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Window Glossary

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.

Boxed mullion. Hollow mullion between two double-hung windows to hold sash weights.

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 of Glass U- and R-values. Glazing system U- and R-values measured from the center of the glass to 2-1/2" from the frame.

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.

Clerestory. A window in a gable or in an outside wall of a room or building that rises above an adjoining roof.

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 gazing 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.

Extruded aluminum. Developed by forcing aluminum through a die to form a specific shape.

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 Door. 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. Also called French windows.

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.

I.G. Unit (Insulating Glass Unit). Two or more lites of glass separated by a spacer and hermetically sealed at the glass edges.

Impact glass. A laminated glazing product, produced by bonding plastic vinyl layers (sometimes in combination with polyester film) between two pains of glass into a single sheet. Laminated glass looks like ordinary glass, but protects against object impact, forced entry, and (to some extent) noise. Tests show that specially designed laminated glass products resist wind-borne debris impact better than unprotected glass. When broken, the glass fragments tend to remain integral, adhering to the plastic interlayer, helping to preserve the integrity of the building envelope.

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.

Insulating glass. Double- or triple-glazing with an enclosed, dehydrated, and hermetically sealed gas space between the panes.

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 which 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 (also 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.

Low-e. Shorthand for “low emissivity,” a measurement of how much heat a material radiates. Window manufacturers use invisible low-e coatings—made of metal oxide or even semiconductor material—on window panes to reduce the amount of heat radiating from the glass.

Emissivity. The ratio of heat radiation emission from a surface to the maximum possible emission. A low-emissivity coating is also a high reflectivity one. 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.

Operable window. Window which can be opened for ventilation.

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 the 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.

Shading coefficient. The solar heat gain coefficient of a glazing system divided by the solar heat gain coefficient for a clear single pane of the same size. Alternatively: The shading coefficient (SC) is the ratio of solar heat gain through a glazing system under a specific condition of interest to solar gain through a single pane of the reference glass (double -strength sheet glass with 0.86 transmittance, 0.08 reflectance, and 0.06 absorptance at normal incidence) under the same conditions. This coefficient can vary with changing solar spectral distributions, especially if the glazing system it applies to is strongly spectrally selective. It can also vary with direction of incidence. ASHRAE now recommends calculating solar heat gain using a spectral-based method for calculation of the solar heat gain coefficient, giving the fraction of incident beam solar radiation transmitted by a window for the specific solar spectrum incident on the window and for the specific direction of incidence.

SC = (solar heat gain of the glazing system) ÷ (solar heat gain of standard reference glass)

For direct solar beam radiation having a standard spectral distribution incident normally on the window (perpendicular to the glass), the shading coefficient is the solar heat gain coefficient divided by 0.87 and has a value of 1.0 for clear single pane glass with the above specifications and a SHGC value of 0.87, but this is not exactly true for other angles of incidence and for non-standard solar spectral distributions.

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.

Solar Heat Gain Factor. The solar heat gain through a sunlit double strength sheet glass at any specified orientation and any time of day in units of watts per square meter. This value varies with the direction of incidence and, in general, with the spectral distribution of the incident radiation. The maximum (clear sky condition) value of this quantity is often tabulated for different latitudes and different times of day. Before the introduction of strongly spectrally selective glazing systems, one could multiply the solar heat gain factor by the shading coefficient of the glazing system to determine the solar heat gain through the glazing system alone, excluding opaque framing elements. With modern spectrally selective glazings, it is now recommended to determine solar heat gain through a spectral calculation of the solar heat gain coefficient, which gives the fraction of incident beam solar radiation transmitted by a window for the specific solar spectrum incident on the window and for the specific direction of incidence.

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. In the SI system has units of Joules per square meter and per second and per degree Celsius, J m-2 s-1 C-1. In the IP system has units of Btus per square foot and per hour and per degree Fahrenheit, Btu ft-2 hr-1 F-1.

Visible Light Transmittance, VT 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.

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