Fog, moisture or a milky haze trapped between the two panes of a window is one of the most common window problems in Ontario homes. Unlike condensation on the inside surface of the glass, fog between the panes cannot be wiped away.
Understanding why it happens can help you decide whether the window needs a new sealed glass unit or a complete replacement.
1. What Causes Fog Between Window Panes?
A Failed Insulated Glass Seal
Double- and triple-pane windows are built as sealed insulated glass units (IGUs). The space between the panes is sealed at the edges and often filled with an insulating gas. When the edge seal fails, humid outdoor air enters the space and condenses on the inside surfaces of the glass.
Common reasons a seal fails include:
- Age and normal wear of the sealant.
- Repeated heating and cooling through Ontario seasons.
- Direct sun exposure on south- or west-facing windows.
- Water sitting in the frame or a blocked drainage path.
- Pressure or movement from an out-of-square frame.
2. Is Foggy Glass a Problem or Just Cosmetic?
A foggy glass unit is more than a visual issue. Once the seal has failed, the unit has lost much of its insulating value, and over time mineral deposits can permanently stain the inner glass surfaces.
- Reduced view and natural light.
- Lower energy efficiency and colder glass in winter.
- White streaks or stains that become permanent.
- More condensation on the room side in cold weather.
3. Can You Clean or Defog the Glass?
Because the moisture is sealed inside the unit, it cannot be reached by normal cleaning. Some services drill and flush the glass, but this does not restore the original seal or insulating gas, and results vary.
In most cases, the reliable fix is to replace the sealed glass unit itself.
4. Replace the Glass Unit or the Whole Window?
If the window frame, hardware and operation are still in good condition, replacing only the sealed glass unit is usually the more economical option. The frame stays in place, and a new unit is made to the exact size of the opening.
Full window replacement may make more sense when:
- The frame is rotted, cracked or warped.
- The window no longer opens, closes or locks properly.
- Several units in the same window have failed.
- You want to upgrade the style or energy performance of the whole window.
5. Foggy Glass vs Condensation on the Inside
Condensation on the room-side surface of the glass is usually caused by high indoor humidity and can often be reduced with ventilation. Fog that sits between the panes and cannot be touched is a sign of seal failure.
For more information on window performance and upgrades, see Natural Resources Canada’s guide to upgrading windows and exterior doors.
Foggy Window Glass Replacement in Toronto & the GTA
AEO Services Inc. replaces foggy and failed sealed glass units in windows and patio doors for homes and businesses across Toronto, the GTA, York Region, Durham Region and Simcoe County.
Explore our Foggy Window Glass Replacement service or submit a service request online.
Frequently Asked Questions
Why is there moisture between my window panes?
Moisture between the panes means the edge seal of the insulated glass unit has failed and humid air has entered the space between the glass.
Can I replace just the glass instead of the window?
Often yes. If the frame and hardware are in good condition, a new sealed glass unit can be made to size and installed in the existing frame.
Will foggy glass go away on its own?
It may look clearer on some days, but once the seal has failed, the problem usually returns and gets worse over time.