Double-pane windows are one of the better ways to keep the cold air outside in the winter time, but do you notice that your windows have become foggy and not keeping the heat in like they used to? If so, your window might have a failed seal. Using thermal imaging, you would see a noticeable difference between a window that has intact seals and one where they failed.
The windows are sealed to keep a gas inside of the windows in place so that it can work as an insulator. Unfortunately, over time when this gas expands and contracts due to being heated up by the sun, it wears out the seal and air makes it inside. This will make for a big difference in the efficiency of the window to keep the elements at bay.
While there are methods to fix this issue, they will never bring the window back to its original state. If that is what you want, you will need to have them replaced with new double-pane windows.