What Causes Window Condensation?
The difference between normal interior condensation and a sign your windows are underperforming.
Condensation on the interior of your window glass happens when warm, humid indoor air meets a cold glass surface — the glass temperature drops below the air's dew point, and moisture condenses out, the same physics as a cold glass of water sweating on a summer day.
When it's normal
Some interior condensation during very cold weather, especially in rooms with higher humidity (bathrooms, kitchens, near indoor plants), is normal even with quality windows — it reflects your indoor humidity level more than a window defect.
When it signals a problem
Frequent or heavy condensation, especially on newer or supposedly efficient windows, can mean the glass's insulating performance isn't adequate for your indoor humidity level — either an underperforming U-factor for your climate, or (if it's between the panes rather than on the interior surface) a failed seal.
What you can control
Reducing indoor humidity (ventilation fans in bathrooms and kitchens, a dehumidifier in damp basements) reduces interior condensation regardless of window quality. If condensation persists on newer windows even with reasonable humidity control, that's worth discussing with an installer.
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