About Interior Pane Condensation:
Q: What causes condensation to form on the interior glass of my windows?
A: Whenever there is excess humidity in a home, it manifests itself in the form of condensation on very cold surfaces which, in winter, will be your window glass. The warmer the air in your home, the more moisture it will retain. When this moist air comes in contact with your relatively cold window glass, it is quickly cooled, resulting in droplets of water.
Q: Do windows themselves cause condensation?
A: No, condensation is not the fault of the window. All window surfaces except for the glass are at a relatively warm temperature. Even the best insulating glass is still the weakest link in the insulation chain of any window. In fact, replacement of old drafty windows and doors with new, better sealing units reduces air flow through the home, making it tighter, and so able to retain more moisture.
Q: Where on a window does condensation normally form, and why?
A: Condensation always begins to form at the bottom of the glass pane. This happens because when the warm (and moist) air from the room contacts the cold glass surface, it begins to cool. Cool air tends to fall and by the time it hits the bottom of the glass it has cooled enough to condense. This has a cumulative effect.