Q How do we prevent condensation on windows during cold nights? When we wake up, there is enough condensation on the windows to mop up with a towel. We have new windows, a new furnace and carbon monoxide detectors. We don't have very many houseplants, don't cook an unusual amount or have a dehumidifier. The house is at 47 percent humidity according to the little hygrometer. What can we do to prevent this condensation that builds up during the night?
A When warm moist air in the home contacts the cool window glass, the vapor in the air turns to liquid on the glass. The same thing happens in the summer when condensation forms on the outside of a glass holding a cold drink.
To prevent or minimize condensation (you might not be able to stop it), lower the indoor humidity and raise the temperature of the window glass. For many, the first step is to turn off the humidifier. Then:
• Keep window curtains and shades open at night so that air circulates and warms the glass.
• Operate the kitchen exhaust fan every time you cook to eliminate moisture. Make sure the fan actually blows to the outdoors and doesn't just recirculate. Look in the cabinet above the fan. If you see ductwork, then the fan exhausts to the outdoors. If not, you have a recirculating fan, which does not eliminate moisture. You'll want to crack the window when cooking. (Some homes have downdraft exhaust fans with the vent housing under the cooktop.)
• Operate bathroom exhaust fans while bathing or showering and for 15 minutes afterward. You can increase ventilation in the home (bringing more fresh air into the home) by operating the kitchen or bathroom fan for a period of time every day, beyond when you're bathing and cooking.
• Do not vent the clothes dryer to the indoors.
• Watch the hygrometer. (They're available at hardware and home stores.) The colder it is outdoors, the lower the humidity level should be indoors if you want to prevent condensation. Many Minnesota homes in winter can't handle humidity levels higher than 50 percent. But if the humidity is too low, it can lead to dry skin, bloody noses and shrinkage in wood floors and chairs. A compromise is best. Lower the humidity by increasing ventilation and watch the windows. If you have a little ribbon around the bottom, no problem. If you have to mop it up, it's too much. That can lead to mold and indicates possible larger problems in attic and wall cavities. Free publications that relate to the issue are available at Minnesota's Energy Center. They include "Home Moisture," "Indoor Ventilation" and "Windows and Doors." To download copies, go to www.commerce. state.mn.us. Click on Energy Info on the blue banner. Then click on Home Energy Guides. For copies sent by mail, call 651-296-5175 or 1-800-657-3710.
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