You may find dark, damp sections of your windows this time of year.  But how to treat it? 

Moldy window sills

Q We have real wood window frames, and during the winter they seem to have grown mold on the bottom of the frame, where the glass and wood come together. What’s the best way to get rid of it? How can I prevent it in the future?

A To remove the mold, wash the area with an all-purpose cleaner and then treat with a weak bleach solution (10-1, water to bleach). As with any treatment, be sure to test in an inconspicuous area first, to see if there’s a negative reaction.

Then plan to prevent mold from occurring again next winter. Recognize that mold growth indicates a persistent condensation problem. During winter, the window glass is cool. When warm, moist indoor air hits the glass, it cools and the moisture in the air condenses on the glass (like it does on a glass of ice water on a summer’s day). This moisture repeatedly wets the wood, which never gets dry, and eventually mold grows.

To prevent this, you can raise the temperature of the glass or reduce the moisture in the air. Often a combination of the two processes works best. Some methods for doing this are more complicated and costly than others:

Warm the glass

• Simply leaving the drapes or shades open so air circulates, especially at night, will keep the glass warmer.

• Add another layer of glass (storm windows) or put plastic over the windows.

Lower the humidity

• Shut off the humidifier, if one is used.

• Operate an exhaust fan or fans (not the recirculating types) for an extended period every day.

• Consider adding a mechanical ventilation system such as an air-to-air heat exchanger (energy recovery ventilation). It effectively lowers moisture levels in a house while providing a steady supply of fresh air. Contact a heating, air conditioning ventilation (HVAC) contractor for information.

Send your questions to Fixit in care of the Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488, or call 612-673-7032, or e-mail fixit@startribune.com. Past columns are available at www.startribune.com/fixit. Sorry, Fixit cannot supply individual replies.