An insulated glass unit (IGU) is a type of window that consists of two or more panes of glass separated by a spacer. The spacer is typically made of metal or plastic and is filled with a gas, such as argon or krypton, to provide insulation between the panes. IGUs are also sometimes referred to as "double-pane" or "triple-pane" windows, depending on the number of panes they contain.

The main purpose of an IGU is to improve the energy efficiency of a building by reducing the amount of heat transfer between the interior and exterior of the building. IGUs work by creating a barrier between the inside and outside of the building, which helps to prevent heat loss in the winter and heat gain in the summer. This can help to keep the building more comfortable and reduce energy costs.

In addition to their energy-saving benefits, IGUs also offer other advantages, such as noise reduction, improved durability, and increased security. They are commonly used in both residential and commercial buildings.