5 min read · Cost
Fog between window panes is one of the most common window complaints. It signals a specific failure mode. Here's what's happening and what to do.
What fog between panes actually means
Dual-pane (and triple-pane) windows have a sealed air or inert gas (argon, krypton) space between panes. When the seal fails, moisture from outside air enters the space and condenses on the inside surfaces of the panes — visible as fog or moisture. The seal failure is permanent; it doesn't self-repair.
Why the seal fails
Manufacturer seal failure: original sealing inadequate (some manufacturer batches have higher failure rates). UV degradation: over 15-25 years, sealants break down. Thermal cycling: extreme temperature changes stress the seal. Improper installation: install stress on the unit. Most fogging is in older windows where seal life has aged out.
Can the seal be repaired?
Honestly, no — at-home or aftermarket 'window defogging' services exist but typically aren't permanent solutions. The seal can be cleaned and have moisture drained, but the underlying seal failure doesn't repair. Fog returns within months to a few years. Replacement is the durable answer.
Glass-only replacement option
If the window frame is sound, sometimes only the IGU (insulated glass unit) can be replaced while keeping the existing frame. IGU replacement cost: $200-$500 per window typically. Requires the frame to accept the new IGU; not all windows support this.
Full window replacement when warranted
Older windows with multiple fogging panes typically warrant full window replacement rather than IGU-only repairs. The whole window's seals are aging similarly; replacing just one delays the same failure in others. Full replacement also captures energy performance improvements.
Insurance and foggy windows
California homeowners insurance typically doesn't cover seal failure (excluded as gradual deterioration). Window damage from impact or storm is covered; seal failure isn't. Replacement is typically out-of-pocket.
How fog timing tells you about the window
Fog appears within first few years: manufacturing defect (warranty if still in period). Fog appears 10+ years in: normal aging. Fog appears in multiple windows in same time period: batch issue or universal aging. Fog only on south/west: UV-driven; expected aging direction.
Decision framework
Single foggy window in otherwise newer install: IGU replacement makes sense. Multiple foggy windows in older install: full window replacement is usually better total value. Foggy windows in older home with other comfort/energy issues: full window replacement project.
Foggy window options
| Option | Cost | When right |
|---|---|---|
| At-home defogging service | $50-$200 | Not durable; rarely recommended |
| IGU-only replacement | $200-$500 per window | Newer frame; single window |
| Full window replacement | $850-$2,500+ per window | Older windows; multiple issues |
| Whole-home window project | $13,000-$30,000+ | Comprehensive upgrade |
Key takeaways
- Fog means seal failed; not self-repairable
- IGU-only replacement possible if frame sound
- $200-$500 per IGU; full window replacement is typically broader
- Insurance doesn't cover seal failure
FAQ
Quick Answers
All dual-pane seals eventually fail — typically 15-25 years on quality windows; sooner on budget windows.
On newer windows yes; on older windows with other issues, full replacement is usually better.
Sources
Authoritative references
- National Fenestration Rating Council (NFRC) — window performance ratings
- ENERGY STAR — Residential Windows, Doors & Skylights
- Contractors State License Board (CSLB) — verify a California contractor
External links to government, code, and manufacturer sources. Sierra Siding is not affiliated with these organizations; references are provided for verification.