Double-glazed windows, also called double-paned, greatly reduce the amount of home heating and cooling lost through your windows. As such, they are often promoted as a way to improve your home's energy efficiency and are required to qualify for certain building certifications. Though double-glazed windows are superior to many other types of windows, they are not minus their faults.
The first sign of a problem with a double-glazed window is condensation between the panes. When working properly, double-glazed windows should not collect any condensation between the panes because the layer is sealed. If the layer appears misty or foggy, the seal has worn out.
If the seal on your double-glazed windows has worn out, you might see slight grooves running down the inside panes. This phenomenon, called "riverbedding," happens when accumulated vapour in between the panes forms water droplets that follow the same narrow paths as they run down the glass. Over time --- about six to 12 months --- these droplets etch permanent grooves in the glass much like running water forms a canyon. Because water accumulates as the droplets reach the bottom of the pane, riverbedding is more apparent in the lower third of your window.
You might see white, chalky spots on your double-glazed windows before you see any condensation at all. These snowflake-like patterns are called "silica haze." They form when the silica gel in your window seal has become saturated with water and becomes eroded by passing air currents. The silica then accumulates on the pane in cross-hatch patterns that form at 45-degree angles or as a chalky haze over the entire surface of the glass.