How to Repair a Failed Seal on Insulated Glass Windows

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

The symptoms are easy to spot: windows made from insulated glass with two or three panes develop condensation or fogginess between the glass panels that can't be wiped off from either the inside or outside of the window. The cause: a failure in the edge seal that secures the individual panes of tempered glass. But the question then becomes, is there anything you can do about this problem, other than replace the entire window? And is it really necessary to do anything?

Anatomy of an IGU (Insulated Glass Unit)

In windows that feature two or three panes of glass with open space between the panes, the individual glass panels are known in the industry as IGUs (Insulated Glazing Units or Insulated Glass Units), a construction that is sometimes known as a thermopane or simply insulated glass. The space between the glass panes is filled a vacuum or with an inert (or noble) gas like argon or krypton, serving as a thermal break separating inside and outside temperatures. Sometimes one or two panes are covered special films to reduce energy transmission. Windows with IGUs are considerably more energy efficient than standard single-pane windows. A single pane of glass has an insulating value of about 1, but with the addition of a second pane, the value doubles to R-2. When special coatings such as low-E films are added, the insulating value of the window can jump to as high as R-6. This is still a long way from the normal insulating value of a stud wall, which is about R-11, but it is a significant improvement that can lead to noticeable energy savings in very cold or very hot climates.

But if the seals that protect the edges of the IGU develop a break, then the inert gases can escape and outside moisture can enter the space between the panes, and the IGU loses its extra insulating value. The visible symptom 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 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 high-efficiency 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. Certain actions can hasten the failure of the glass seals, including housepainters improperly using heat guns while stripping paint, or homeowners using pressure washers to clean windows. Improper installation of an IGU window can also cause the seal to fail. But even when there is no such dramatic event, the seal is still destined to fail—eventually. Glass manufacturer PPG estimates that "[e]ven when an IGU is perfectly constructed, the gas will escape at a rate of about one percent per year, and that rate is much faster when the IGU is poorly made."

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

This is one of the examples of why it makes sense to hold onto warranty information for any household product you buy. Premature window seal failure means a defective product, and manufacturers will be obligated to honor whatever warranty carried by the product. For example, Pella expressly guarantees that their nonlaminated windows will not experience "premature failure or permanent material obstruction of vision due to a failure of the glass seal for twenty (20) years from the date of sale." Other window manufacturers offer similar warranties, which often provide for complete replacement of the IGU should failure occur. Other manufacturers may offer prorated compensation in the case of failure, scaled according to the age of the window.

Upon purchasing a home, you should ask the seller to hand over warranty information about the windows. And make sure to keep your warranty information and proof-of-purchase in a safe place.

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

If there is no warranty 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, though it does not restore the insulating R-value to a window. 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 until the inner portion is dry. An anti-fog solution is applied to the inside of the IGU, liquid sealant added to the bottom, then a seal is installed in the drill holes.

Reviews of defogging are mixed. Argon and krypton gasses are not replaced with this process, so the insulating value of the window is not restored. And the initial seal problem is not addressed unless the seal failure was at the very bottom of the IGU, so fogginess and condensation may well return. Although there are DIY kits available that allow homeowners to try this repair itself, these are hard to use successfully—the better option is to hire a specialty company to do this repair.

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 to your specifications that can be installed in existing window frames in some instances. There are companies that specialize in doing this work on-site, but you may be able also do it yourself, provided the window frames are constructed in a way that allows them to be taken apart. Not every window is constructed this way, so you'll need to examine the corners of the window to see if there are screws that join the frame pieces together. Or, with other types of windows, there are removable stop moldings that hold the IGU in place within the frame. If you find a means of disassembling the window frame or removing the IGU, you're in luck. Simply contact one of the many glass replacement companies available, order an IGU to the size and specifications you need, then remove and disassemble the window frame to replace the IGU panel.

This is a fairly complicated DIY project, but it can be considerably cheaper than having an entire window replaced.

Leave the Window in Place

If you live in a mild climate, you may never reap the benefits that double-paned, low-E windows have to offer. In a mild climate, one failed window seal in a house with 20 windows will make little difference in your 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.

Preventing Seal Failure

To prevent seal failure in IGU windows, follow these guidelines:

  • Buy windows with long warranties. While 10- and 20-year warranties are common, some companies now even 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 IGU and frame. 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 IGU and frame.
  • Don't use heat guns to remove paint from window frames. Careful scraping and sanding is the better method when repainting windows with IGU panels.