Windows Foggy Inside: Why It Happens and What You Can Actually Do About It

What's Actually Causing the Fog Between the Panes

Modern double-pane and triple-pane windows are built as insulated glass units (IGUs). The IGU consists of two (or three) glass panes separated by a spacer bar, with the space between them sealed airtight and filled with insulating gas — typically argon or krypton. This sealed gas space is what gives modern windows their thermal performance. It's why a double-pane window in a Salmon Arm home dramatically outperforms a single-pane window on cold Shuswap winters. The fogging you see between the panes is caused by this seal failing. When the perimeter seal degrades — due to age, UV exposure, water infiltration at the frame, or thermal cycling stress — outside air (and moisture) begins to infiltrate the space between the panes. Moisture-laden air entering the sealed unit condenses on the cooler inner glass surfaces and appears as fog, haze, or in advanced cases as visible water droplets between the panes. The seal failure process is progressive. In early stages, fogging appears only on cold mornings and clears as the glass warms — because the moisture evaporates back out when temperatures equalize. As the seal degrades further, the fogging becomes more persistent. Eventually, mineral deposits from the condensed water leave a permanent haze on the inner glass surfaces that doesn't clear even in warm weather. At this stage, the glass appearance is permanently impaired regardless of exterior temperature. Argon gas escaping from a failed seal contributes to the problem beyond aesthetics. Argon provides roughly 25-30% better thermal insulation than air. A compromised IGU with escaped argon performs like a much older window, with higher heat loss in winter and heat gain in summer. The foggy appearance is often the visible symptom of a larger energy efficiency problem.

• IGU seal failure allows outside air and moisture into the sealed space between panes
• Moisture condenses on cooler inner glass surfaces, creating visible fog or haze
• Early-stage failure: fog clears when glass warms; advanced failure: permanent haze
• Argon gas escapes when seal fails, reducing window thermal performance by 25-30%

Why Cleaning Can't Fix This Problem

This is the critical point that homeowners most often misunderstand: the fog is not on the glass surface — it is between the panes, on the inner faces of the glass that cannot be accessed without disassembling the window unit. Cleaning the exterior glass surface, cleaning the interior glass surface, or applying any cleaning solution to the accessible faces of the glass has zero effect on fog that is physically located in the sealed space between the panes. Scrubbing harder, using stronger solutions, or hiring a professional cleaner will not change what you're seeing. The condensation or mineral deposit is on the interior faces of the glass, sealed inside the unit. This is not something a window cleaner — no matter how experienced — can address through cleaning services. It's a different problem entirely, requiring either IGU replacement or full window replacement. Understanding this distinction saves homeowners money and frustration: calling a window cleaner for foggy double-pane windows results in paying for a service that can't help, with the fog still present afterward. The only cleaning-adjacent service that occasionally has marginal benefit is the use of specialized defogging kits, which involve drilling small holes in the spacer bar and injecting a desiccant compound. This is sometimes offered by window restoration companies and produces mixed results — the haze may improve temporarily, but the underlying seal failure continues, and the long-term outcome is typically continued degradation. Most window professionals do not recommend defogging kits as a permanent solution.

If you've hired window cleaners and the fog persists, that's not a sign the cleaners did poor work. The fog is inaccessible to cleaning. This conversation should have been had at the quote stage — if a window cleaner doesn't identify failed seals during assessment, that's a service quality gap worth noting.

How to Tell a Failed Seal from Surface Condensation

Not every foggy-looking window has a failed seal. Surface condensation — moisture that forms on the accessible faces of the glass — is a different, benign phenomenon that doesn't require repair. Surface condensation on the interior glass face occurs when warm, humid interior air contacts cold glass in winter. It appears as droplets or a misty film on the room-facing surface of the glass — the surface you can touch. It wipes away easily with a cloth and is gone by midday as the room warms. This is not a defect; it's normal physics. It can be reduced by increasing ventilation, using a dehumidifier, or adding window insulation film to raise the glass surface temperature. Surface condensation on the exterior glass face happens on cool mornings when warm, humid outdoor air contacts glass that has been cooled overnight. This wipes away from outside and clears as the sun warms the glass. Again, not a defect. A failed seal shows characteristics that distinguish it clearly: the fog or haze appears to be inside the glass, not on a touchable surface. Wiping the interior and exterior surfaces doesn't affect it. It may change appearance with temperature but doesn't wipe away. In advanced cases, there are visible mineral deposit patterns — white cloudy areas or 'tide marks' — within the glass unit that remain even in warm, dry weather. The simplest test: on a dry indoor day, fog the glass with your breath and immediately check if the haze you see wipes away with your hand. If it does, it's surface condensation. If the haze is behind where your breath condensed, and doesn't wipe, it's inside the unit.

• Interior surface condensation: touchable, wipes away easily, a normal physics phenomenon
• Exterior surface condensation: occurs on cool mornings, clears as temperature rises
• Failed seal fog: appears inside the glass, does not wipe away from either surface
• Breath test: fog your breath on the glass and check whether the haze is on or inside the surface

IGU Replacement vs Full Window Replacement: The Cost Calculation

If you've confirmed a failed seal, the decision comes down to whether to replace the glass unit alone (IGU replacement) or the entire window assembly (frame, hardware, and glass). **IGU replacement** is the lower-cost option when the window frame is in good condition. A glazier removes the failed glass unit from the existing frame and installs a new IGU. In BC, IGU replacement for a standard residential window typically costs $150-$400 per unit depending on size, glass type, and whether upgrades are specified. The existing frame, hardware, and installation are retained. IGU replacement makes sense when: the frame is structurally sound, the seals around the frame are intact, the hardware operates correctly, and the window is less than 20-25 years old. It's a cost-effective repair that restores the window's appearance and thermal performance. **Full window replacement** is warranted when: the frame is rotting or damaged, the window is approaching end of life (typically 20-30 years for a well-maintained unit), or when multiple IGUs in the same window assembly are failing simultaneously (suggesting a frame or installation issue rather than an isolated glass defect). In Salmon Arm and the Shuswap, the freeze-thaw climate is relatively hard on window seals compared to warmer BC regions. Homes built in the 1990s with original windows may be approaching the point where multiple IGUs are starting to fail — at which point full window replacement on a room-by-room basis makes more economic sense than repairing individual units. For specific quotes on IGU replacement in the Salmon Arm area, contact a local glazier or window company. This is not a service Wagon Windows provides — our expertise is in window cleaning and exterior maintenance, not glass repair. We're happy to confirm whether what you're seeing is a cleaning issue or a failed seal when we visit.

• IGU replacement: $150-400 per unit in BC, viable when frame is sound
• Full replacement: warranted when frame is damaged, window is aging, or multiple IGUs failing
• Salmon Arm's freeze-thaw climate is harder on window seals than warmer BC regions
• 1990s-era homes may be at the point where multiple seals are failing simultaneously