Insulated glazing, otherwise known as double glazing or triple glazing, refers to the layers of glass in a window. A double glazed window is two panes of glass (also called a lite) with a sealed air (or gas) space in the middle. The edges are sealed together with an air-tight spacer. This is used so that heat cannot escape through the air space by conduction or convection. Heat radiation is not affected as it can travel through a vacuum space. To increase the R-value of insulated windows are sealed with argon gas.