Useful Window Terminology

Argon. An nontoxic gas that many window manufacturers use instead of air between panes of insulated glass. Argon has higher insulation value than air and won't degrade the low-e coating like air will.

Basement Window. When replacing a basement window, the harsher, damper basement environment should be considered. Use basement  replacement windows that are made of inorganic materials, such as all-vinyl windows.

Design Pressure Rating. DPR refers to the amount of wind pressure (in pounds per sq. ft.) a window or door will withstand when closed and locked. It also establishes performance factors such as water penetration, air infiltration and operational force. Higher DPR numbers mean better performance, but these details are primarily of concern in coastal areas, where building codes (and homeowner's insurance policies) establish minimum DPRs.

Impact glass. Glass that is strengthened to resist object impact and forced entry, usually by sandwiching clear plastic or polyester between two sheets of glass. If impact glass does break, glass fragments tend to adhere to the plastic, helping preserve the integrity of the building envelope.

Insulated glass. Glazing manufactured with insulating air space(s) between two (double-pane) or three (triple-pane) layers of glass.

Jamb. A part of the window frame. A rectangular window will have a top or head jamb that joins two side jambs. The side jambs join the sill, which forms the bottom of the window frame.

Low-e. This coating technology improves the energy efficiency of windows, skylights and doors that contain glass by reflecting heat back into the living space during the heating season and by reducing interior heat gain during hot weather. Click for more details on low-e window technology.

NFRC. The National Fenestration Rating Council administers an independent rating and labeling system for the energy performance of windows, doors and skylights. Visit the website for more details: www.nfrc.org.

Replacement window. A window unit that is designed to fit inside the jambs or frame of an existing window after the old sash has been removed. Read more about energy efficient replacement windows.

Rough Opening. The framed opening where a window, door or skylight unit will be installed.

Sash. The part or parts of the window that contain the glass, including the exterior frame (made up of vertical stiles and horizontal rails) and smaller interior divider pieces (called muntins).

Shim space. The narrow clearance space between the window frame and the window's rough opening. To improve energy efficiency, the shim space should be sealed and filled with fiberglass or low-expanding foam insulation. It's difficult to tell if the shim space has been sealed and insulated because it's covered by interior and exterior casing.

Solar Heat Gain Coefficient. SHGC measures how well a window blocks solar heat. SHGC is expressed as a number between 0 and 1, and lower numbers indicate better blocking ability. Limiting solar heat gain is desirable in hot climates, but not in cold ones.

Tempered glass. Glass that has been heat-treated to improve its strength. Tempered glass is sometimes called safety glass because when it breaks, it creates pebble-sized particles rather than sharp shards and slivers.

U-factor. The inverse of R-value, U-factor is a measure of heat conduction through a material. The lower the U-value, the better, in terms of energy efficiency. Windows are given a U-value rating by the NFRC.

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