Q. With the first cold snap in fall, we begin a long season of fighting condensation in our house. The water drips off the window glass onto the sills, then puddles on the floor below. This will be our third winter in this house. Can you tell me what causes this and what we can do to get rid of the excessive wetness?

A. When the first cold days of fall arrive, outdoor humidity drops. In many climate zones, summer weather is humid and hot, and moisture builds up in our houses, penetrating wood, dry wall and concrete.

With less humid weather the moisture begins to dissipate. A window, the temperature of which has been lowered by the cooler weather, is the first place the trapped warm, humid air condenses.

Windows are usually the coldest part of the house, having the least insulating value. Installing insulated windows may help, but it won't cure the problem.

The moisture that has penetrated your house during the summer usually dissipates as the cool, dry weather continues, and less condensation is apparent. Overnight condensation, which builds up as temperatures drop outside, is normal. When daylight returns, raising the temperature outside, the condensation should dissipate.

Condensation is harmless if it dries up, but when it remains it is a sign of trouble. Persistent high levels of moisture can damage your house structurally, resulting in dry rot, paint problems, and mold and mildew growth.

If you have a continuing problem with excess condensation on your windows, you are likely to have unseen problems in your walls and attic, too.

Newer houses are much more prone to problems of this nature. With efforts to make homes more energy efficient, we have sealed up the interiors too tightly. There just isn't enough air entering to flush out moisture. The rate of escaping warm interior air is greatly reduced, resulting in condensation on cooler wood sheathing, framing, attic interiors, etc.

When warmer weather returns, the moisture-laden interior (wall cavities don't dry out easily) is a breeding ground for fungus. Older houses with air leakage--cooler, less humid air entering and warm, humid air flowing out--aren't damaged as much. When indoor air flows out, warming interior structural parts, less condensation collects and any fungus-producing moisture dries up much faster.

Basically, the way to reduce condensation is to control indoor humidity. Taking showers, cooking, doing laundry, watering indoor plants, feeding pets and operating aquariums are all humidity-producing activities.

A backdrafting furnace or water heater is a problem. Gas and oil-fired appliances release a lot of water vapor. A high level of condensation on these units may indicate that the appliances are not vented properly and should be checked. Many furnaces have a central humidifier, designed to keep interior air from getting too dry. These can be out of adjustment, releasing too much moisture.

Damp basements and crawl spaces are another source of unwanted moisture. Make sure that rain runoff is guided away from the foundation and that concrete basement walls and floors are treated with a masonry sealer and crawl spaces are covered with six-mil plastic.

Limit the number of indoor plants. Your bathroom and any whirlpools, saunas and hot tubs need adequate ventilating fans to draw the air outside. Clothes dryers and any combustion device should vent outdoors. Staggering activities that produce moisture--no simultaneous showering, cooking, laundry tasks--can help.

Often the solution can be as simple as opening a window for five to 10 minutes to air out the house when condensation builds up. Fans operating on reverse cycle are helpful in circulating the air and keeping warm air moving off the ceiling and back down walls and windows.

If excess condensation continues after reducing activities that produce moisture, you will have to use a dehumidifier or install venting fans to force moist air out.

More expensive retrofits to combat the problem would be installation of tight-fitting storm windows, insulated windows with two panes or even a third pane (triple glazing). With forced-air heat you can use automatic methods, too. A heating professional can hook up a humidistat (a device that measures relative humidity) to a fresh-air vent coupled to your furnace duct work. The humidistat then opens the vent when humidity rises too high.

Send e-mail to copleysd@copleynews.com or write to Here's How, Copley News Service, P.O. Box 120190, San Diego, Calif. 92112-0190. Only questions of general interest can be answered in the column.