Insulated glass units are sometimes called thermal panes, or double pane glass. All three names mean the same thing.
They come in a single unit: two pieces of glass are glued together with a special sealant with a spacer between the panes. A vacuum is used to form a dead-air space between the two panes.
This spacer is a formed, square aluminum tube that has moisture absorbent desiccant inside to help the units last longer. (Not all insulated units have this feature.) Higher-end units are sometimes filled with a gas, like Argon, that improves the insulating ability of the unit but can only be used for higher altitude areas.
Low Emissivity (Low E) units are manufactured with a special coating on the inside surface of the exterior pane to improve the insulating ability of the unit. These windows are a bit more expensive, but you recoup the extra cost in energy savings.
Seal failure can occur when a window is exposed to constant moisture, such as a water sprinkler pointed toward the window or when the window is in a steam room/sauna /hot tub area.
But even if the window is not exposed to the above conditions, over time, the seal fails, allowing moisture to form between the two panes, making the window appear foggy. This foggy appearance will only get worse and will turn to a milky appearance. This can be especially unsightly on the front of your house!
The best solution to fogged window issues is to replace the unit with a new one. There is no need to replace the entire window frame unless it’s damaged. Replacing the glass units is a much more efficient way to improve your homes appearance and efficiency.
Most insulated glass units come with a standard 5-year or an optional 10-year warranty against seal failure. If the seal fails on an insulated unit, you should be covered under warranty.
Be sure that the glazer who comes to install your unit has enough experience and skill to handle both a sensitive insulation and any subsequent servicing you might require. I wouldn’t hurt if they were meticulous in their attention to detail either. Checking on their testimonials helps insure great installation.