Why Are My Windows Foggy?

portrait photo of Charles

Charles E. Whitley, P.E.

For those who live in hot or cold environments, double pane windows are a great way to increase the energy efficiency of your home. Double pane windows consist of two panes of glass that have been sealed together in a frame, with a pane of glass on the interior and exterior surface of the window, and a gap between the panes. In some windows, the gap is filled with an inert gas such as argon, or air is withdrawn from the space to create a vacuum. The inert gas or the vacuum is intended to act as insulation to minimize the transfer of thermal energy through the window.

Double pane window cross section

Different manufacturers use different types of material to seal the space between the panes of glass. Most double pane windows have two seals, an inner and an outer seal, and an aluminum spacer to maintain the gap between the panes. Regardless of the methods and materials used in the construction of the seals, all seals are subjected to multiple forms of abuse. The seals experience stresses that result from the expansion of the glass during hot weather and the contraction of the glass during cold weather. They are subjected to degradation from ultraviolet rays of the sun, and to deflection of the glass due to wind, hail, and the random impact of baseballs, frisbees, and other toys. The seals must remain flexible during periods of cold weather and must maintain their integrity during periods of hot weather.

If the seals are damaged or fail, the most common result is the distortion of the view through the window, known as fogging. When the seals fail and gaps develop along the seals, humid air can enter the space between the two panes. Once in the space between the panes, the moisture in the air often condenses on the inside face of the glass, resulting in the distortion of the view. With the condensation on the inside of the windows, it cannot be removed by cleaning the outside, accessible surfaces of the windows.

The most common cause of seal failure on double pane windows is aging of the seal material. After years of exposure to ultraviolet light and heat, the seals typically become brittle. Once the material becomes brittle, it loses the ability to flex and maintain a bond to the glass when the glass expands or contracts. Window manufacturers recognize that this condition can occur, as shown by their warranties on the windows. A review of the warranties provided by leading window manufacturers shows that most guarantee that their windows will remain fog-free for a period of 15 to 20 years. A portion of the manufacturers reduce that fog-free guarantee to 10 years if the windows are installed in a facility that is not owner-occupied. One of the most common locations for fogging of windows is in beach condominiums. The windows in beach condominiums typically have high exposure to ultraviolet light (sunshine), and are regularly exposed to the oceanfront winds. A good thing to remember next time you are reserving a place for that beach trip: Ask about the quality of the view through the windows.


About the Author

Charles E. Whitley, P.E. is a consulting engineer with our Birmingham Office. Mr. Whitley provides technical consultation related to the design and construction evaluation of industrial, commercial, and residential structures; failure analysis; damage assessment; and expert testimony. You may contact Charles for your forensic engineering needs at cwhitley@edtengineers.com or (205) 838-1040.

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