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Window Replacement Glossary of Terms


Double Hung window: A window consisting of two sashes operating in a rectangular frame, in which both the upper and lower halves can be slide up and down. A counterbalance mechanism usually holds the sash in place.

Grids: the decorative patterns on a window pane locked between the panes of glass can be colonial, craftsman, or prairie style.

Sliding window: A window fitted with one or more sashes opening by sliding horizontally in tracks provided by frame members.

Bow window: A rounded bay window that projects from the wall in an arc shape, commonly consisting of five sashes.

Bay window: An arrangement of three or more individual window units, attached so as to project from the building at various angles. In a three-unit bay, the center section in normally fixed, with panels operable as double-hung or casement windows

Casement window: A window containing one or more sash hinged to open from side, that project outward or inward from the plane of the window in a vertical plane.

Picture window: A large, fixed window framed so that it is usually, but not always, longer horizontally than vertically to provide a panoramic view

Awning window: A window, with one or more sash that rotate about its top hinge and projects outward.

Sash: The portion of a fenestration assembly that is installed in a frame and includes the glazing, stiles and rails, a sash may be operable of fixed.

U-Value: is a measure of heat transmission through a wall or window. The lower the U-Factor the better the insulation value.

Low-emittance (low-E) coating: Microscopically thin, virtually invisible, metallic oxide coating on the glass surface primarily to reduce the U-factor by suppressing radiative heat flow. The most amazing characteristic of Low-E technology is its “spectral selectivity.  Acting as a filter, it differentiates between the longer wave lengths constituting the heat portion of the electromagnetic spectrum and the shorter wavelengths that reach us in the form of visible light, and thus provides for the best of both worlds: maximum illumination with minimal warming effect.

Solar Heat Gain Coefficient (SHGC): indicates the percentage of solar heat entering a window (sash, frame and glass) relative to a hole of the same size in the wall. Express as a number between 0 and 1, the lower a window’s SHGC, the less solar heat it transmits.

Visible Light Transmittance (VT): The percentage of the visible spectrum weighted by the sensitivity of the eye, that is transmitted through the glazing. Numbers range from 0 to 1, with the higher numbers allowing in more light.

Ultraviolet light (UV): The invisible rays of the spectrum that are outside of the visible spectrum at its short-wavelength violet end.  Ultraviolet rays are found in everyday sunlight and can cause fading of paint finishes, carpets, and fabrics.

Argon: An inert, nontoxic gas used in insulating glass units to reduce heat transfer.

Fusion weld: The process of connecting the ends of vinyl extrusions together by heating them to a certain temperature and compressing them together. The result of this process is much stronger to chemical welding or screws.

Tempered glass: (also called “safety” glass) is designed to crumble into small, rounded pieces when broken, rather than jagged shards.

Obscure glass: Any textured glass (frosted, etched, fluted, etc.) used for privacy, light diffusion, or decorative effects.

Laminated glass: Two or more sheets of glass with an inner layer of transparent plastic to which the glass adheres if broken.  Used for safety glazing and sound reduction.

Sound Transmission Class (STC): The sound transmission loss rating of a material over a selected range of sound frequencies.  The higher the number the less sound transmitted.

National Fenestration Rating Council (NFRC): is a non-profit organization that administers the only uniform, independent rating and labeling system for the energy performance of windows and doors. For more information visit their web site at

ENERGY STAR®: ENERGY STAR® is a voluntary partnership among the U.S. Department of Energy, the U.S. Environmental Protection Agency, product manufacturers, local utilities, and retailers. Partners help promote efficient products by labeling with the ENERGY STAR® logo and educating consumers about the benefits of energy efficiency. By choosing ENERGY STAR®-labeled products, you'll keep your utility bills down, and help the environment at the same time. For more information visit



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