Why do double pane windows lose their seal?


Editorial team Published on

Double pane windows lose their seal because constant temperature swings cause the glass and sealants to expand and contract over time.

Main Causes of Seal Failure

  • Solar Pumping (Thermal Stress): Direct sunlight heats the air inside the window, causing it to expand and push out against the seals. Cooling at night creates a vacuum that pulls in outside air and humidity, stressing the adhesive.
  • Age and Material Breakdown: Over 10 to 20 years, rubber and polymer sealants (like butyl or silicone) dry out, harden, and crack from UV exposure.
  • Improper Installation: If a window is racked, improperly shimmed, or exposed to pooling water from blocked weep holes, the perimeter seal degrades much faster.
  • Manufacturing Defects: Misaligned spacer bars or uneven sealant application can create weak spots that break down prematurely.

Signs and Solutions

  • The Sign: Fogging, hazing, or mineral streaks trapped between the panes that cannot be wiped away.
  • The Fix: You cannot effectively repair a broken factory seal with DIY caulk or drill-and-defog kits. The only permanent solution is replacing the insulated glass unit (IGU) or the entire window.