Why Do Some Windows Fog Up Between the Glass Panes?
Windows fog between panes when the sealed insulating glass unit fails and moisture enters the space between the glass layers.
Double-Pane Window Design
Most modern windows use two panes of glass separated by a sealed air or gas-filled space.
This design improves insulation.
Sealed Air Space
The space between panes is sealed around the edges.
This prevents outside air and moisture from entering.
Seal Failure
Over time, seals can degrade due to temperature changes and expansion.
Once the seal breaks, humid air can enter the gap.
Condensation Formation
When moist air becomes trapped between panes, it condenses as temperatures shift.
This creates fog or visible droplets inside the window.
Gas Loss
Many insulated windows contain inert gas such as argon.
Seal failure allows this gas to escape and reduces insulation performance.
Seasonal Effects
Temperature swings can make fogging more noticeable.
Condensation may appear and disappear as conditions change.
Building Envelope Practice
Fogging between panes indicates a compromised seal rather than surface condensation.
The moisture is trapped inside the insulated glass unit.
