5 min read · Cost
Fog or moisture trapped between the panes of a window is one of the most common complaints California homeowners bring to us, and it points to a single specific failure: the sealed glass unit has lost its seal. It is not dirt you can wipe away and not something a good cleaning fixes. Here is what is actually happening inside the glass, why repair rarely lasts, and how to decide between glass-only and full window replacement.
What fog between the panes actually means
A dual-pane or triple-pane window is a sealed insulated glass unit: two or three panes spaced apart with a perimeter seal trapping dry air or an inert gas like argon or krypton between them. That sealed cavity is what gives the window its insulating value. When the perimeter seal fails, humid outdoor air migrates into the cavity, and as temperatures swing, the moisture condenses on the inner faces of the glass where you cannot reach it. That is the fog. The failure is permanent and progressive — it does not dry out and reseal itself, and once it starts the cavity will keep cycling between clear and foggy with the weather.
Why the seal fails
Several mechanisms end a seal's life, often in combination. Some units fail early because the original factory seal was marginal — certain manufacturing batches simply have higher failure rates. The far more common cause is age: over roughly fifteen to twenty-five years the sealant chemistry breaks down under ultraviolet exposure and the relentless thermal cycling of expansion and contraction. Poor installation that stresses the unit, and pooling water at the sill that attacks the seal from outside, both shorten the clock. In California, the south and west windows that take the most sun and heat are typically the first to fog, which is itself a clue that ultraviolet aging is driving the failure.
Can the seal be repaired?
Honestly, no — not durably. Aftermarket window defogging services do exist; they drill the unit, draw out the moisture, and sometimes inject a coating. The fog clears for a while, but the original perimeter seal is still broken, so humid air keeps entering and the fog returns within months to a few years, and the lost gas fill and insulating value never come back. We do not recommend defogging as anything more than a short cosmetic stopgap. The seal cannot be genuinely restored in place, which is why glass or window replacement is the only lasting answer. For how glass work folds into a larger exterior budget, see our overview of window and siding cost together.
Glass-only replacement when the frame is sound
When the window frame and sash are structurally sound and weather-tight, you can often replace just the insulated glass unit and keep the existing frame in place. The frame has to be a type that accepts a replaceable unit — many vinyl and aluminum frames do, some welded or older units do not — so a glazier or window professional verifies it first. Glass-only replacement is the efficient path for an isolated foggy pane in an otherwise newer, well-functioning window, because it solves the visible problem without the cost and disruption of pulling and reframing the whole opening. We confirm the frame is worth keeping before recommending this route.
When full window replacement makes more sense
If several windows have fogged in the same era, the smart read is that the whole batch of seals is aging together, and replacing one glass unit just postpones the same failure in its neighbors. In that situation full window replacement is usually the better total value, because it captures meaningful gains beyond the fog: tighter modern weatherstripping, better frame materials, and improved energy performance you can verify against ENERGY STAR criteria and the independent ratings published by the NFRC. On an older home that already has comfort and draft complaints, a coordinated window project resolves the fog and the underlying performance issues in one pass rather than chasing failures one pane at a time.
Insurance and foggy windows in California
Homeowners are often surprised to learn that California policies generally do not cover seal failure. Insurers treat a failed glass seal as gradual wear and deterioration, which is an excluded cause of loss, so the cost of fixing foggy windows is typically out of pocket. What policies do cover is sudden, accidental glass damage — a thrown rock, storm impact, vandalism — which is a different event entirely. If your fog arrived without any impact, expect to pay for the glass or window work yourself, and budget accordingly rather than assuming a claim will offset it.
Reading the timing and building a decision
When the fog appears tells you what you are dealing with. Fog within the first couple of years usually signals a manufacturing defect that may still be under warranty, so check your paperwork before paying anyone. Fog at ten-plus years is normal end-of-life aging. Fog showing up across several windows in the same window of time points to a batch issue or simply uniform aging of the whole install. From there the decision is straightforward: a single foggy unit in a newer install favors glass-only replacement, while multiple fogged windows in an older home — especially one with other comfort or energy gripes — favors a full window replacement project for better long-run value.
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 between panes means the sealed glass unit's perimeter seal has failed — it is permanent
- Aftermarket defogging is a cosmetic stopgap, not a durable repair; the seal stays broken
- Glass-only replacement works when the frame and sash are sound and accept a new unit
- Multiple fogged windows in one era usually favor full replacement for better total value
- California insurance treats seal failure as excluded wear, so the cost is typically out of pocket
- Fog within the first couple of years may be a warranty defect — check your paperwork first
FAQ
Quick Answers
Yes — all insulated glass seals eventually fail, typically at fifteen to twenty-five years on quality windows and sooner on budget units or in heavy sun exposure.
No. Defogging can temporarily clear the moisture, but the broken perimeter seal remains, so humid air re-enters and the fog usually returns within months to a few years.
On a newer window with a sound frame that accepts a new insulated unit, yes; on an older window with other comfort or energy issues, full replacement is usually the better value.
Those elevations take the most sun and heat in California, so ultraviolet aging and thermal cycling break their seals down first — it's an expected direction of failure.
Generally no — California policies exclude seal failure as gradual deterioration. Sudden impact or storm glass breakage is covered, but ordinary fogging is not.
A single foggy pane in a newer install favors glass-only work; several fogged windows in an older home, especially with drafts or comfort issues, favor a full replacement project.
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.