Foggy Windows: Why the Seal Failed and Whether to Repair or Replace

Fog Between the Panes vs. Fog on the Glass

Start by finding out which side the moisture is on.

On the room side of the glass. This is ordinary condensation. Andersen explains that interior condensation happens because indoor humidity is too high, "not because there is something wrong with the window," and recommends 30 to 35 percent humidity in winter. Wipe it off, run a fan, and it goes away.

Between the panes. If the fog or haze won't wipe off from either side, it's inside the sealed unit. Andersen and Pella both describe condensation between the panes as a sign of a failed seal, and lowering humidity won't change it.

What a Failed Seal Means

A double-pane window is two sheets of glass held apart by a spacer and sealed around the edge, often with argon gas between them. The U.S. Department of Energy describes these multiple panes, gas fills, spacers and low-E coatings as what make efficient windows work.

When the edge seal fails:

• Moist air gets into the space between the glass and condenses as fog.
• The argon leaks out, and the window loses part of its insulating value. Window + Door magazine describes the loss of inert gas as degrading thermal performance.
• The view stays cloudy and usually gets worse.

Some slow gas loss is normal even in a good window. The Efficient Windows Collaborative puts it at about 0.5 percent of the gas fill per year. A failed seal is different: it's an open path for moisture.

Why Seals Fail, and Why Albuquerque Is Hard on Them

• Sun and heat cycling. As the glass heats and cools, the air space expands and contracts and stresses the seal. InterNACHI notes sun-facing windows see bigger swings, and Albuquerque's strong sun and big day-to-night temperature changes make this a daily cycle.
• Altitude. A sealed unit holds the air pressure of where it was made. Shipped to a higher elevation, the pressure difference makes the glass bow, which Cardinal Glass says stresses the edge seal. Andersen offers high-altitude glass with capillary tubes to equalize pressure, and notes this can mean some argon loss. Marvin's warranty excludes insulating glass installed above 5,000 feet without capillary tubes unless its high-altitude supplement applies. Much of the Albuquerque metro sits around or above that elevation, so how a window is built for altitude matters here. That doesn't mean altitude caused every failed seal; age, sun and installation play a part too.
• Age and manufacturing. Pella notes seal failure typically occurs in older windows, and Window + Door points to fabrication defects as a common cause.
• Installation. A window forced into an out-of-square opening, or flexing in a poorly supported frame, puts stress on the glass edge.

Does Defogging Work?

Defogging services drill a small hole in the glass, clean and dry the space, and seal or vent the hole. The window can look clearer afterward. But:

• InterNACHI notes skepticism about how effective and cost-effective this is, and that removing the water doesn't repair the seal or restore insulation.
• Bob Vila calls it a temporary fix that can fog again, and notes the gas-filled unit's added insulating value is gone.
• Pella says a failed seal "cannot be effectively repaired," and the window or insulated glass unit typically needs replacing.

Repair or Replace: Your Options

1. Replace just the glass unit. If the frame and sash are sound, a new insulated glass unit can often go into the existing window. It's the smallest fix.

2. Replace the sash. Andersen notes that on some windows, replacing a sash rather than the whole window can solve the problem when the frame is sound.

3. Replace the window. Makes sense when several windows are failing, the frames are deteriorating, the windows are old, or you want better glass for New Mexico's climate. Our guides to choosing windows for New Mexico's climate and U-factor and SHGC explain what to look for. On a stucco home, also see replacing windows in a stucco home.

Check the Warranty First

Glass seal failure is a standard warranty item. For example, Pella's vinyl window warranty covers insulating glass seal failure for 20 years from sale, with labor only in the first two years, and Marvin's warranty covers seal failure for 20 years on most sizes. Warranties generally cover obstruction of vision, not the amount of argon, and they exclude condensation from indoor humidity.

Look for the manufacturer's name on the window's spacer or a label, then find your warranty. If the glass is covered, you may only pay for labor, or nothing at all.

When Replacement Windows Make Sense

Timberwolf installs the Climate Solutions 8100 Series window, with argon-filled glass and a lifetime transferable warranty. If your windows are fogging and out of warranty, a window quote will tell you what replacement would look like. See Windows & Doors.