Energy adviser: Warm, moist indoor air can be issue



It happens often here in the Northwest. When warm moist air inside homes settles on chilly surfaces such as glass windows or outer walls, you get condensation.

Water runs down the window panes or moisture collects on other surfaces, and that can contribute to dry rot, mold and mildew problems.

Industry research shows that visible indoor mold and mildew may be present in as many as one-third of all Northwest homes. Most vulnerable are older houses with single-pane, metal-framed windows, those with blinds and drapes that are often closed, homes with high indoor relative humidity, and multifamily and manufactured housing. But mold and mildew can occur in both new and older structures.

While the science of the phenomenon is simple, there are no easy solutions to water condensation in winter, says Bob West, an energy counselor at Clark Public Utilities.

“Bedrooms with attached baths are notorious for condensation because bedroom doors are often kept closed and they’re not always well ventilated,” West noted. “Using an exhaust fan or even a dehumidifier can help lower moisture levels and keep mold or mildew from developing.”

Even more serious problems may occur when two-by-four rafters in the ceiling or wooden wall studs are exposed to colder exterior air. Cold rafters attract moisture that becomes a breeding ground for mold and mildew embedded in the wall or ceiling.

“Sometimes you can see ‘shadowing’ on sheet rock in a ceiling where rafters are acting as moisture condensers,” West said. “The solution is to cover the rafters in insulation.”

Additional tips for managing indoor humidity:

• Get rid of moisture in the air by making sure your exhaust fans are operating at top performance. Always use your kitchen fan when steaming vegetables or simmering other food.

• Use bathroom fans to pull hot, moist air outside. “Many ceiling fans make a lot of noise but don’t do a very good job of removing moist air,” West said. Test your fan by holding a single square of toilet paper to the fan. If the fan isn’t holding the tissue to the outlet, it is not working like it should, West said. You may want to install a high-volume bathroom fan rated to draw a minimum of 100 cubic feet of air per minute. Quality fans also are rated for noise.

• There are trade-offs when conserving heat by closing mini-blinds and using heavy curtains. The blinds and curtains keep the heat in, but they also allow windows to become colder, which can result in more condensation on the glass. If your windows are vinyl, make sure the “weep holes” at the bottom of the frames are clean and functioning to allow moisture to drain.

• House plants put a lot of moisture into the air around them. Consider reducing your indoor plant population or go with plants that need little watering.

• Avoid stacking damp fireplace wood inside. Soggy logs can contribute to indoor humidity as they dry out.

• Make sure that clothes dryers are vented outdoors.

nFix leaky faucets as soon as possible.

• Measure indoor relative humidity using a hygrometer, a device available in most hardware stores. Homes should have humidity levels less than 50 percent to avoid mold and mildew. For best health and comfort, humidity levels should be less than 30 to 40 percent, experts say.

Humidity is not the only contributor to indoor mold and mildew. Greasy film from cooking oils and soap scum can create a welcoming environment for mold spores, so it’s important to keep walls and other surfaces clean. Use white vinegar in a water solution to wipe down surfaces after they’ve been cleaned of mold to prevent a return.

West sees indoor humidity problems as a Catch-22, because keeping humidity down may mean pulling warmed air out of your house with fans, leaving windows uncovered at night or turning up the thermometer.

“All of that can mean energy loss,” West said. “You either must remove the humidity from the room or warm the surfaces collecting moisture to cut down on condensation. There’s no easy answer, but homes are healthier when mold and mildew are kept at bay.”

Energy Adviser is written by Clark Public Utilities. Send questions to or to Energy Adviser, c/o Clark Public Utilities, P.O. Box 8900, Vancouver, WA 98668.