The glass panes are separated by a ” spacer “. A spacer is the piece that separates the two panes of glass in an insulating glass system, and seals the gas space between them. Historically , spacers were made primarily of metal and fiber, which manufacturers thought provided more durability. However, metal spacers conduct heat ( unless the metal is thermally improved ), undermining the ability of the IGU to reduce heat flow. It may also result in water or ice forming at the bottom of the sealed unit because of the sharp temperature difference between the window and surrounding air. To reduce heat transfer through the spacer and increase overall thermal performance, manufacturers may make the spacer out of a less – conductive material such as structural foam. A spacer made of aluminum that also contains a highly structural thermal barrier reduces condensation on the glass surface and improves insulation, as measured by the overall U – factor.