Why Insulated Window Glass Fogs

When insulating glass goes bad.


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 2×4 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, extra panes of glass 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 (Fig. A). 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, resulting in a broken window seal, 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.

FH00OCT_02120001-1200 insulated windowsFamily HandymanFigure A: Insulated glass windows

Insulating glass seals have to withstand the onslaught of heat, cold, rain, wind and impact. All in all, they do a darn good job. But even with a failure rate of only 1 to 3 percent, there’s a good chance that at least one glass unit in your house will bite the dust.

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 manufactured by their members, and properly installed, have a 1 percent failure rate after 10 years and a 3 percent rate after 15 years.

The troublemakers are older and improperly installed units—they generate the most problems. The leading causes of failure or a broken window seal are:

  1. Seals breaking down from exposure to water (Fig. B). Windows without the proper safeguards to keep water from puddling around the perimeter seals will fail sooner.
  2. Excess heat (Fig. C). Talk to companies that replace insulating glass and they’ll tell you most of their work takes place on windows with direct sun exposure. 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.
  3. Old age (Fig. D). Even the most elastic, flexible seal can’t last forever. Eventually a seal will allow moisture to enter the window.

Impact will rarely break the seal of a healthy window, but it can be the last straw for one already weakened from one of the above situations.

When windows fog and fail, the only viable option is replacement. It’s extremely difficult to separate the old panes, clean them up and reseal them again: The glass becomes “etched” from minerals in the moist air, the old seals are difficult to remove in order to get a tight new seal and a repair is just not cost effective. And the heck of it is, there’s not much you can do to prevent window failure.

FH00OCT_02120002-1200 insulated windowFamily Handyman

Figure B: Water damage

Perimeter seals can deteriorate when they sit in water; bottom seals are particularly vulnerable. Glass should be elevated up and away from the window sash (and any infiltrating water) with small setting blocks. Weep holes or channels should be included to promote drainage.FH00OCT_02120003-1200 insulated windowsFamily Handyman

Figure C: Heat damage

Perimeter seals can soften and panes can literally bow outward from hot air expansion between panes. Lots of expansion and contraction can actually turn windows into “mini-pumps” that pull in and push out outside air, a real problem if it’s moist air. South-facing windows are the hardest hit.FH00OCT_02120003-1200 insulated windowFamily Handyman

Figure D: Damage from old age

Perimeter seals can dry out, crack and allow moist outside air to infiltrate. Most windows today carry 10- to 20-year warranties—not bad for all the abuse they have to put up with.

Your best insurance against window failure

Your best defense against a broken window seal is to buy a window with a good, long warranty right from the start; 10 to 20 years is standard these days. Some companies even offer “lifetime” warranties. Insulating glass mounted in the frame at the window manufacturing plant has all the seals and safeguards necessary for long life.

Direct-set glass (Fig. B), glass manufactured to a specific size and shape then set into a site-built frame by carpenters (or you), has a strong chance of failure if installed wrong. The seal is likely to deteriorate if setting blocks aren’t installed to hold the bottom seal up and away from moisture and if the window isn’t properly caulked and sealed where it meets the stops. Often, the company that manufactures the direct-set glass will warranty the window if it’s installed by them. Installation will be more expensive, but you may get a longer-lasting window.

Homeowners should examine the window, frame and seals periodically. Separated sash frames (which allow moisture to reach the seals) and deteriorating perimeter seals can be caulked to increase the longevity of insulating glass. You can also help protect your double-pane windows by preventing excess moisture from accumulating on them; the best way to do this is to ensure good air circulation both inside and out. Keeping them clean and well sealed with paint will also help. Most glass manufacturers include their name or ID, as well as a manufacturing date or date code, near the spacer or bottom of the glass so you can determine whether a failed glass unit is still under warranty.

But again, the single best safeguard against window failure in the future is to buy a top-notch window now. You get what you pay for.

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