Insulated glass units (IGUs) are designed to provide thermal efficiency and clarity by sealing two or more panes of glass with a spacer and a gas fill. However, over time, the seal can degrade, leading to performance loss and visible damage. Recognizing the early indicators of seal failure is crucial for homeowners and building managers to address issues before they escalate.

The most common sign of IGU seal failure is fogging or condensation that appears between the glass panes. This occurs when the seal breaks, allowing moisture-laden air to enter the cavity. Unlike surface condensation, which wipes away, internal fogging indicates a compromised seal. Over time, this moisture can lead to mineral deposits, permanent staining, and reduced visibility.

Another indicator is a noticeable distortion or wrinkling of the glass surface. When the seal fails, the gas inside may escape, causing the panes to bow under atmospheric pressure changes. This creates a rippled or wavy appearance.

Additionally, temperature differences between the inner and outer panes might become less pronounced. A properly sealed IGU maintains thermal insulation; a failed unit will feel cold or drafty near the glass, increasing heating or cooling costs.

Water droplets, dirt accumulation between panes, or a milky-white film are further signs of seal degradation. In advanced cases, mold or algae growth may appear inside the unit.

Regular inspection of windows for these indicators can save money and energy. Once seal failure is confirmed, replacement of the IGU is typically required, as repairs are rarely effective. Proper installation and quality materials extend the life of insulated glass units.