Common Window Seal Failure Symptoms and What They Mean

If a window in your home has a cloudy haze between the panes that does not wipe away, the insulating glass unit has failed. The four classic window seal failure symptoms show up in a predictable order, and recognising them early decides whether you replace just the glass or end up replacing the whole window unit. Our window replacement service in Toronto and the GTA handles both options, and the right call depends on which symptoms you are seeing and how far the failure has progressed.

This article walks the four symptoms in order, explains what each one means about the unit, and tells you which ones are cosmetic versus which ones are quietly costing real money. None of them fix themselves. The fog does not evaporate. The seal does not reseal. Time only makes it worse.

What a Window Seal Actually Is

A modern Canadian window is built around an insulated glass unit, or IGU. Two or three panes of glass sit in a spacer at the edges, separated by an inert gas fill, usually argon or krypton. Two seals hold it all together. The primary seal, a thin butyl rubber bead, is the gas-tight barrier. The secondary seal, a thicker silicone or polyisobutylene layer, is the structural bond between the spacer and the glass.

When the primary seal degrades, the argon leaks out and humid outdoor air leaks in. The water vapor in that air condenses on the cold inner surface of the glass during the first cold night of the season and cannot escape. That is the first symptom and the diagnostic moment. Once you see fog between the panes, the seal is gone and replacement is the only fix.

Symptom 1: Foggy or Hazy Glass That Does Not Wipe Away

The earliest sign of seal failure is a thin haze that appears between the panes on cold mornings and slowly burns off through the day. You can wipe the inside and outside surfaces of the glass clean and the haze is still there because the moisture is sealed between the two panes.

The haze starts at one corner, almost always the bottom corner where the gas fill is densest and the seal is closest to freeze-thaw cycling. It spreads across the bottom edge first, then up the sides, then across the entire unit. The progression takes anywhere from 6 months to 5 years depending on how aggressive the primary seal failure is.

Symptom 2: Water Droplets Visible Between the Panes

Once the haze has had time to accumulate, you start to see distinct water droplets clinging to the inside surface of the outer pane. They appear as small beads that you can clearly see when looking at the glass from a slight angle. The droplets sit there because they cannot evaporate back out through the failed seal.

This is the symptom that homeowners often mistake for interior condensation and try to wipe away. The cloth never reaches the droplets because they are sealed inside the glass sandwich. At this point the seal failure is well established and the gas fill is mostly gone. Replacement is the only path forward, and waiting longer means moving to symptom three.

Symptom 3: Permanent Mineral Haze on the Inner Surface

Repeated condensation and evaporation cycles inside the failed unit deposit dissolved minerals onto the inner glass surface. Over time these minerals etch into the glass and create a permanent cloudy white film that no amount of cleaning will remove. This is the point where the unit is not just thermally compromised, it is visually ruined.

The etching takes 3 to 7 years to develop after the initial seal failure, depending on the local water chemistry and how often the unit cycles through temperature extremes. South- and west-facing windows in the Winnipeg and prairie locations with hard water tend to etch faster than north-facing windows in coastal climates. Once the mineral haze is in, the glass replacement cost is the same, but the unit has been looking bad in your home for years longer than it needed to.

Symptom 4: A Noticeably Colder Zone Near the Window

This is the symptom homeowners feel first and often misidentify as a draft. The interior glass surface of a failed unit runs 5 to 10 degrees Fahrenheit colder than a working unit at the same outdoor temperature, because the lost argon fill removed most of the insulating layer. You feel that cold radiate into the room when you sit within three feet of the window.

The diagnostic for this versus an actual air leak is straightforward. Light a candle and hold it 6 inches from the seam where the sash meets the frame. If the flame flickers, you have an air leak from worn weatherstripping. If the flame stays steady but you can feel cold radiating off the glass itself, the IGU has failed and the cold is coming through the glass package, not around it. Both problems are fixable. They need different fixes.

Repair the Glass or Replace the Whole Window

The decision hinges on three things: the age of the window unit, the condition of the frame and operating hardware, and how many other failure modes are present. Use this short list to triage.

• Glass-only IGU replacement makes sense when the frame is intact, the sash operates smoothly, the unit is less than 20 years old, and only the glass package has failed. This applies to about 70 percent of seal failure cases in modern Canadian homes.
• Full window replacement makes sense when the frame shows warping or rot, the operating hardware is binding or broken, the unit is older than 25 years, or you have multiple windows on the same wall that all need work. The labour overhead of a single full replacement is similar to glass-only, so combining work makes sense.
• Wait one season before deciding when you see fog on a single window during a high-humidity week, the fog clears within hours of warming up, and you cannot replicate it. Sometimes interior or exterior condensation gets misidentified. A second observation in a different week confirms whether the IGU has actually failed.

For warranty-eligible cases, the original installer or manufacturer often handles the glass replacement under warranty terms. Check the paperwork from your original installation before paying out of pocket. Many Canadian manufacturers warranty their IGU seals for 20 years, and the warranty often survives ownership changes, even on used homes.

Frequently Asked Questions

Sources and References

• Natural Resources Canada: ENERGY STAR Canada window performance certification
• Canadian Standards Association: CSA A440.4 Window, door, and skylight installation standard
• National Research Council Canada: Codes Canada fenestration and IGU performance requirements