Humidity Control Article

Why is there condensation on the exterior of my energy efficient windows?

Exterior condensation is actually an indication that the insulating glass in your new window is performing as it should!

Why does exterior condensation occur in the summer?

Summer condensation occurs when the outside window surface is cooler than the dew point temperature of the outside air. The outside glass surface in energy efficient windows will be closer to the outside air temperature, while the outside glass of an in-efficient window will be closer to the inside temperature. Low-E coatings help reduce the amount of radiant heat transfer through a window.

How does ‘Low-E’ contribute to summer condensation?

At night during the summer the Low-E coating reduces the heat transfer from inside, so the outside glass surface can cool significantly below outside air temperatures. When the inside temperature is below the outside temperature, a Low-E coating will allow the outside glass temperature to drop to about the same as that of an inefficient window. In cases where the outside air is colder than the inside temperature, a Low-E coating allows the outside glass to get even colder. Windows with Low-E coatings will develop more summer condensation than inefficient windows.

Is summer condensation detrimental to my home and/or family?

Exterior condensation is not a call for concern! For the most part, exterior condensation will not compromise the structural integrity or your home, as most of the surfaces in contact with the moisture are designed to withstand the elements. Likewise, exterior condensation evaporates as the sun warms the air. This type of environment is not conducive for mold growth, therefore eliminating any health hazards typically associated with condensation.

How can I prevent summer condensation?

Some options for preventing summer window condensation problems are to warm the inside surface of the window by raising the thermostat setting or by trimming back trees, shrubs and bushes to allow for maximum sunlight exposure.

In summary, condensation occurs when a surface falls below the dew point temperature of the air. The outside glass in an energy efficient window will be closer to the outside temperature, and the inside glass will be closer to the inside temperature. The glass in an inefficient window will be more heavily influenced by both inside and outside temperatures. A Low-E coating will tend to warm the outside surface in summer sunshine. A Low-E coating will also lower the outside surface temperature at night in the summer. Therefore, a low-E coating will reduce the potential for winter condensation, while creating more potential for summer condensation.