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About This Project
Maybe it’s happened to you. You wake up one morning and notice one of your double-pane windows or patio doors is a little cloudy. You wipe the inside pane with the sleeve of your bathrobe and the cloudiness remains. You wipe the outside pane and get the same results. Then you have a revelation: Eureka! The cloudiness is between the two panes of glass. Some days (and especially at night) the cloudiness disappears, but then it reappears—and over time the cloudy area becomes larger, thicker and more obtrusive. Welcome to the Insulating Glass Failure Club.
How Insulating Glass Works
Insulating glass, often referred to as “IG”, “double-pane” or “Thermopane” glass, was developed to create a more energy efficient window. Glass itself is a lousy insulator; a single-pane window has an R-value of about 1. (R-value is the measure of a material’s ability to retard the flow of heat.) A standard insulating glass pane has an R-value of about 2, a somewhat wimpy number compared to the R-11 value of a 2x4 wall, but still greater than a single-pane window, even with a storm window on it. The R-value is greater, not because there are two panes of glass, but because there is air or gas sealed between the two panes of glass. With the addition of different gases (like argon and krypton) and films and coatings
(like “low-E”) between the panes, the insulating values can reach R-6 or greater.
Just as important as their insulating value is the increased comfort double-pane windows and doors bring. They’re warmer to the touch, and minimize the potential of frost and condensation on the inside pane. And there are other less easily measured benefits, like lower air conditioning costs in summer and free solar heat in winter.
Insulating glass is created by bonding two panes of glass together along their perimeter while maintaining a (usually) 1/2- to 3/4-in. space between them. Most high-quality double-pane windows manufactured today have two perimeter seals, an inner seal that resists water, aging and corrosion, and an outer seal that provides rigidity and strength. If one seal fails, the other can—at least for a while—pick up the slack. Some windows may have just a single seal. A hollow, usually aluminum, tube or spacer is the other element in seals. Its job is to keep the panes rigidly spaced. This tube usually holds small beads of a moisture-adsorbing (similar to absorbing) desiccant to keep windows from fogging up if a small amount of moisture penetrates the seal or is trapped between panes during manufacture. But once the desiccant is saturated and moist air starts entering through a bad or broken seal, it’s a downhill slide. It’s just a matter of time before your window starts to fog.
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