Condensation can form on interior glass surfaces when there is too much moisture in the air. If the interior of a structure exceeds certain limits of moisture in the air, the moisture will condense and show up on comparatively cooler surfaces, such as glass.
Windows are typically the coolest areas of interior walls; even if they have storm panels, are glazed with welded insulating glass, have Low-E4® insulating glass, or use triple pane glass. When the warm, room temperature air comes in contact with the glass surface, the air is cooled and if there is enough moisture in the air, the dew point will be reached and the water in the air will condense.
A good analogy is when you have an iced drink on a warm summer day, and the glass has moisture on the outside of it. The warmer air meeting the cooler surface of the glass causes condensation to form.
Recommended humidity levels in winter months should not exceed 30-35%. If these humidity levels are exceeded, you may want to take measures to reduce the interior humidity level such as:
- Checking your ventilation
- Using a dehumidifier
- Turning the humidifier on your furnace down (or off)
- Making sure blinds or curtains are open during the day
- Leaving ceiling fans on to promote air movement
- Use an exhaust fan in bathroom areas when showering
Additional information can be found in the Guide to Understanding Condensation.