How to Tell If Your Window Seal Is Broken (7 Signs + Quick Tests)
The surest sign of a broken window seal is fog, haze or droplets between the panes that you can't wipe off from either side; white streaks inside the glass, distorted reflections from bowed panes and a unit at its typical failure age are other signs. The fix is a new sealed glass unit, usually in your existing frame, not a new window. Drafts and leaks around the sash point to a different seal, the weatherstripping or caulking, which is a cheaper fix.
'Window seal' can mean two things: the factory edge seal of the sealed glass unit (IGU), or the weatherstripping and caulking around the sash and frame. They fail differently and are fixed differently.
Moisture between the panes that you can't wipe off from either side means the sealed unit has failed. It won't clear up on its own; the fix is a new sealed unit, usually in your existing frame.
Early failure can look like a faint haze. Later stages leave a permanent milky film or mineral streaks, and in some cases the panes bow.
You can't see argon leaking. Gas loss can be detected only with special equipment, so fog is the practical signal.
The spacer bar between the panes often carries a date code, so you can tell whether the unit is still under the manufacturer's 10 to 20-year seal warranty.
Drafts, whistling or daylight around the sash mean worn weatherstripping, not a failed sealed unit.
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The clearest sign of a broken window seal is moisture between the two panes of glass: fog, haze or droplets that you can't wipe away from inside or outside. That means the edge seal of the sealed glass unit (IGU) has failed and humid air is getting into the cavity. It will not resolve on its own.
People also say "window seal" when they mean the weatherstripping and caulking around the sash and frame. Those fail too, with different symptoms and a cheaper fix. Below are seven signs, split by which seal is failing, and a few five-minute tests you can do today.
Two different "seals" on every window
| Sealed-unit (IGU) edge seal | Weatherstripping and caulking | |
|---|---|---|
| Where it is | Around the edge of the glass, between the panes | Where the sash meets the frame, and where the frame meets the wall |
| What it keeps out | Moisture and air from the gap between the panes; keeps argon in | Drafts, rain and insects from your room |
| How it fails | Fog, haze or residue between the panes | Drafts, whistling, daylight, water on the sill |
| Fix | Replace the sealed unit | Replace the weatherstrip or recaulk |
A sealed unit's edge seal is a factory job. A primary seal (polyisobutylene) and a secondary seal (polysulfide or silicone) hold the panes to a spacer filled with desiccant. Even good seals let a little vapour through over the years. The unit fails when the desiccant is saturated and can't absorb any more.
Signs 1–5: the sealed glass unit has failed
1. Fog or droplets between the panes
This is the classic sign. Clean both faces of the glass first, including the outside, to make sure the moisture really is inside the unit and not on a surface. If it's still there, the seal has failed.
2. A faint haze that comes and goes
Failure often starts as a faint haziness before it becomes obvious. Because condensation depends on temperature, you may see it some days and not others. Early haze is still a failed seal. It usually gets worse, not better.
3. White film, streaks or spots inside the glass
As moisture condenses and dries inside the unit over and over, it leaves mineral deposits. The haze turns into permanent cloudiness or a milky appearance. If the glass looks dirty but no amount of cleaning helps, look closer: the dirt may be on an inside face of the unit.
4. Warped reflections from bowed glass
In advanced failure, pressure changes inside the unit can make the panes bow slightly inward. Bowing shows up as a funhouse-mirror reflection. The reflection test below checks for it.
5. The unit is at seal-failure age
Canadian double-pane units typically start failing somewhere between 10 and 20 years. In a 25-year field study of more than 2,400 units, 4.9% had failed by year 10 and 9.2% by year 25. Age alone doesn't mean a unit has failed, but a 20-year-old unit with slight haze almost certainly has.
Signs 6–7: the weatherstripping or caulking has failed
6. Drafts, whistling or daylight around the sash
Moving air at the edge of a closed window means the weatherstripping isn't sealing. It's worth fixing: air leakage accounts for 30 to 40% of a home's heating and cooling loss. Weatherstrip lifespans vary a lot. Felt may need replacing every 1 to 2 years, tubular rubber or vinyl lasts about 5 years, and metal or vinyl V-strip is more durable.
7. Water or ice on the sill, or stains below the window
Water at the joint between the frame and the wall points to failed exterior caulking or blocked drainage. Keep the weep holes at the bottom of the frame clear: when water pools against the glass edge, it also speeds up failure of the sealed unit's seal.
Five-minute tests you can do today
- Wipe test. On a day you can see moisture, wipe the inside face with a dry cloth, then the outside face. If the moisture is still there, it's between the panes.
- Reflection test. Stand at an angle to the window and look at the reflection of something straight, such as a roofline, a door frame or a fence. Compare it with the same view in a neighbouring window of the same type. Strongly curved or stretched lines suggest bowed panes.
- Date-code check. Look along the metal or foam spacer between the panes, near a corner. Certified units carry a permanent mark with the manufacturer and year of manufacture. Photograph it. You'll need it for a warranty claim and it helps whoever quotes the replacement.
- Paper test for weatherstripping. Close the window on a strip of paper. If it slides out with no drag, the weatherstrip isn't compressing at that spot. Repeat around the sash.
- Draft test. On a windy day, move the back of a slightly damp hand slowly around the closed sash. A cool spot shows you where air is getting in.
What you can't test at home is whether argon has leaked. Leaks can be detected only with special gas-detection equipment. In practice, fog is the signal. By the time a unit fogs, the gas is gone.
Is it the seal, or just humidity?
Condensation on the room side of the glass in winter is a humidity problem, not a seal problem. Water on the outside surface on a cool morning is a sign your glass is insulating well. Only moisture between the panes points to a failed unit. If your windows run wet every winter morning, our condensation and humidity guide walks through the fixes.
What a broken seal costs you
A failed sealed unit isn't dangerous and doesn't make the glass weaker. The cost is comfort and energy. Seal failure and gas loss can raise a unit's heat loss (U-factor) by up to 32%. Once the desiccant is saturated, the low-E coating can also degrade, and mineral residue can become permanent. It's worth fixing, but you have time to get proper quotes.
What to do next
- Fog between the panes: replace the sealed unit. NRCan says this is best corrected by replacing the glazing unit. If the frame is sound, you keep the window. See foggy windows: replace the glass, keep the frame. Defogging clears the view but doesn't reseal the unit. Our defogging guide explains why.
- Check the warranty before you pay. Seal-failure warranties usually run 10 to 20 years, but labour may be covered for only 1 to 2 years, and many aren't transferable.
- Drafts or leaks: have the weatherstripping, hardware and caulking checked. Our window and door hardware repair team handles these.
- Renting? In Ontario, landlords must keep a rental unit in good repair. Toronto's property standards by-law requires windows to be kept weather-tight and "free from defects". Report the problem to your landlord in writing. This is general information, not legal advice.
When to call a pro
Call when you've confirmed moisture between the panes, when glass is bowed, or when several windows show drafts and you can't find the source. A glazier can confirm which seal has failed, read the spacer label, measure for a replacement unit and check whether the location needs tempered glass. We give free quotes, and a photo of the glass and the spacer label helps us prepare one.
Next step
If you've spotted fog or haze between the panes, the fix is a new sealed unit, usually in your existing frame. Learn how our foggy window repair works, or contact us with a photo of the window and the spacer label for a free quote.