If condensation has formed on your window, it is time that we figure out what exactly is happening. After all, windows are not wells; they do not make water. When condensation forms on a window, it is trying to tell you something. On the modern window, condensation usually forms in one of three places: the outside of the window, the inside of the window, or between the panes of glass (in double and triple-paned windows). Each has a different cause and a different solution.
Condensation on the inside of a window is a sign that airborne water is trapped in the house due to poor air circulation and exchange. This usually means that the furnace is not getting enough air for proper circulation. To test this, tape a piece of kite string to the molding above the window. Make certain that all the windows and doors are tightly closed. Wait until the furnace goes on, and after it has been on for about two minutes, open the window. If the kite string goes outdoors, against the screen, you have positive air circulation and do not have a problem. In the event the string blows in, your house has negative air pressure. You do not have enough combustible air in the house and, most probably, you do not have sufficient air exchange for good health. If you have a forced air furnace, you can probably eliminate the condensation by turning on the furnace fan for several hours.
Lately, there have been complaints from people reporting condensation on the outside of their double-pane Low E glass windows. This is not a problem, but a sign that the Low E coating is working and reflecting heat back into the house. This condition usually occurs in the spring and fall when there are relatively warm, humid days and cooler nights.
In summary, interior and/or exterior condensation is not the fault of the window, but most often it is the result humidity and temperature. Like a cold aluminum can on a hot humid day, or a mirror after a shower, water vapor will condense into water droplets or fog on colder surfaces.
Causes: Plants, showers, cooking, hot water base-board heating, and damp basements all add humidity. All rooms within a house may not have the exact same issues as others.
Possible Solutions: Reduce humidity levels in home, leave drapes and blinds open so warmer air can reach glass, open fireplace dampers or add roof vents so humidity has an easier escape route.
See the Chart Below for Recommended Humidity Levels to Prevent Window Condensation: