What Are the Sources of Water Vapor in the Home?

Air and Foundation Leaks

• Water vapor enters a home through the walls and the roof at any place that is not airtight. Moist air enters through cracks around windows, air conditioners, electrical outlets and doors. If the ground around a home is not graded properly, the foundation or crawl space becomes wet. Similarly, downspouts wet the foundation if they let out water too close to the house. Water from a damp slab or crawl space then evaporates into the air of the home. The amount of vapor entering the air increases as the temperature rises.

Plumbing Problems

• Poorly maintained plumbing allows water to accumulate and evaporate into the air. Leaky pipes contribute to humidity. Some harmful leaks are in concealed areas, such as above the ceiling, under the floor and inside the walls. Dripping faucets and damaged tile grout in the shower also add to the water vapor in the air. Other common sources of humidity include leaking water heaters and faulty washing machine hoses.

Living Things

• People and pets add to the humidity of the air in the home through their breathing and perspiration. The amount of vapor added depends on the size of the person or animal and the level of activity. Houseplants also add to indoor humidity, both by transpiration and by evaporation from the soil and standing water in saucers.

Routine Tasks

• Showering and bathing increase water vapor in the home. The amount added with a shower depends on several factors, including the length of the shower and whether a fan is used. Crystal L. Pilon of Oak Ridge National Laboratory reports that about half of the water remains in the house even if an exhaust fan is used. Washing dishes and laundry, cleaning and cooking all release some water vapor. Dryers normally are vented outside, but air-drying laundry indoors adds water vapor to the home environment as well.

Controlling Water Vapor

• Control excess water vapor by stopping it at its source and removing unavoidable humidity. The U.S. Environmental Protection Agency recommends grading around the house with a 1/2-inch downward slope for each linear foot of property. Keep downspouts several feet from the house and keep yard drains clear. Caulk and seal any cracks around the foundation. Make sure the roof keeps water out and that air conditioners are in good repair. The University of Florida Extension also recommends repairing faulty plumbing and fixing air leaks in the structure by weatherstripping and caulking. The EPA recommends whole-house mechanical ventilation to blow out excess moisture. At a minimum, exhaust fans should be installed in the kitchen and bathrooms.