Excessive Moisture in Home

Problem: Excess humidity causes condensation that leads to dripping on windows, wet sills, damp walls, mold, wet insulation, wood rot, and metal corrosion.

Background: Humidity is water vapor absorbed in air. Condensation forms when vapor cools enough to convert to liquid, such as the fog, water, or ice seen on a cold window, or the dampness on cold wall surfaces. Excessive humidity can come from sources such as humidifiers, bathing, washing and drying clothes, cooking, washing dishes, mopping floors, plants, pets, plumbing leaks, humans, whirl pools, steam baths, saunas, and attached greenhouses and aquariums. Sources also may include moisture migrating through foundations, brought inside by air leaks, from combustion venting failure, or from building materials and furnishings.

What to do: A 30% to 50% humidity level is enough to control dry skin problems, dry throats, coughing, and static electricity. Higher levels may increase potential for mold and mildew. Even reasonable indoor humidity, however, can cause condensation and wet insulation in cold climates if water vapor penetrates into cool parts of the home, such as attics. (Holes where wires, pipes, ducts, or light fixtures enter attics should be sealed to prevent humid air from entering.) Wood rot may occur in studs, wall sheathing, or roof decking if condensation is pro longed.
Solutions include using bath room and kitchen exhaust fans or whole house ventilating systems, and controlling excessive moisture sources, such as wet basements, un vented clothes dryers or kerosene heaters, indoor firewood storage, overly frequent showers, or misuse of humidifiers. Installing an interior- side vapor retarder and a continuous air barrier system to keep vapor- laden air from penetrating into cool areas, are helpful as well as installing adequate corner insulation and thermally insulated windows and doors. In bathrooms, using triple-insulated glass, and well-insulated walls and ceilings installed with vapor retarders, can reduce condensation.

Special advice: Excessive condensation can also be caused by having improper vents for fuel-burning equipment, such as furnaces, water heaters, wood-burning stoves, and fireplaces. If this is the case, take action immediately because this situation can be dangerous. To be properly vented, the appliance area should be kept at neutral air pressure with a constant supply of outside air. All exhaust fans, exhaust hoods, and appliance exhausts should be balanced with an adequate supply of outside air brought into the house or directly to the equipment.

Helpful hint: New houses often have a higher indoor humidity the first few years due to the moisture in building materials. Opening windows or extra use of ventilation fans on warm days may help. If moisture problems occur when an automatic set-back thermostat (automatically adjusts the system to save energy) is used, adding a timer to operate the furnace fan several times an hour, or leaving the heat on during the night, may solve the problem.