Musty odors and dampness in basements are common summer problems in many houses. Sometimes, the dampness comes from summer rains that result in storm water finding its way into the basement. Sometimes, it doesn’t enter as water, but as water vapor coming through the basement wall from saturated soil on the outside of the wall. More often, it comes from condensation against cool surfaces in the basement. Because basement walls are in contact with the soil, and soil temperature several feet below the surface remains at a constant temperature of 15.5 degrees C. or less, basement walls and floors tend to remain cool.
While walls in newer basements are insulated, floors generally are not. Cool air can hold less water vapor than warm air. When outside air at 26.6 degrees Celsius with a relative humidity as low as 60 % enters a basement and cools to 18.3 degrees C., condensation begins to occur on cool surfaces. The higher the outside temperature and relative humidity, the more moisture will be available to condense. Similarly, the cooler the basement, the more moisture is available for condensation. You might think opening windows and letting more warm air into the basement will warm the air and decrease the condensation problem. Unfortunately, it is difficult to provide enough warm air to increase the surface temperatures of uninsulated basement walls and floors.
The earth behind the walls absorb whatever warmth the added air provides, with little increase in the temperature of the walls and floors. What the added air does provide is an increased supply of moisture, which can actually make condensation worse. If the moisture problem results primarily from condensation, the best solution is to close the basement off as much as practical to minimize the amount of warm humid air that enters it. Then, use a dehumidifier to reduce the moisture level of the basement air.
Dehumidifiers work like air conditioners. A blower circulates the warm, humid air over refrigerated coils. Some of the moisture in the air condenses on the coils and is collected in a water tank or is discharged through a hose to a floor drain. Dehumidifiers are rated by the number of pints of water they collect per day. Small ones remove 20 to 25 pints per day, while large ones can remove 40 to 50 pints a day. The Association of Home Appliance Manufacturers provides recommendations for selecting the appropriate size dehumidifier. For moderately damp basements of less than 1,500 square feet that have a musty smell all summer and damp spots on walls, a dehumidifier rated at 25 pints per day should do the job. Consumer Reports, on the other hand, suggests there is no advantage in going with the minimum size to do the job since some of the small ones it tested were less efficient than the larger ones. The larger ones would also operate less to do the same job, which could be an advantage since many models are quite noisy.
Conventional dehumidifiers are meant to operate with temperatures at 18.3 degrees C. and warmer and only drop humidity levels to about 50%, which should be fine for summer basement conditions. When the temperatures drop much below 18 degrees, the coils freeze and the units cease to operate. Dehumidifiers are relatively expensive to operate, so you should do what you can to minimize their use.
Check to be sure there isn’t any water leaking in or coming through the walls. Check your grading around the house so that water clearly flows away from the foundation. Down spouts that don’t direct the roof runoff away from the house should also be corrected.
If you are running air conditioning in your home, you should turn off the dehumidifier. The air conditioning should do an adequate job of dehumidifying, and it is likely that the air conditioning may drop basement air temperatures to the point where the dehumidifier will freeze up.
Finally, try to keep warm humid air from leaking into the basement. This means keeping doors and windows closed and caulking or otherwise sealing any obvious air leaks.