What Are the Causes of Condensation on the Outside of Double-Pane Windows?


Window condensation can occur for a number of reasons. Some causes are natural and the condensation will disappear, while other causes are an indication of what industry experts refer to as “failure.” While natural condensation is nothing to worry about, if your window develops and retains condensation for other reasons, it may require repair or replacement.


  • Condensation is an accumulation of water on the windowpane. Condensation on the outside surface of a window most often occurs during cold weather. Water vapors develop as the temperature lowers, and the molecules inside the vapors move closer and closer together until they form water droplets. When the air temperature reaches what is called the “dew point,” the condensation process begins, and the droplets collect as a visible stream or build up of liquid, on the outside or in between the double windowpanes.

Structural Problems

  • A double-pane window is not made of glass alone. Gas is enclosed in between the panes. The gas layer provides desirable insulation properties to reduce heat loss and during cold weather and heat gain during warm weather. Some double-pane windows also have a thin layer of film covering the inner panes to further reduce heat loss and gain. When the inside layer is compromised through faulty manufacturing, improper maintenance or aging, condensation can develop.

Natural Condensation

  • Double-pane windows go through a natural daily cycle of expansion and contraction, which is called solar and thermal pumping. Solar pumping occurs as sunlight shines on the windows it heats the gas inside of the two glass panes. Thermal pumping occurs when the sun goes down and the glass contracts. Natural condensation, on the outside of windows, due to solar or thermal pumping is okay, and usually evaporates as the temperature rises.

Silica Desiccant Deterioration

  • In addition to gas and film, double-pane windows have a material called silica desiccant inside the frames of the window. The desiccant is composed of tiny pellets that absorb moisture. As the desiccant ages, it loses its capability to absorb moisture and prevent condensation if the moisture from the outside seeps through the double-pane windows to the inner areas. This results in condensation forming on the outside and in between the double-pane windows.


  • Homeowners should continually examine double-pane windows. Even newly installed windows might have batch issues, which if left undetected can lead to early failure. Check windows on bright and sunny days by inspecting the edges closest to the frames to see if there are any visible signs of condensation. If so, you should notify your retailer immediately to have the windows repaired or replaced. Every two yeas you should re-caulk the exterior seam where the glass and the window frame meet the exterior material of the home, such as wood or brick mortar. This reinforces the integrity of the double-pane window structure and helps prevent the development of condensation.

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