Why Is There Moisture Forming Inside My Double-Pane Windows?

A double-pane window has two glass panels with a sealed space between them. That space is filled with dry air or an insulating gas like argon. The seal around the edge is critical. It maintains the dry environment inside the gap, preventing outside air and humidity from getting in. When this seal fails, it allows the outside atmosphere to infiltrate the insulated chamber.

Window seals degrade over time from exposure to sunlight and temperature swings. Physical damage from slamming windows or pressure washing can also compromise the seal. Poor initial installation or manufacturing defects are other potential causes. The problem is more noticeable in climates with significant seasonal temperature changes, as the expansion and contraction of materials stresses the sealant.

• Natural aging and degradation of the sealant material over 10 to 20 years.
• Prolonged exposure to direct sunlight and UV radiation, which breaks down the seal.
• Extreme and repeated temperature fluctuations that cause the window frame to flex.
• Physical impact or stress from improper cleaning, painting, or window operation.
• Manufacturing flaws where the seal was not properly applied during assembly.
• Pressure changes, such as from high winds or significant differences in indoor/outdoor air pressure.

Condensation inside the panes is primarily an efficiency issue, not a direct leak. However, persistent seal failure can lead to other problems. If you see water staining or damage on the surrounding wall or sill, the window frame itself may be leaking from a separate issue. In that case, the moisture is coming from outside your home and requires a different repair approach.

• Foggy windows reduce insulation, raising your heating and cooling bills.
• Prolonged interior condensation can lead to mold growth on the window frame.
• If the window frame is wood, trapped moisture can cause it to rot.
• Water pooling on the interior sill indicates a leak, not just seal failure.
• Damaged seals may allow more outside noise to enter your home.
• Consider an inspection to differentiate between seal failure and a structural leak.