Condensation on interior window surfaces is a problem for many homeowners, particularly those who own newer, tightly sealed homes. Older homes were not as well insulated, and the drafts of colder air that enter them, although bad for heating bills, are actually good for decreasing condensation. The colder, drier air absorbs much of the moisture in the indoor air, and thus it doesn't condense on the windows.
Air contains a lot of water. The warmer the air is, the more water it can hold, which is why many people experience clammy discomfort in hot, humid weather and chapped skin in dry, winter weather. When a home's heated interior air comes into contact with the cold of a window pane, the temperature of the air drops quickly and the water content is forced out of it. This is called condensation. Contrary to popular belief, the condensation is not leaking in through the windows, but is coming out of the air inside the home.
Although high-tech windows involving dual panes, vacuums and argon-filled spaces will minimize the passage of heat and cold through the windows and thus reduce condensation, they will also decrease the passage of moisture between inside and outside, thus increasing the humidity in the house and making it more susceptible to condensation. The net result may be no change at all!
The dew point is the temperature at which air can hold a certain amount of water. If the air inside the home is 68 degrees Fahrenheit and the dew point is 50 degrees Fahrenheit, condensation will form any time the air comes into contact with anything that is cooler than 50 degrees Fahrenheit. On a cold winter day, the surface of the window glass will probably be cold enough that condensation will form because the temperature of the glass is below the dew point of the air.
A little bit of condensation is not a problem, but too much of it can cause problems with mold and mildew and even rotting wood around the windows. Excessive condensation can run off the windows and pool on floors and other surfaces, causing cosmetic damage.
One solution to condensation is to equalize the temperature of the home with the outside, obviously not a good solution on a bitter winter day. Another solution is to install a dehumidifier in the home to lower the water levels in the air. This is a tricky business in the winter because many people find the air already too dry during cold weather. Like many household problems, it is a matter of finding the happy medium that will minimize one problem without making another problem worse.