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Window Glossary


Choosing a replacement window type and style is a big decision. A&B Improvements Ltd. has assembled a list of words and terms you may hear when replacing your home’s windows.
Air Infiltration. Amount of air that passes between a window sash and frame. In windows it is measured in terms of cubic feet or air per minute, per square foot of area. The lower the number, the less air the window lets pass through.
Aluminum-clad window. Window with wood construction covered with aluminum sheet. Has a factory-applied finish to protect from weather and solar degradation.
Apron. Horizontal trim board under a window stool.
Argon Gas. An odorless, colorless, tasteless, nontoxic gas which is six times denser than air. It is used to insulate better and reduce heat transfer.
Awning window. Hinged at the top and swinging out at the bottom, operated by a cranking mechanism. Usually rectangular, and wider than they are long.
Bay window. A series of usually three windows assembled in a polygon shape that projects outward from the side of a building, normally extending to the ground.
Bow window. Projects from the side of a building, normally with opaque elements extending to the ground, like a bay window, but in this case usually composed of a series of five window units assembled in an arc, rather than a polygon.
Casement window. A window unit hinged at the side and swinging outward, often operated by a cranking mechanism. Usually more energy-efficient because they have fewer moving parts and seal tighter than double-hung windows. In-swinging are French in origin while out-swinging are from England.
Center-hung sash. A sash that pivots on pins in the middle of the sash stiles and sides of the window frame to allow access for cleaning from the inside.
Check rail (also meeting and lock rail). Horizontal members of a double-hung window which come together.
Condensation Resistance Factor. A measure of the effectiveness of a window or glazing system to reduce the potential for condensation. The higher the condensation resistance factor, the more energy efficient the window and glazing system, generally.
Desiccant. Material used in insulating glass to absorb water vapor which causes fogging.
Diffusing glass. Glass with an irregular surface for scattering light; used for privacy or to reduce glare. Also called “obscure” glass.
Divided Lites. Division of glazing by the use of muntin bars.
Dormer window. Window in a wall that either projects from a sloping roof, or is recessed (inset dormer) into the roof, or a combination of both.
Double hung window. Has two vertically moving sashes, each opening and closing a different part of the window. A single hung window has only one moving sash, usually the bottom half of the window.
Double-hung Sash Window. A design invented in 17th-century Holland comprising two panels that slide up and down in vertical grooves with the aid of cords or chains concealed in the window jamb.
Double-strength Glass. Glass with a thickness of approximately 1/8".
Edge of Glass U- and R-values. The U- and R-values measured from the frame of the glass to 2-1/2" from the frame.
Fixed light (also fixed sash). Window which is non-operative (doesn't open).
Flat Casing. The exterior trim applied to the side jambs and header on wood and clad units.
Frame. The stationary portion of a window installed into the rough opening in a wall, enclosing the sash (operating and/or stationary).
French Window. A casement window that extends from the ceiling to the floor and features glass panes that run its entire height. Introduced at Versailles in the seventeenth century.  
Gas-fill. An inert gas, usually argon, sealed between the panes of glass in a window instead of air. The gas is a far better insulator than just air, thus further increasing the thermal value of a window.
Head casing. Top or upper member of any element or structure. In windows, it refers to the top of the frame.
Header (also lintel; beam). Supporting member or beam above window opening which transfers building weight above to the supporting wall structure on each side of the window. The term header is generally in reference to a wood beam, whereas "Lintel" often refers to a steel or masonry beam.
Header. The horizontal top of the frame.
Horizontal sliding window (also horizontal slider). Windows which slide horizontally.
Insulated glass. Two or more panes of glass separated by insulation at the edges and air (or a more insulating gas) in the middle to provide greater thermal efficiency to a window.
Jalousie windows (also louvered windows). A window composed of overlapping narrow glass, metal, or wooden louvers, operated with a crank handle for adjusting the louver angles.
Jamb. A side jamb is the vertical molding of a window; the head jamb is the horizontal molding at the top. The bottom framing member is referred to as a sill.
Krypton Gas. An inert, odorless, colorless, tasteless, nontoxic gas that is about 12 times denser than air. It is used to replace air between the glass panes to reduce heat transfer and deter convection. Used when a higher performance is desired than that produced with Argon gas.
Laminated Glass. Two or more pieces of glass bonded together over a plastic interlayer. (See impact glass.)
Light or Lite. A window; a pane of glass within a window. Double-hung windows are designated by the number of lights in their upper and lower sashes, as in “6-over-6.”
Light-to-Solar-Gain Ratio (LSG). Ratio of visible transmittance to solar heat gain coefficient.
Lintel. Horizontal member (wood, steel, or stone) over a window opening to support the weight of the wall above. A header.
Low E (Emissivity) Glass. Glass with a transparent, usually multi layer, metallic oxide coating applied onto or into a glass surface. The coating allows short-wave energy to pass through but reflects long-wave infrared energy which improves the U-factor. Properly placed in a window, a “high-solar-gain” low-e coating can cut the loss of heat during the winter. A “low-solar-gain” low-e coating cuts the admission of solar radiant heat during the summer by reflecting this heat back to its source, thus providing year-round savings by lowering utility bills.
Mullions. Vertical members between window units. They are sometimes confused with muntins, which are secondary framing members that hold multiple panes of glass in the sash. Other parts of the sash include stiles (the outside vertical members) and rails (the top and bottom horizontal members).
Muntin (also sash bar; window bar; glazing bar). A secondary framing member (horizontal, vertical, slanted) to hold the window panes in the sash. Often confused with "mullion".
NFRC label. NFRC stands for the National Fenestration Rating Council. This non-profit organization sets energy certification and labeling standards for windows in the United States.
Obscure Glass. Glass that has been made translucent instead of transparent, usually by its diffuse optical quality.
Palladian Window. A window with three openings, the central one arched and taller and wider than the others. Popularized by the Renaissance Italian architect Andrea Palladio.
Pivot window units. Window units in which the sash hardware is located near the midpoint of the stile or rail to permit sash rotation.
R-Value. A measure of insulation value. The higher the R-value, the better insulated are windows, walls, and ceilings. R-Value is the reciprocal of the U-factor.
Rough Opening. The hole in the wall where a window or door unit will be installed. Openings are larger than unit size to allow room for insulation and shimming the unit square.
Sash. The entire window, including the glass and the surrounding pieces that hold it together. The sash fits into a frame that is actually tied into the surrounding wall and holds the sash in place.
Sill. The horizontal bottom of the frame.
Single-strength Glass. Glass with a thickness of approximately 3/32".
Slider Window. A window in which the sash moves horizontally. Sliders are available in a 2- or 3-lite configuration, with the 3-lite having operable end vents.
Solar Heat Gain Coefficient (SHGC). Ratio of the solar heat gained through a fenestration to the solar radiant heat incident on the fenestration. Solar gain per unit area of window divided by average solar irradiance incident on the window.
Tempered glass. Special heat-treated, high-strength safety glass which shatters into pebble-sized particles and not in slivers.
Total Unit U- and R-values. The U- and R-values of the window calculated from the average of the center of glass, edge of glass and frame U- and R-values.
U-factor. A measure of the rate of heat conduction through a surface—a wall, ceiling, or window. The lower the U-factor, the better job a window does in keeping out heat and cold by conduction.  
Visible Light Transmittance or VT. The fraction of light that is transmitted through glass in the visible spectrum (380 to 720 nanometers). The higher the number the higher the fraction of visible light incident on the window that is transmitted.