You look at your windows and see haze. Is it dirt? Condensation? A failed seal? The answer determines whether you need a $15 cleaning or a $400 window replacement. Here's how to tell — and what to do either way.
The 30-Second Test
Can You Wipe the Fog Away?
It's dirt or condensation. Professional cleaning solves it. Cost: $10-16 per window. Problem solved same day.
It's between the panes — failed thermal seal. The Insulated Glass Unit (IGU) has lost its argon gas fill and moisture has entered. Cleaning won't fix this.
The 5 Signs of a Failed Seal
- Fog appears and disappears — Comes with temperature changes. Warm day + cold night = fog. Midday sun = clears. This cycle is diagnostic.
- Fog is between the panes — You can see it but can't touch it from either side. It's trapped in the sealed airspace.
- White mineral residue — Over time, repeated condensation leaves calcium deposits between panes. This is permanent even if you replace the gas fill.
- Distortion or waviness — Failed seals allow panes to flex inward (vacuum effect) or outward, creating visual distortion.
- Age of windows — Thermal seals last 15-20 years. If your windows were installed 2000-2010, you're in the failure window for seal degradation.
Why Toronto Kills Window Seals
Toronto's climate is uniquely destructive to thermal window seals:
- 60°C temperature range — From -25°C winter to +35°C summer. This expansion/contraction cycle stresses seal adhesive thousands of times per year.
- 80+ freeze-thaw cycles — Each crossing of 0°C stresses seals. Toronto has more than Calgary, Winnipeg, or Vancouver.
- Intense summer UV — South and west-facing windows get brutal UV that degrades butyl and silicone seal compounds. These windows fail first.
- Lake effect humidity — Higher humidity = more moisture available to penetrate weakened seals.
Your Options — Cost Comparison
| Option | Cost/Window | Duration | Best For |
|---|---|---|---|
| Professional cleaning | $10-$16 | Permanent* | Dirty glass (not failed seal) |
| IGU replacement (glass only) | $150-$400 | 15-20 years | Failed seal, good frame |
| Defogging service | $75-$150 | 3-5 years | Budget fix, selling soon |
| Full window replacement | $400-$1,200+ | 25-30 years | Failed seal + damaged frame |
* Until windows get dirty again — typically 3-6 months depending on environment.
The "Clean First" Strategy
Before spending $150-$400 per window on IGU replacement, get a professional cleaning first. Here's why:
- 50% of "foggy" windows are just dirty. Winter grime, furnace exhaust film, and mineral buildup create a haze that looks like a failed seal but wipes right off.
- Professional cleaning reveals the real problem. Once exterior and interior surfaces are clean, genuine failed seals become obvious — the fog is clearly between panes.
- MANTLE identifies failed seals during cleaning. We'll point them out during your walk-around and explain your options. No upsell — we don't do window replacement. Just honest assessment.
Condensation vs Failed Seal — The Other Confusion
Interior condensation (water droplets on the inside surface) is NOT a failed seal. It's a humidity problem. Common in Toronto winters when indoor humidity is high and window surface is cold.
Fix: reduce indoor humidity (exhaust fans, dehumidifier), improve air circulation near windows, or upgrade to triple-pane.
Exterior condensation (fog on the outside surface in spring/fall mornings) is actually a sign of GOOD windows — they're insulating well enough that the exterior pane stays cool. This clears by mid-morning.
Not Sure If It's Dirt or a Failed Seal?
Book a cleaning — we'll clean your windows AND identify any failed seals. Honest assessment included.
Get Free Estimate →