Why do double pane windows fog up between the panes?
Because the edge seal has failed. A double pane window is two panes sealed around a spacer, with dry air or gas inside. Sun, heat, water and temperature swings wear that seal down until moist outdoor air seeps in, then condenses on the inside faces of the glass whenever they turn cold.
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What wears the seal out
- Sun and heat: glass facing south or west runs hottest, and its seal ages faster.
- Temperature swings: the air inside expands and contracts with the weather, pushing and pulling on the edge.
- Water: a DOE Building America Solution Center guide says seal failure is commonly a result of the seals sitting in direct contact with water for extended periods. That happens where water collects along the bottom edge of the glass and cannot drain.
Why the fog shows up on cold mornings
Once the seal leaks, the space between the panes breathes with the weather. Moist air is drawn in, and when the glass cools overnight that moisture condenses where no cloth can reach it. Low-E coatings and gas fills do not cause the fog; they simply stop working as well. To see which windows are most at risk, read how long double pane windows last. When one is fogged, foggy window repair replaces the sealed unit.
Related questions
- Can foggy windows be repaired?
- Will condensation between window panes go away?
- Can you get rid of fog between window panes yourself?
- How much does it cost to fix a foggy window?
- Does homeowners insurance cover a broken window seal?
- If one window is foggy, do I need to replace all of them?
- All glass repair questions
Seal gone on one of yours?
Send us a photo for a free estimate, or call (202) 600-6220.