Questions and Answers

Condensation on windows is an alarming signal of excess humidity in a home, right away most people put the blame on the window.

Windows do not cause condensation.

Condensation is becoming more and more of a concern because it is a product caused by progress. With newer technology in-home insulation and thermal improvements, today’s houses are becoming “tighter” and more energy efficient. The tighter houses mean that humidity levels can no longer balance out between the inside and outside of homes. Windows are often to blame for the condensation as they are first place that it is seen. Your new windows are more airtight and less air is entering your home from the outside. The air leaking from older windows evaporated the moisture before it could collect.


What is condensation?

Condensation occurs when air that is saturated with moisture hits a cooler surface. Warm air can hold more moisture than cool air. When warm air that is saturated hits a cool surface the air’s temperature is lowered. This lowering of the air’s temperature causes the release of the excess moisture in the form of condensation.

What causes winter condensation?

Winter condensation is caused by excess humidity inside the home and is seen on the inside of the windows. This excess humidity can be caused by a number of factors including cooking, showering, laundering, houseplants, pets or humidifiers. Temporary condensation is not considered detrimental. Temporary condensation forms during baths, cooking, dishwashing, laundry and other activities that put steam in the air. Condensation that appears at the start of each heating season occurs because homes absorb moisture through humid summers and the condensation should cease after a few weeks of heating. Sharp temperature changes may also lead to temporary condensation. Moisture levels inside the home need to be reduced if the condensation remains on the windows throughout the day, even when the outside temperatures have warmed up. When condensation is forming and running down walls or mold/ mildew are visible. Though windows are often the first place it is seen, excess moisture may also be in between walls and in attic insulation. The windows may be acting as an early warning sign of excess moisture in the home. To reduce excess moisture insure humidifiers are correctly set according to outside temperatures. Use exhaust fans and check that all ventilation equipment is adjusted properly. Make sure attic louvers remain open all year round and that crawl spaces are properly ventilated.

Why do I see condensation near the bottom of each sash?

Each window sash is self-contained unit with a sealed atmosphere. The air in the atmosphere becomes layered just as air does in any closed space. The coolest air settles at the bottom while the warmer air rises to the top. This means the glass surface will be cooler near the bottom of each sash, thus condensation will first appear on the cooler surface.

What causes summer condensation?

Summer condensation is on the outside of the windows and can occur for a number of reasons. The glass temperature drops below the dew point temperature of the outside air or there is a high relative humidity. As the summer sun warms a window, a low-E coating treatment reduces the amount of heat that moves inward. This is what energy efficient windows are supposed to do. However, at night during the summer, heat is radiated from the outside glass to the cold outside air. The LowE coating reduces the heat transfer from inside so the outside glass surface can cool significantly below outside temperatures. Under the right combination of inside temperature, outside temperature and outside humidity, windows with LowE coatings can develop summer condensation. Relative humidity outside the home cannot be controlled and usually evaporates during the day. Exterior condensation is a sign that your windows are doing their job; keeping the inside and outside separate.