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 “thermal”- meaning they have insulated glass and either wood frames or aluminum frames with thermal breaks - are designed to function well to a range of +10F exterior and +70F interior, with an interior relative humidity (RH) of 25-30%. If any of these variables change, condensation may result.

Window treatment should help, shouldn’t they?

One of the most surprising aspects of condensation can be that it frequently take place behind fixed, insulated, or tightly fitting window treatments such as heavy drapes or honeycomb-style window treatments.

This is not what one would expect. We correctly think of these treatments as adding insulating value to the house. Why would condensation take place here?

The answer is simple. While it is true that window treatments can reduce the flow of heat through a window opening, there is an unfortunate by-product:

Window treatment prevents the flow of roomside heat from warming the glass surface. Glass surface temperature will cool. An environment may be created for condensation.

While window treatments retard the flow of heat, they don't do much for retarding the flow of water vapor. You need a fully taped foil or plastic vapor barrier for that.

Remember osmosis and diffusion from high school biology? Of course you do. These forces of nature are playing themselves out on your windows and draperies. To refresh your memory:

Osmosis: The force in nature wherein water naturally migrates from a place of greater concentration to a place of lesser concentration. This is your roomside humidity equally dispersing itself throughout your house.

Diffusion: Osmosis through a semi-permeable membrane. The membrane is your window treatment. Diffusion is also known as "vapor drive".

What can be done in my home? # Wipe off the condensation. By doing this, you are physically reducing roomside humidity.

Open the windows an inch or so. The dry outside air will mix with humid inside air and reduce roomside humidity.

Lower your humidifier settings to 25% or below.

Open up your blinds and drapes to a level which will allow the roomside heat to warm the glass surfaces.

Use exhaust fans during and after showering and cooking.

If you are painting or drywall taping open the windows to allow moisture to dissipate.

Look at your clothes dryer. Make sure it vents to the exterior. “Vent-less” clothing dryers can bring large amounts of moisture into living spaces.

It could be the windows too…

After excluding non-window sources, look for these possibilities on the windows themselves:

Weather-stripping not tightly compressed allowing air infiltration into the space. Although we know that air infiltration can actually reduce interior humidity, it can also focus a cold spot on a metal window or glass edge.If there is a cold spot you could see condensation or even frost which could be window related.

There could be a similar cold spot where caulk is missing at the perimeter of the window.

Your windows could be single-glazed, where only one layer of glass, rather than two, separates you from the exterior.

Deal with humidity first

Good strategy would be to deal with the living space issues first. If you attack them as we describe above, condensation can disappear in 24-48 hours.

If condensation persists, look to the windows and caulking.