Condensation

Condensation on windows is one of the most common calls window repair companies receive. But not all condensation is equal: where it forms tells you whether you have a normal humidity issue or a more serious window problem.

Types of Window Condensation

1. Interior Surface Condensation (Normal)

Moisture forming on the room-side surface of the glass is caused by high indoor humidity, not a window defect. It is most common in winter when the cold glass surface meets warm, moist indoor air. Solutions: reduce indoor humidity (exhaust fans, dehumidifier), increase ventilation, or improve window insulation to raise the interior glass surface temperature.

2. Exterior Surface Condensation (Normal)

On cool mornings, dew forms on the exterior of high-performance Low-E windows. This actually indicates the window is working correctly—the interior heat isn’t reaching the outer glass surface, so it gets cold enough for dew to form outside. It evaporates on its own.

3. Condensation Between the Panes (Seal Failure)

Fog, haze, or moisture trapped between the glass panes that cannot be wiped away is the definitive sign of a failed window seal. The IGU’s hermetic barrier has been breached, humid outside air has entered, and the desiccant in the spacer bar is saturated. The only fix is IGU replacement.

Dew Point and Windows

Condensation forms when a surface temperature drops below the dew point of the surrounding air. In Ohio winters (indoor RH 35%, indoor temp 70°F), the dew point is approximately 43°F. A poor single-pane window glass surface may be 30°F—well below the dew point, guaranteeing condensation.