Foggy windows can be repaired using a defogging procedure. While still a newer repair option, window defogging can produce cosmetic results that help improve visibility in your windows. On the downside, defogging does nothing to restore the window's insulating ability (R-value) to its original level.
However inconvenient fogging is, it isn’t the only issue commonly seen in double pane windows. There are times that a window may appear stained or glossy from the exterior of the home. This condition happens whenever the seal of a Low-E window fails. Low-E, or low-emissivity, glass was created to minimize the amount of infrared and ultraviolet light that comes through your glass, without minimizing the amount of light that enters your home. Low-E glass windows have a microscopically thin coating that is transparent and reflects heat. While it might be possible to replace the glass with a low-E/argon-filled glass, you should consider if the repair is worth the investment. The better option might be replacing the entire window with a newer window because it is labor intensive, which adds up to more cost.
If you have concerns about about the condition of your windows, contact a licensed Home Inspector to learn more about your window condition and possible repair or replacement options.