Thursday, Sep 27, 2012

From the inside, looking out
Window terminology explained
By MARK SALERNO, CMHC, Special to QMI Agency

Windows play a key role in the overall performance of your home. In addition to enhancing the aesthetic beauty of your house, windows can provide fresh air and ventilation, allow daylight to brighten interior spaces and keep out harsh outdoor elements such as wind, rain and snow.

Homeowners may not be familiar with window terminology, given that they are not a frequent purchase item (since they last 20-25 years) and given that the technologies and performance standards for windows have changed significantly in recent years.

The basic components of a window include the frame, glazing and, in the case of operable (or openable) windows, the sash. Frame, glazing and sash design and construction are all important for both energy efficiency and appearance.


Materials commonly used for window frames include wood, clad wood, aluminum, vinyl and fiberglass. Each has advantages and disadvantages. For example, wood frames have high insulating properties and can be painted or stained any colour and can be less expensive than frames made of other materials. However, they require ongoing maintenance to keep up their appearance and prevent moisture damage.

Vinyl is another frame material that is fairly common in the Canadian residential marketplace. Depending on the size of the window, extruded vinyl frames incorporate internal air cavities, which may or may not be reinforced with another material such as wood or metal. Vinyl frames are very durable and have low maintenance needs. Vinyl windows provide good or excellent insulating properties, particularly if the cavities in the sash and frame are insulated.


Window glazing (or glass) is typically composed of two sheets of glass separated and sealed with a spacer bar. The glazing in a window can be a single unit or several panes divided by a mullion. A mullion is a secondary frame that holds the window panes in the sash.

For greater energy efficiency, select windows with low-emissivity (low-E) glazing. Low-E consists of a thin layer of metal oxide applied to the exterior face of the interior glazing in a double-glazed window. This coating allows sunlight to pass through but blocks heat from escaping. Low-E glazing also filters out the sun’s ultraviolet (UV) rays, which can fade furnishings.

The coating can reduce wintertime condensation on windows by keeping the indoor surface of the glass and frame warmer. Argon gas in a sealed window unit will further improve the energy performance. A double-glazed low-E and argon-filled window provides similar insulation value approaching that of a triple-glazed unit with air only, but it costs and weighs less.


Windows come either fixed or operable (openable). Operable units are available as sliding, single-hung, double-hung, awning, hopper or casement units.

Operable windows have a sash, which is a unit assembly of stiles and rails for holding the glass that moves when the window is opened. Sashes can be made of wood, vinyl, metal or fiberglass and should have weatherstripping to achieve a tight seal with the frame when the window is closed.

Be sure to look at the type of weatherstripping used for operable windows. It is generally the most vulnerable component in an operable window, as it receives the most wear and tear. It is designed to prevent both air leakage and water infiltration. The better the weatherstripping, the better the window performance.

To help you find the right windows for your home, Canada Mortgage and Housing Corporation (CMHC) has a free About Your House fact sheet called “Understanding Window Terminology.” To order your copy, visit us online or call our toll free number: 1-800-668-2642.

Mark Salerno is district manager for the Greater Toronto Area at the Canada Mortgage and Housing Corporation.

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