What Are the Causes of Moisture on Window Sills?

Condensation collects on window glass and runs down into the window sill.

Moisture developing on the window sills is a sign of a larger problem in the home. If this is a chronic occurrence it means the house is too humid inside. Wiping down the sills with a dry towel will take care of the obvious problem, but it doesn't get to the causes.

Causes

Moisture on window sill is caused by condensation. This occurs when moist air comes in contact with the cool surface of a glass window. This commonly happens to the outside surface of a cold can of pop on a warm summer day. The air dew point is warmer than the temperature of the glass, bringing the water vapor in the air down to a low enough temperature to condense into a liquid. Condensation can occur on any surface which is colder than the surrounding air dew point.

What is Dew Point?

The air dew point is the temperature when air can't hold anymore water vapor. Any excess vapor must condense into a liquid. Hotter air can hold more water vapor than cool air. When the air temperature cools suddenly the water in it releases into a liquid form. The dew point is always lower than the actual air temperature. Dew point is different from humidity, which measures the amount of water vapor currently present in the air.

Damage it Causes?

Condensation on window sills is damaging if chronic and left alone. Water condensing on the windows is a sign it's also condensing in other harder to see areas such as the walls and roof. It causes mold or mildew growth, wood rot, warping, roof ice build-up, damp insulation and discolored, blistered or bubbling paint. Humidity build-up usually occurs in new homes which are built to air-tight and energy efficient standards. The water in the home evaporates and builds up, and has no way to escape.

Prevention

To stop moisture from collecting inside the home you need to reduce the humidity in the home and encourage air movement out of the home. Open the windows and air out your home at least once a day for a few minutes. Use gas, oven, bathroom and fireplace vents to allow humid air to escape the house. Stop using any humidifiers. Check that the vents in your attic are clear and large enough to allow air to properly escape.

References

• Gorell Windows and Doors: Understanding Condensation
• Weather Questions: What is Dewpoint Temperature
• The Weather Prediction: What is Humidity?

Resources

• Marvin Windows and Doors: Condensation