Why Do Windows Fog Between the Panes? Seal Failure Explained

That cloudy appearance between your window panes isn't dirt you can clean—it's seal failure, and understanding why it happens helps you make the right repair decision.

How Insulated Glass Works

According to the FGIA (Fenestration & Glazing Industry Alliance), an insulated glass unit (IGU) consists of:

1. Two (or three) panes of glass
2. A spacer separating the panes
3. Sealed edges preventing air/moisture entry
4. Insulating gas (usually argon) filling the space
5. Desiccant material absorbing any residual moisture

The system only works when completely sealed.

What Causes Seal Failure

According to GANA research, seals fail for several reasons:

Age: According to FGIA, typical seal life is 15-25 years. No seal lasts forever.

Thermal pumping: Glass expands and contracts with temperature changes. According to NFRC, this constant flexing stresses seals over time—especially in climates with extreme temperature swings like Idaho.

UV degradation: Sunlight breaks down seal materials. South and west-facing windows often fail first.

Manufacturing defects: Some units fail prematurely due to quality issues.

Physical damage: Frame settling, impact, or installation stress can compromise seals.

What's Actually in There

According to GANA, that foggy appearance is:

• **Moisture:** Humid air has entered the unit
• **Mineral deposits:** When moisture evaporates and re-condenses repeatedly, minerals accumulate on glass surfaces

The permanent haze is typically mineral buildup that can't be cleaned because it's between sealed panes.

How to Identify Seal Failure

According to FGIA, signs include:

• Cloudiness that doesn't wipe away
• Fog that comes and goes with temperature
• Visible moisture droplets between panes
• Distorted reflections through the glass

The Bottom Line

Seal failure is a natural aging process, not necessarily a defect. Once it occurs, the insulating gas has escaped and the window has lost significant efficiency—even if the fogging doesn't bother you visually.

*For repair vs. replacement guidance, see: [Foggy Windows: Repair vs. Replace](/guides/foggy-windows-repair-vs-replace)*