Insulated glass unit (IGU) is a type of window that seals multiple tempered, flat, or curved glass panels with spacer materials and gas in a frame to reduce heat transfer and improve energy efficiency. The number of layers in an IGU varies depending on the application and insulation requirements, but typically, the IGU has two pieces of glass surrounding a gap filled with air or an inert gas such as argon or krypton.

The insulation effectiveness - R value and RSI value, as well as the thermal resistance of windows, depend on the thickness of the glass, the air between the panes, and the type of gas used in the spacers. For example, the hollow effect of a glass unit with two glass panels surrounding the air space is twice that of a unit with one glass panel but no air space. Increasing the thickness of insulating glass will increase the R value and RSI value. However, this increases the cost of glass.

If you want the highest possible R-value, a good choice is to have windows with eight or more layers of film between the outer and inner panes. Compared to standard single pane glass windows, these windows are lighter, easier to transport and frame, making them a more practical choice for households in both cold and hot climates.