Every year, usually in January, we (Builders Architectural - a top Chicago-based window contractor) receive phone calls from customers who complain of sudden condensation on their windows.

This condensation can be surprising because of its sudden onset and large volume. Condensation is widely misunderstood but actually it is quite simple.

Think of a glass of cold beer on a muggy summer day when you are outdoors. The glass in your hand is dripping water on its outer surface to such extent you might almost think the liquid is actually leaking through the glass.

Unless you are drinking from a practical joker's dribble glass, leaking glass is probably not the cause. The real cause is airborne water vapor(a gas) converting to liquid when it cools while contacting the cold glass. The water vapor has cooled to its "dew point" and condensed into a liquid.

This happens on your windows during a winter cold snap in the exactly same way.

Lets say you live in Chicago and so far the winter has been moderate. Your humidifier is set at 35-40% relative humidity(RH). One day the outside temperature drops from +30F to +10F.

Suddenly your windows are fogged. In extreme cases water drips down the face of the windows. Left unattended, water can damage window frames and finishes.

Then you call your favorite window contractor and complain that your windows are defective. Chances are they are not. The elevated humidity in your home has come in contact with very cold glass surfaces. The water vapor within the air has changed to a liquid form and is now visible on your windows.

Generally, most windows which are