The Deptment of Energy's Advice on Finding the Cause of Your Damp Basement
The source of your problem could be a water leak or high humidity. Both can lead to mold, mildew, or other biological growth. Depending on the severity of the problem, conditions can lead to rot, structural damage, premature paint failure, and a variety of health problems.
Water can seep into your house from the outside through a leak in the foundation or small gaps around windows or doors. Water can also come from inside your house from a leaking water pipe, toilet, shower, or bathtub. High indoor humidity caused by normal activities of everyday living, such as showering, cooking, and drying clothes can also be a source of the problem.
A damp basement is commonly caused by moisture migrating through a concrete foundation. There may not be a sign of any leak or standing water, but the moisture evaporates, increasing indoor humidity. Another common cause is condensation on the cold concrete walls and floors during humid months.
Note whether the problem occurs below a bathroom, on the ceiling, or at the corners. Where the problem occurs can lead to what is causing the problem. If the problem is localized, such as a spot on the ceiling, wall, or corner, it is possibly caused by a water leak. If the problem is in a large area, like a whole wall or room, then it might be caused by humidity.
If you plan to remodel your basement, it is important to control moisture problems before doing anything else. Taking corrective action can be relatively easily, but depending on the severity of the problem, they can be difficult and expensive to repair.
Stop Water Leaks
If a leak is the source of your problem, have it fixed first. If you have standing water on the floor of your basement after a heavy rain then it is likely from a leak in the foundation. Clean the rain gutter and redirect the downspout runoff away from the foundation.
Make sure the ground around the house slopes away from the foundation. If necessary, re-grade it so that the ground slopes away. If you have a sump pump, make sure it is working properly.
If you have water stains on the ceiling or wall around a bathroom, it could be a leak from a water pipe, toilet, bathtub, or shower. Hire a plumber to repair the leak. If the leak has caused substantial water damage or mold, you should hire a contractor who specializes in mold remediation and water damage repairs.
Reducing Indoor Humidity
If your basement has a dirt floor, cover it completely with plastic to slow water vapor coming through the soil. Use ventilation fans in kitchens and baths to control moisture. Check to make sure that the ventilation fans vent directly outside. In some cases, the vent fan may have been installed to vent into the attic, become disconnected, or been blocked.
Your clothes dryer should be vented directly to the outside. Inspect the vent duct. Make sure it is attached securely to the dryer. Check that it is clear of obstructions. Check for holes that leak air. If the vent duct is damaged, replace it with a metal duct. The vent duct should be cleaned at least once a year.
Ask a heating and cooling contractor to check your system to make sure it is sized and operating properly to remove humidity. If the system is too big, or if the airflow is incorrect, your air conditioner will not remove humidity like it should. Ask the contractor to check your duct system for air leaks, proper size, and air flow to each room.
Sealing air leaks prevents high humidity levels in your home. During hot, humid months, using a dehumidifier in the basement can reduce condensation on the walls. It may work better after you've sealed air leaks to reduce the amount of humid outdoor air you are bringing into the basement.