Insulating glass units, or IGUs, are designed to keep homes warmer in the winter and cooler in the summer.

An insulating glass unit commonly consists of two (sometimes more) panes of glass separated by a spacer material and sealed together at the edge. The insulating airspace is filled with air or a noble gas, such as argon or krypton inside. Each glass pane has two surfaces, so typical double-paned IGUs have four surfaces.

Using argon or krypton in the air space between the two panes of glass provides further insulation, as these gases are denser than air and less likely to let heat conduct through the IGU. Combined with a low-e glass coating, these gases work to improve the window’s overall u-value, which is a measure of the heat transmission through a building part (such as a wall or window). The lower the u-value, the better the insulating glass unit.

When 90% argon gas fill is used in a low-e IGU instead of air, the window’s u-value can be improved by up to 16%. Similarly, krypton improves the u-value in a low-e IGU by up to 27%. However, using a noble gas instead of air in an IGU can add both time and cost to the window’s construction.