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The following is a list of terms related to window replacement. Each term is followed by a definition in brief to help you better understand the meaning of that term. Like all areas of home remodeling, window replacement loves its jargon. Read through our terms and you, too, can talk like a pro.


Airspace is the space between the panes of a multi paned glass window. Airspace between panes creates thermal insulation, much like the airspace maintained between the outside of the exterior framed wall and the inside of the exterior brick veneer wall covering does for the exterior wall of a building. Air (or argon or krypton in certain windows) acts as an excellent insulator in these cases.


The material used to absorb moisture from within the sealed airspace of an insulating glass (IG) unit, also known as a multi paned glass window.

Energy Star

Energy Star is a joint program of the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Energy helping consumers to save money and protect the environment through energy efficient products and practices. In 1992 EPA introduced ENERGY STAR as a voluntary labeling program designed to identify and promote energy-efficient products to reduce greenhouse gas emissions. Energy Star windows promote lower heating bills and energy efficiency.

Insulating Glass

When multiple glass panes or lites are assembled together into unites, they are commonly called insulating glass units. These window units use the thermal and acoustic insulating properties of a gas contained in the airspace formed by the unit. They are able to provide good insulation without having to give up visibility. Most insulated glass units are double glazed, but more and more are becoming triple glazed as time goes by.

Low E Glass

Low e is a common term used to refer to glass which has low emissivity due to a film or metallic coating on the glass or suspended between the two lights of glass to restrict the passage of radiant heat.


Muntins are strips of wood or metal separating and holding panes of glass in a window. Muntins are also sometimes called glazing bars, muntin bars, or sash bars. Muntins can be found in windows, doors, and furniture. They create a grid system used to divide small panes of glass, known as lites, into a single window sash or casement.

Picture Window

A picture window is a large window with no moveable sash. Picture windows are normally much longer than they are wide to provide a panoramic view outside.

Rough Opening

The framed in opening, slightly larger than the actual window or door it is built in to house, that replaces wall studs to support the structure and accommodate a window or door during window replacement.