In order to do something about a damp basement, you have to determine what the source of the moisture is. Here's a low-tech yet effective way to find where the moisture is coming from: Cut a few 12x12-inch squares of aluminum foil or thick plastic and tape them to the walls and floor. Wait several hours, then check for condensation on the outside of the foil or plastic. If it's there, it's also in the air — a problem you can often solve with a dehumidifier. If the underside of the foil or plastic is wet, moisture is passing through the wall or floor from the soil. An interior coating can provide some relief, but you should also attack the source — outside.