Commonly called thermal windows, insulated double- and triple-pane windows, officially known as IGUs (insulated glass units), offer far better energy efficiency than single-pane glass windows.
Whether it is just a simple sealed glass sandwich of two glass panes or a more sophisticated IGU with coatings and inert gas filling the space between panes, these windows play an important role in the energy efficiency of the home.
As with any closed system, one problem sometimes does emerge: trapped moisture. When water gets trapped between the glass, it manifests itself as fog on the inside. This moisture cannot escape on its own. But there are a couple of solutions for foggy windows that run far short of total window replacement: IGU replacement and defogging.
Understanding IGUs (Thermal Windows)
Nearly all windows today, whether for new construction or replacement, have, at their heart, an insulated glass unit, or IGU. This means two or sometimes three panes of glass that are factory-sealed together to form a single unit; they cannot be separated. Single pane windows are difficult to find and are mainly confined to older homes (pre-1980s) or outbuildings (sheds, etc.) where energy saving does not matter.
A basic IGU in which the space between the sealed panes is filled with air will offer better energy efficiency than a single-pane window, but thermal energy still is transmitted fairly easily through the air, in which the molecules are easily activated when subjected to heat. Thus, while an IGU filled with air is much better than a single-pane window at forming a thermal break, there are other options that can do a much better job.
The better solution is to fill the space between panes of glass in the IGU with a heavy, inert gas such as argon or krypton. In these dense gases, the molecules move very much slower under the impact of thermal energy, which means windows with IGUs filled with such gas will create a much better barrier against heat loss. For this reason, nearly all insulated thermal windows now use one of the inert gases to fill the space between panes.
What Causes Foggy Glass?
A common problem with IGUs occurs when the seals around the edges of the glass unit begin to fail, allowing the inert gas to escape and outside air to enter the space between the glass panes.
As a result, moisture in the infiltrating air can condense when conditions are right (colder outer glass against warm air inside the glass panel), causing the glass to develop the hazy fogginess that most everyone has seen.
The fogginess may come and go, depending on weather conditions. In the summer, when outdoor temperatures keep the glass warm, the moisture inside the IGU generally does not condense into visible condensation, but as the weather cools, the moist air touching the cold outer glass causes moisture to condense into visible water droplets. Anytime you see this kind of fogginess that comes and goes in a window, you are dealing with an insulated window that has lost its seals.
The reality is that all IGUs gradually lose the inert gas filling the space between panes, and the seals themselves have a limited life expectancy and will eventually break down.
Research shows that a host of factors including temperature and atmospheric pressure fluctuations, wind loads, working loads, sunlight, water, and water vapor all negatively affect their life expectancy. The International Association of Certified Home inspectors estimates that all units naturally lose 1% of their gas per year.
In other words, you can eventually expect your IGU windows to lose their effectiveness and for the seals to fail. After 15 or 20 years of service, it should be no surprise when some of your thermal windows develop foggy glass. If it happens within the window's warranty period, however, your first option should be to contact the window manufacturer to discuss replacement under the conditions of the product warranty.
Foggy Glass Solutions: IGU Replacement and Defogging
Up until recently, fogginess in insulated glass was something you either lived with or dealt with by window replacement. This can involve the replacement of the entire sash—the IGU plus the wooden or fiberglass frame around the glass—or the removal and replacement of just the IGU panel inside the sash frame. The second solution is to defog the glass from the inside-out.
New window glass
Inert gasses retained
Less expensive than total window replacement
Few companies provide service
More expensive than defogging
Poor frames aren't replaced
While replacing IGUs is not an inexpensive process, it is cheaper than having the entire window unit replaced. And new IGU panels come with a warranty against failure for some amount of time after installation.
Replacement of the IGU or IGU-plus-sash is usually cheaper than total window replacement because less labor and fewer parts are required. However, it can be difficult to find a company that will replace your IGUs or sashes only.
Window companies make their real money with whole-house or substantial partial-house window replacements, not by switching out the occasional sash. Per-window costs will be best if you have many window sashes to replace since this is more attractive to contractors.
Windows can be kept
No disposal of heavy materials
Less expensive than IGU or window replacement
Some R-value lost from the original
Few companies offer defogging services
Fog will return if window seal is broken
Alternatively, foggy windows can be repaired using a defogging procedure. Window defogging is capable of producing cosmetic results that help improve visibility in your windows.
One benefit of window defogging is that the windows stay in place; they are simply repaired. No replacement means almost no mess and no landfilling of non-biodegradable window parts.
On the downside, defogging does nothing to restore the window's insulating ability (R-value) to its original level. In fact, since inert gas is replaced with air, some R-value is lost.
Defogging is criticized by some experts but lauded by some homeowners who are happy with the cosmetic improvements.
Defogging costs less than replacing an entire IGU. While it does get rid of the fog and make your window look better, it does not replace the inert gas between the glass panes.
Nor does it restore the thermal performance of the window to its original specs. On the other hand, defogging does dry out the window, and a dry window is always warmer than a moist window.
How Window Defogging Works
The window defogging procedure expels the water vapor from inside the window. Then the window is quickly sealed up again before more moisture-laden air can infiltrate the IGU.
Professional defogging contractors follow a simple procedure that is usually done from within the home. Warm weather is more conducive to defogging windows. Windows cannot be defogged in freezing or below-freezing temperatures.
Drill Holes in Glass
The technician drills tiny holes in the glass to expel the moisture from between the glass panes. Usually, two holes are drilled. It then takes anywhere from a few hours to several days for the moisture to clean out from inside the window.
Apply Anti-Fog Solution
An anti-fog solution is applied to the inside of the window. With the moisture gone, most of the stains (calcium and magnesium deposits) are removed, too.
Apply Liquid Sealant
A liquid sealant is added to the bottom of the glass on any of the holes that will not be retained for venting.
After all of the moisture is gone, vents are added to the window to allow the window to naturally expel moisture and air.
Starman, Bojan, et al. Primary Seal Deformation in Multipane Glazing Units. Applied Sciences, vol. 10, no. 4, 2020, p. 1390. doi:10.3390/app10041390
Window Gas Fills: What Inspectors and Consumers Should Know. International Association of Certified Home Inspectors.