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 are:

1. Seals breaking down from exposure to water (Fig. A). Windows without the proper safeguards to keep water from puddling around the perimeter seals will fail sooner.
2. Excess heat (Fig. B). 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. C). Even the most elastic, flexible seal can’t last forever. Eventually a seal will allow moisture to enter the window.

Figure A: 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.

Figure B: 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.

Figure C: 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.

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.