Why Windows Fog Between Panes — and What the Fix Costs
Updated July 6, 2026 · By paveking_admin · LightGate Glass Field Team
Quick answer: Fog between panes means the insulated glass unit’s perimeter seal failed. The fix is replacing the sealed unit — $250–$600 installed — not the whole window, and not a drill-and-vent ‘defogging’ service.
Why Seals Fail
- Age: IGU seals last 15–25 years; thermal cycling works them loose.
- Sun exposure: south- and west-facing units fail first.
- Trapped water: clogged weep holes soak the unit’s edge seal.
- Cheap spacers: builder-grade units from the 2000s fail early and in batches.
Repair Options Compared
| Option | Cost |
|---|---|
| Defog service (drill + vent) | $100–$250 — fog usually returns, insulation not restored |
| IGU replacement (our recommendation) | $250–$600 — full performance restored, 10-yr seal warranty |
| Full window replacement | $650–$1,600 — only needed if the frame failed too |
Signs It’s Spreading
Seal failures cluster: if one unit fogged, its siblings from the same production run are next. Whole-house checks catch them early, and multi-unit replacement pricing (10–15% off at 5+ units) beats fixing them one by one.
Count your foggy units and get per-unit pricing here, or use the calculator.
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LightGate Glass is a licensed, insured glass & window contractor serving Philadelphia, the PA suburbs, South Jersey and Delaware. Every guide is written and reviewed by our field team — the people who measure, fabricate and install this glass every day.
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