Insulating glass refers to glass that is made to prevent significant heat transfer into or out of a building through glass. It consists of multiple pieces of glass separated by spacers made of either metal, such as aluminum, or structural foam. The space between the glass is sometimes filled with a noble gas, such as argon or krypton. Insulating glass is often abbreviated IG and is sometimes called double-glazed or double-pane glass. More recently, with additional lites of glass being added, it now can also be triple-paned or quadruple-paned glass.