Technical Glossary of Window Terms
Air infiltration. The amount of air leaking in and out of a building through cracks in walls, windows and doors.
Annealed glass. Standard sheet of float glass which has not been heat-treated.
Argon. An inert, nontoxic gas used in insulating glass units to reduce heat transfer.
Awning. Window similar to a casement except the sash is hinged at the top and always swings out.
Balance. A mechanical device (normally spring-loaded) used in single- and double-hung windows as a means of counterbalancing the weight of the sash during opening and closing.
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 is normally fixed, with the end panels operable as single-hung or casement windows.
Bottom rail. The bottom horizontal member of a window sash.
Bow window. A rounded bay window that projects from the wall in an arc shape, commonly consisting of five sashes.
Brick molding. A standard milled wood trim piece that covers the gap between the window frame and masonry.
Btu (B.T.U.). An abbreviation for British Thermal Unit, the heat required to increase the temperature of one pound of water one degree Fahrenheit.
Cam Lock, A single point locking mechanism that uses a cam action to lock and pull the window sash against the frame to form a tighter seal. Large windows may have more than one cam lock.
Casement. A window sash that swings open on side hinges.
Casing. Exposed molding or framing around a window or door, on either the inside or outside, to cover the space between the window frame or jamb and the wall.
Caulking. A mastic compound for filling joints and sealing cracks to prevent leakage of water and air, commonly made of silicone, bituminous, acrylic, or rubber-based material.
Center Hinge Door, A 2 panel patio door system with the operable panel hinged from the center of the door.
Check rail. The bottom horizontal member of the upper sash and the top horizontal member of the lower sash which meet at the middle of a double-hung window.
Clerestory. A window in the upper part of a lofty room that admits light to the center of the room.
Condensation. The deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as a cold window glass or frame that is exposed to humid indoor air.
Conduction. Heat transfer through a solid material by contact of one molecule to the next. Heat flows from a higher-temperature area to a lower-temperature one.
Convection. A heat transfer process involving motion in a fluid (such as air) caused by the difference in density of the fluid and the action of gravity. Convection affects heat transfer from the glass surface to room air, and between two panes of glass.
Daylight Opening, The visible area of glass in a window or door.
Dewpoint. The temperature at which water vapor in air will condense at a given state of humidity and pressure.
Divided Lite, A window opening divided into smaller sections by a grid system on the interior or exterior of the glass, or between the glass panes.
Double glazing. In general, two thicknesses of glass separated by an air space within an opening to improve insulation against heat transfer and/or sound transmission. In factory-made double glazing units, the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties.
Double-hung window. A window consisting of two sashes operating in a rectangular frame, in which both the upper and lower halves can be slid up and down. A counterbalance mechanism usually holds the sash in place.
Double-strength glass. Sheet glass between 0.115" and 0.133" (33.38 mm) thick.
Emergency exit window. Fire escape window (egress window) large enough for a person to climb out. In U.S. building codes, each bedroom must be provided with an exit window. The exact width, area, and height from the floor are specified in the building codes.
Extrusion. The process of producing vinyl or aluminum shapes by forcing heated material through an orifice in a die. Also, any item made by this process.
Eyebrow windows. Windows with a shallow arched head with the arch using only a section of a full ½ circle radius. These are different then either an ellipse or a ½ circle. Can be made with or without extended legs.
Fixed panel. An inoperable panel of a sliding glass door or slider window.
Fixed window. A window with no operating sashes.
Flashing. Sheet metal or other material applied to seal and protect the joints formed by different materials or surfaces.
Float glass. Glass formed by a process of floating the material on a bed of molten metal. It produces a high-optical-quality glass with parallel surfaces, without polishing and grinding.
Fogging. A deposit of contamination left on the inside surface of a sealed insulating glass unit due to extremes of temperatures or failed seals.
Frame. The fixed frame of a window which holds the sash or casement as well as hardware.
French Door, A style of door in which 2 panels open to provide a clear opening almost as wide as the door frame.
Gas fill. A gas other than air, usually argon or krypton, placed between window or skylight glazing panes to reduce the U-factor by suppressing conduction and convection.
Geometric Window, Windows of various shapes including: triangles, trapezoids, octagons, and pentagons.
Glass. An inorganic transparent material composed of silica (sand), soda (sodium carbonate), and lime (calcium carbonate) with small quantities of alumina, boric, or magnesia oxides.
Glazing. The glass or plastic panes in a window, door, or skylight.
Glazing bead. A molding or stop around the inside of a window frame to hold the glass in place.
Header. The upper horizontal member of a window frame. Also called head.
Heat-absorbing glass. Window glass containing chemicals (with gray, bronze, or blue-green tint) which absorb light and heat radiation, and reduce glare and brightness. See also Tinted glass.
Heat gain. The transfer of heat from outside to inside by means of conduction, convection, and radiation through all surfaces of a house.
Heat loss. The transfer of heat from inside to outside by means of conduction, convection, and radiation through all surfaces of a house.
Hinged windows. Windows (casement, awning, and hopper) with an operating sash that has hinges on one side. See also Projected window.
Horizontal slider. A window with a movable panel that slides horizontally.
Insulating glass. Two or more pieces of glass spaced apart and hermetically sealed to form a single glazed unit with one or more air spaces in between. Also called double glazing.
Insulating value. See U-factor.
Insulation. Construction materials used for protection from noise, heat, cold or fire.
Interlock. An upright frame member of a panel in a sliding glass door which engages with a corresponding member in an adjacent panel when the door is closed. Also called interlocking stile. Also the check rails of single and double hung windows.
Jalousie. Window made up of horizontally-mounted louvered glass slats that abut each other tightly when closed and rotate outward when cranked open.
Jamb. A vertical member at the side of a window frame, or the horizontal member at the top of the window frame, as in head jamb.
Level, A condition that exists when a surface is exactly horizontal.
Lift. Handle for raising the lower sash in a double-hung window. Also called sash lift.
Light. A window; a pane of glass within a window. Double-hung windows are designated by the number of lights in upper and lower sash, as in six-over-six. Also spelled informally lite.
Lintel. A horizontal member above a window or door opening that supports the structure above.
Long-wave infrared radiation. Invisible radiation, beyond red light on the electromagnetic spectrum (above 3.5 micro meters), emitted by warm surfaces such as a body at room temperature radiating to a cold window surface.
Low-conductance spacers. An assembly of materials designed to reduce heat transfer at the edge of an insulating window. Spacers are placed between the panes of glass in a double- or triple-glazed window.
Low-emittance (Low-E) coating. Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. A typical type of low-E coating is transparent to the solar spectrum (visible light and short-wave infrared radiation) and reflective of long-wave infrared radiation.
Meeting rail. The part of a sliding glass door, a sliding window, or a hung window where two panels meet and create a weather barrier.
Mulled Unit, 2 or more windows structurally joined together to fit into one rough opening
Mullion. A major structural vertical or horizontal member between window units or sliding glass doors.
Nailing fin. An integral extension of a window or patio door frame which generally laps over the conventional stud construction and through which nails are driven to secure the frame in place.
NFRC. National Fenestration Rating Council.
Obscure glass. Any textured glass (frosted, etched, fluted, ground, etc.) used for privacy, light diffusion, or decorative effects.
Operable window. Window that can be opened for ventilation.
Operator. Crank-operated device for opening and closing casement or jalousie windows.
Pane. One of the compartments of a door or window consisting of a single sheet of glass in a frame; also, a sheet of glass.
Panel. A major component of a sliding glass door, consisting of a light of glass in a frame installed within the main (or outer) frame of the door. A panel may be sliding or fixed.
Picture window. A large, fixed window framed so that it is usually, but not always, longer horizontally than vertically to provide a panoramic view.
Pivot Bars (pins), The component on the bottom corner of an operable sash in a single or double hung window that allows the sash to be tilted in.
Plate glass. A rolled, ground, and polished product with true flat parallel plane surfaces affording excellent vision. It has been replaced by float glass.
Plumb, A condition that exists when a surface is exactly vertical.
Polyvinylchloride (PVC). An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum windows.
R-value. A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value.
Radiation. The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches the earth by radiation, and a person's body can lose heat to a cold window or skylight surface in a similar way.
Rail. Horizontal member of a window sash.
Relative humidity. The percentage of moisture in the air in relationship to the amount of moisture the air could hold at that given temperature. At 100 percent relative humidity, moisture condenses and falls as rain.
Retrofitting. Adding or replacing items on existing buildings. Typical retrofit products are replacement doors and windows, insulation, storm windows, caulking, weatherstripping, vents, landscaping.
Reveal, The space between the window sash or door panel and the frame.
Rough opening. The opening in a wall into which a door or window is to be installed.
Safety glass. A strengthened or reinforced glass that is less subject to breakage or splintering.
Sash. The portion of a window that includes the glass and the framing sections directly attached to the glass, not to be confused with the complete frame into which the sash sections are fitted.
Screen. Woven mesh of metal, plastic, or fiberglass stretched over a window opening to permit air to pass through, but not insects.
Sealant. A compressible plastic material used to seal any opening or junction of two parts, such as between the glass and a sash, commonly made of silicone, butyl tape, or polysulfide.
Shading coefficient (SC). A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8-inch clear, double- strength, single glass. It is being phased out in favor of the solar heat gain coefficient, and is approximately equal to the SHGC multiplied by 1.15. It is expressed as a number without units between 0 and 1. The lower a window's solar heat gain coefficient or shading coefficient, the less solar heat it transmits, and the greater is its shading ability.
Sheet glass. A transparent, flat glass found in older windows, now largely replaced by float glass.
Short-wave infrared radiation. Invisible radiation, just beyond red light on the electromagnetic spectrum (between 0.7 and 2.5 microns), emitted by hot surfaces and included in solar radiation.
Sill. The lowest horizontal member in a door, window, or sash frame.
Sill track. The track provided at the sill of a sliding glass door. Also, the sill member incorporating such a track.
Single glazing. Single thickness of glass in a window or door.
Single-hung window. A window consisting of two sashes of glass, the top one stationary and the bottom movable.
Single-strength glass. Glass with thickness between 0.085" and 0.100" (2.162.57 mm).
Skylight (operable or pivot). A roof window that gives light and ventilation.
Sliding glass door. A door fitted with one or more panels that move horizontally on a track and/or in grooves. Moving action is usually of rolling type (rather than sliding type). Also called gliding door, rolling glass door, and patio sliding door.
Sliding window. A window fitted with one or more sashes opening by sliding horizontally or vertically in grooves provided by frame members. Vertical sliders may be single- or double-hung.
Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.
Solar radiation. The total radiant energy from the sun, including ultraviolet and infrared wave lengths as well as visible light.
Solar spectrum. The intensity variation of sunlight across its spectral range.
Spectrally selective coating. A coated or tinted glazing with optical properties that are transparent to some wavelengths of energy and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared radiation.
Square, A condition that exists when 2 surfaces are perpendicular.
Stile. The upright or vertical edges of a door, window, or screen.
Tempered glass. Treated glass that is strengthened by reheating it to just below the melting point and then suddenly cooling it. When shattered, it breaks into small pieces. Approximately five times stronger than standard annealed glass; is required as safety glazing in patio doors, entrance doors, side lights, and other hazardous locations. It cannot be recut after tempering.
Thermal break. An element of low conductance placed between elements of higher conductance to reduce the flow of heat. Often used in aluminum windows.
Threshold. The member that lies at the bottom of a sliding glass door or swinging door; the sill of a doorway.
Tilt window. A single- or double-hung window whose operable sash can be tilted into the room for interior washability.
Tinted glass. Glass colored by incorporation of a mineral admixture. Any tinting reduces both visual and radiant transmittance.
Transmittance. The percentage of radiation that can pass through glazing. Transmittance can be defined for different types of light or energy, e.g., visible light transmittance, UV transmittance, or total solar energy transmittance.
Transom, A window, usually rectangular, placed over a door or window.
U-factor (U-value). A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-°F (W/sq m-°C). Values are normally given for NFRC/ASHRAE winter conditions of 0° F (18° C) outdoor temperature, 70° F (21° C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value.
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.
Vent. The movable framework or sash in a glazed window that is hinged or pivoted to swing open.
Vinyl. Polyvinyl chloride material, which can be both rigid or flexible, used for window frames.
Visible light. The portion of the electromagnetic spectrum that produces light that can be seen. Wavelengths range from 380 to 720 nanometers.
Visible transmittance (VT). The percentage or fraction of the visible spectrum (380 to 720 nanometers) weighted by the sensitivity of the eye, that is transmitted through the glazing.
Warm-edge technology. The use of low-conductance spacers to reduce heat transfer near the edge of insulated glazing.
Weatherstripping. A strip of resilient material for covering the joint between the window sash and frame in order to reduce air leaks and prevent water from entering the structure.
Weep hole. A small opening in a wall or window sill member through which water may drain to the building exterior.
Window. A glazed opening in an external wall of a building; an entire unit consisting of a frame sash and glazing, and any operable elements.
Window hardware. Various devices and mechanisms for the window including catches, fasteners and locks, hinges, pivots, lifts and pulls, pulleys and sash weights, sash balances, and stays.