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