How to Repair a Failed Seal on Insulated Glass Windows

Small flowers in glass vase on windowsill
ULTRA F/Photodisc/Getty Images

The symptoms of failed window seals are easy to spot. The multi-pane glass develops condensation or fogginess that can't be wiped off from either side of the window. The cause is a failure in the edge seal that secures the individual panes of glass. This common problem raises two equally common questions: 1) Is there anything you can do about this problem, other than replace the entire window? and 2) Is it really necessary to do anything?

Anatomy of a Thermal Window

A thermal window features two or three panes of glass with open space between the panes. This window assembly is known in the industry as an IGU, for insulated glazing unit or insulated glass unit. Thermal windows are sometimes called thermopane or insulated windows.

The space between the glass panes of an IGU is vacuumed to remove air and is often filled with an inert (or noble) gas, such as argon or krypton, to slow the passage of heat through the window unit. Inert gasses are less heat-conductive than air or a vacuumed space.

However, if the seals that protect the edges of the IGU develop a break, then the inert gases can escape and ambient air and moisture can enter the space between the panes, resulting in the IGU losing its extra insulating value. The visible symptom of this failure is the tell-tale fogginess or condensation inside the IGU unit, on the inside surface of the glass. Not only do you lose the aesthetic value of a clear window, but the energy-saving value of the window is likely to be cut in half.

IGU Seals

The edges of the glass panes in double- or triple-pane IGU window are embedded in a sealing material. While it appears to be one seal, it is actually two seals working together. The inner sealant is typically made of polyisobutylene (PIB). The outer sealant is an elastic rubber-like seal that, according to AkzoNobel (a maker gasses used to fill thermal windows), "functions as an adhesive, holding the glass unit together and keeping it tight during the service life."

Although these window seals are meant to be long-lasting and may hold up for decades, they can and do fail. Seals can be damaged by house painters using heat guns to strip paint or by homeowners using pressure washers to clean the windows. Improper installation of an IGU window can also cause a seal to fail. But even when there is no such dramatic event, the seals are still destined to fail—eventually. Glass manufacturer PPG estimates that the gas escapes at a rate of approximately one percent per year under ideal conditions. This loss of gas can be faster if an IGU is poorly manufactured.

Remedies for Failed Window Seals

Can window seals be repaired if they fail? There are four possible actions to take when you notice the fogginess, condensation, or wavy appearance that signifies the seals have failed on an IGU window.

Call In the Warranty

Premature window seal failure indicates a defective product, and window manufacturers may offer partial or complete replacement of the IGU if the failure occurs within a stated time frame. Other manufacturers may offer prorated compensation in the case of failure, scaled according to the age of the window.

Fix the Fog and Condensation Problem (But Not the Seal)

If there is no warranty protection on the window, there may still be options for making the window look better. Specialty companies have captured this niche by offering defogging services that can make the window more aesthetically pleasing without actually restoring the sealed window spaces. Windows are not removed and replaced; all materials stay in place. Instead, the company drills a tiny hole in the glass and expels the moisture from between the glass panes. Then, an anti-fog solution is applied to the inside of the IGU, a liquid sealant is added to the bottom, and a seal is installed in the drilled hole.

Reviews of defogging are mixed. Inert gasses are not replaced with this process, so the original insulating value of the window is not restored. And the initial seal problem is not corrected unless the seal failure was at the very bottom of the IGU (where the new sealant is applied), so fogginess may well return. Although there are DIY kits available that allow homeowners to try this repair themselves, these are hard to use successfully—the better option is to hire a specialty company. Also, many windows are made with tempered glass, which is very easy to break when drilling, if you don't know what you're doing.

Replace the IGU Within the Frame

When an IGU seal fails, you may be able to replace the glass unit itself without replacing the entire window and frame. Glass replacement companies make replacement IGUs that can be installed in existing window frames. Typically, this work is done by professionals, but it's possible to do it yourself if the window frames are constructed so they can be taken apart. Some windows have screws that join the frame pieces together; others have removable stop moldings that hold the IGU in place within the frame. This is a fairly complicated DIY project, but it can be considerably cheaper than having an entire window replaced.

Leave the Window As Is

If you live in a mild to moderate climate, you may never reap the full benefits that thermal windows have to offer. In many areas, one or two failed window seals in a house with 20 windows will make little difference in the heating or cooling bills. You may well decide to simply live with the window that has a failed seal, provided you can live with the aesthetics of a window that fogs up occasionally.

Tips for Preventing Seal Failure

There are several ways you can help prevent problems with failed seals in thermal windows:

  • Buy windows with long warranties. While 10- and 20-year warranties are common, some companies offer lifetime warranties.
  • Have the windows installed by the manufacturer. With companies that manufacturer direct-set IGUs, where the insulated panels are installed within the frames at the factory, the full warranty may be offered only if the manufacturer's technicians install the windows.
  • Examine the windows periodically for signs of separation between the IGUs and their frames. Caulk any gaps you find. Keeping the seam clean and well-painted can also help.
  • Don't use pressure washers to clean windows. The high-velocity water stream may cause gaps between the IGUs and frames.
  • Don't use heat guns to remove paint from window frames. Careful scraping and sanding is the preferred method when repainting windows with IGU panels.