat gain. Because cool air is dense, it is less capable of holding moisture, or water vapor, than warmer air. When the air temperature is cooler, it is not only denser, but also less “active.” Molecules in a less active state are more likely to be solid or liquid than vapor or gas. Therefore, since basements tend to be cool and colder air has less ability to hold gaseous water molecules — basements will have higher relative humidity.
Moisture is the most common problem in basements — either entering from outside sources or being produced inside by the occupants’ activities. The soil around the walls can contain a large amount of moisture from surface water that is seeping down or from a high water table. Water can find its way inside by gravity or through a crack or flaw in the water protection layer of the foundation. Water can also be pulled up by a “wicking action” or “pushed up” by hydrostatic pressure from the soil under the walls or floor.