An insulated glass unit (IGU) is the glass in a window or door made up of multiple panes separated by a spacer and sealed together. This space is designed to create a thermal barrier. The glass panes can be treated with low emissivity (low-e) coatings or tints to improve thermal and optical performance. The space between the glass panes can be filled with an inert gas such as argon to provide an insulating layer and diffuse heat transfer.

 

IGUs are also known as double-pane glass windows. They are designed to keep buildings and homes warmer in the winter and cooler in the summer.

 

When the glass seal fails in a IGU, the window can take on a "foggy" appearance. Replacing the unit with a new IGU will restore the window's clarity and insulating properties.