Insulating Glass Windows - How They Work
Insulating glass, often referred to as “IG”, “double-pane”, "dual-pane" or “Thermopane” glass, was developed to create a more energy efficient window. Insulated glass is essentially a “sandwich” of two pieces of glass separated by a spacer bar and sealed on all four sides to create a dead airspace between the two panes. This technology was actually invented in 1935 by the Libbey-Owens-Ford Company (under the trade name Thermopane™). However, it has been significantly enhanced and improved over time.
When there is a difference between inside and outside temperatures, heat transfers through a window. It's lost to the outside during the heating season and is gained from the outside during the cooling season.
Glass is a very good heat conductor. Using two layers of glass with an air space between dramatically cuts the heat flow. Single-pane windows can quickly conduct heat to the inside or outside. Dual-pane windows, with a 1/2 inch to 3/4 inch air space between sheets of glass, work like a Thermos bottle to cut down the flow of heat. If you replace the air between the panes with an inert gas like carbon dioxide, argon or krypton, the window will transfer even less heat and be even more efficient.
A window's thermal performance - which can be measured at the center of glass, the edge of glass and the frame - is rated with a U-value, its overall ability to resist heat flow.
Even the glass itself has been coated to reflect heat. Low emissivity (low-E) coatings can help to increase U-values. A low-E coating is a microscopically thin layer of metal or metal oxide deposited on window glass. The coating reflects warmth back into the home in the winter and prevents unwanted heat from entering the home in the summer. The lower the U-value, the better the window's energy performance will be.
Why Insulating Glass Windows Fail
Insulating glass in windows and doors has to put up with a lot of abuse. The seals have to withstand slamming and banging. They have to be flexible enough to allow the panes to contract in cold weather and expand in hot. The seals can’t stiffen and become brittle in the cold or soften and ooze when it’s warm. They have to stand up to wind, hail, rain, damaging ultraviolet rays, old age, atmospheric pressure changes, errant Frisbee discs and suicidal birds. Still, with all these challenges, double-pane windows are remarkably reliable. Studies by the Sealed Insulating Glass Manufacturers Association show that high-quality units, properly installed, have a 1 percent failure rate after 10 years and a 3 percent rate after 15 years. When seal failure does occur, moisture begins to form between the two window panes and the unit must be repaired or replaced. Foggy window repair is a viable option at less than 1/4 of the cost of replacement.
The leading causes of failure are seals breaking down from excessive exposure to water, excessive heat, (heat causes the panes to expand and contract, and it softens and weakens the seals until they develop a crack in their armor and allow moist air in) and old age. Even the most elastic, flexible seal can’t last forever. Eventually a seal will allow moisture to enter the window.
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