Air chambers. Small spaces shaped like a honeycomb located within the sash and frame that help insulate and strengthen a window.

Air gap (also air space). The space in the cavity between two panes of glass in an insulated glass unit.

Air leakage. The flow of air which passes through cracks in closed and locked fenestration products.

Argon gas (argon filled). An inert, nontoxic gas placed between glass panes in insulated glass units in order to improve the insulating value of sealed glass units.

Awning window. A window with a sash hinged at the top, which projects outward from the plane of the frame. See also Projected or projecting windows.

Bay window. Window made up of three windows joined at 30- or 45-degree angles that protrudes from the exterior wall of a house.

Bow window. Similar to a Bay window with the difference being in the number of windows. Bow windows are an angled combination of windows in 3, 4, 5 or 6 lite configurations. The windows are attached at 10 or 15 degree angles creating a semi-circular look.

Brickmould . Exterior casing around a door or window that cover the jambs.

Btu. An abbreviation for British thermal unit – a standard measure of the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit.

Building envelope. The outer elements of a building, both above and below ground, that divide the external and internal environments.

Camlock and keeper. Mechanisms to keep the window sashes together when locked.

Casement window. A window containing one or more side-hinged sashes hinged that project outward or inward from the plane of the window in the vertical plane. A conventional casement window in North America swings outward, while in Europe it swings inward.

Caulk, caulking compound. 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 of glass. U- and R-values are measured from the center of the glass to 2-1/2″ from the frame.

Coating. A thin layer applied to the surface of a glass in either a chemical deposition technology (i.e., vapor, liquid, etc.) or a vacuum sputtering process. After application it is converted to a solid protective, decorative, or functional adherent film.

Color of transmitted light. The human eye’s and brain’s subjective interpretation of the spectral distribution of transmitted visible radiation. Transmitted light is said to be colorless (white) if it matches the spectrum of the external incident light, while any imparted color is due to the subtraction of the complementary wavelengths by absorption or reflection of those wavelengths by the glazing system.

Composite frame. A frame consisting of two or more materials – for example, an interior wood element with an exterior fiberglass element.

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 or outdoor air.

Conduction. Transfer of heat through a material via molecular contact; heat flows from a higher-temperature area to a lower-temperature one.

Conductivity, thermal. The time rate of steady state heat flow through a unit area of homogenous material induced by a unit temperature gradient in a direction perpendicular to that unit area.

Convection. A heat transfer process involving motion of a fluid (such as air) caused by either the difference in density of the fluid and the action of gravity (natural convection) or by mechanical forces such as blowers, fans, etc. (forced convection). Convection affects heat transfer from the surface to air, whether it is for enclosed spaces (like insulated glazing unit cavity) or open spaces (like indoor glass surface to room air).

Cool daylight glazing. Spectrally selective glazing that employs tinting and/or surface coatings to achieve a visible transmittance that exceeds the solar heat gain coefficient (total solar energy transmittance). See also Light-to-solar-gain ratio.

Daylight distribution. The distribution of illuminance due to sunlight and sky light within a room, generally measured on a horizontal plane at typical work plane height (0.8 m, or 2.5 feet above the floor). Units: lux (lx=lm/m2) or foot-candles (fc) where 1 fc=10.764 lx.

Daylight factor. The ratio, in percent, of workplace illuminance (at a given point) to the outdoor illuminance on a horizontal plane. It is only evaluated under cloudy sky conditions (no direct solar beam).

Desiccant. An extremely porous crystalline substance (hygroscopic or water-absorbing) used in granulated or bulk form inside the spacer of an insulating glass unit in order to keep the gas(es) within the sealed space dry and prevent condensation and fogging.

Dew point (temperature). The temperature at which water vapor in air will condense at a given state of humidity and pressure.

Distortion. The optical effect due to the variation of sheet glass thickness.

Divided light. A window with a number of small panes of glass separated and held in place by muntins.

Double glazing. In general, any use of two layers 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. It also allows for between-glass shading options such as muntins, blinds, and pleated shades. See also Insulating Glass, Dual-seal Unit.

Double glazing unit. Two panes of glass separated by a permanently sealed cavity.

Double-hung (window). A window consisting of a pair of vertical sliding sashes with either sash opening independently of the other. It can use either a counterbalance mechanism to hold the sash in place or spring-loaded side bars that keep sash in place by friction. See also Single-hung window.

Drip. A projecting fin or a groove at the outer edge of a sill, soffit, or other projecting member in a wall, designed to interrupt the flow of water downward over the wall or inward across the soffit.

Egress window. A window providing egress, as defined in applicable building codes. Also referred to as emergency exit window, escape window, and fire-escape window.

Effective thermal conductivity. The combined effects of conduction, convection, and radiation in fluid-filled (gas-filled) enclosures and cavities, converted into an apparent or effective conductivity of a solid.

Emissivity. The relative ability of a surface to reflect or emit heat by radiation. Emissivity factors range from 0 to 1; the lower the emissivity, the less heat is emitted through a window system. Emissivity is typically measured by U-factor (or its inverse, R-value).

Etch. To attack the surface of glass with hydrofluoric acid or other agents, generally for marking or decoration.

Exterior stop. The removable glazing bead that holds the glass or panel in place when it is on the exterior side of the light or panel, as contrasted to an interior stop located on the interior side of the glass.

Extrusion. The process of producing shapes by forcing heated material through an orifice in a die by means of a pressure ram. Also, any item made by this process. An example is the complex cross-section of an extruded PVC window frame.

Fenestration. The placement of openings in a building wall, such as windows, doors, skylights, etc., designed to permit the passage of air, light, or people; one of the important aspects of a building’s exterior appearance. Also, associated interior or exterior elements, such as shades or blinds. From the Latin word, fenestra, meaning “window.”

Fiberglass. A composite material made by embedding glass fiber in a polymer matrix. It may be used as a diffusing material in sheet form, or as a standard sash and frame element.

Fixed (window). A single sash fastened permanently in a frame so that it cannot be raised, lowered, or swung open; a non-venting or non-operable window unit.

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 molten glass (at approximately 1000 degrees Celsius) on a shallow bed of molten tin. Thickness is controlled by the speed at which the solidifying glass ribbon is drawn off the tin bath. The surfaces of the glass do not come into contact with any rollers or mechanisms that could cause damage until the glass has solidified; therefore it produces a high-optical-quality glass with parallel surfaces without polishing and grinding.

Flush glazing. A method of glazing wherein the surfaces of the glass retaining members (stops or beads) are in the same plane normal to the glass as the side faces of the frame members; often achieved by providing pockets in these faces.

Foam spacer. Nonconductive, foam material (often closed-cell silicone foam) used to separate the double- and triple-pane insulating glass units; improves the thermal performance of the window.

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, enclosing structure of a window or other fenestration system which holds the sash, casement, door panels, etc., as well as hardware. Frames can be constructed from aluminum extrusions, steel, PVC extrusions, wood, composite materials, or a combination of these materials.

Gas fill, Gas-filled IGU. A gas, usually argon or krypton, placed between window or skylight glazing panes to reduce the U-factor by suppressing conduction and convection.

Gasket. A pre-formed section, generally of neoprene or rubber-like composition, that provides a continuous sealing for the glass or frame members. It provides a weather-tight seal when compressed.

Gas retention. The ability of a sealed insulating glazing unit to retain its original gas-filled composition. In the long term, diffusion through frame and edge-seal materials allows air to progressively replace the original gas(es).

g-factor. Same as solar heat gain coefficient (SHGC). This quantity is related to total solar energy transmittance (TSET). In some countries, it is formally applied to only the glazing, but generally applies to both transparent and opaque parts of a fenestration system. See Solar Heat Gain Coefficient (SHGC).

Glass. An inorganic, hard, brittle substance, usually transparent, that is made by fusing silicates (sand), soda (sodium carbonate), and lime (calcium carbonate) with small quantities of alumina, boric, or magnesia oxides under high temperatures, without crystallizing. Contrary to common belief, glass is not solid, but is rather a very hard fluid which flows slowly.

Glazing. A generic term used to describe an infill material, such as glass or window assemblies in general. Also refers to the process of applying or installing glass into a window or door sash.

Glazing bead. A small, applied molding used to hold a pane of glass, or substitute for it, in a frame.

Hard coat(ing). A low-emittance (low-E), thin-film surface coating on sheet glass which is deposited at a high temperature during the final stage of glass production. It is resistant to abrasion and attack by moisture, atmospheric pollutants, etc. See also Pyrolytic coating.

Haze. The scattering of visible light, resulting in a decrease in the transparency of a window system and a cloudy appearance.

Head. The main horizontal member forming the top of a window or door frame.

Heat Gain. Instantaneous rate of heat gain at which heat enters into and/or is generated within a space. Latent heat gain occurs when moisture is added to the space (from occupants or equipment). Sensible heat gain is added directly to the space by conduction, convection, and/or radiation.

Heating degree-day. Term used to relate the typical climate conditions to the amount of energy needed to heat a building. The base temperature is usually 65 degrees Fahrenheit. A heating degree-day is counted for each degree below 65 degrees that the average daily outside temperatures reach in the winter.

Heat loss. The transfer of heat from inside to outside by means of conduction, convection, and radiation through all surfaces of a building.

Heat loss rate. The rate at which heat is lost from a system or component of a system, per degree of temperature difference between its average temperature and the average ambient air temperature

Horizontal-sliding window (horizontal slider). A window fitted with one or more sashes that opens by sliding horizontally in grooves provided in horizontal frame members. An operating sash with a fixed light (comprising a unit) is termed a single slider.

Humidity, absolute. The mass of water vapor per unit of volume.

Humidity, relative. The percentage of moisture in the air in relation to the amount of moisture the air could hold at that given temperature.

Inert gas. Refers to the use of chemically nonreactive gas(es) within the cavity of a sealed insulating glass unit for the purpose of reducing conductive/convective heat transfer. See Gas fill.

Insulating Glass (IG) Insulating Glass Unit (IGU). A combination of two or more panes of glass with a hermetically sealed air space between the panes of glass, separated by a spacer. This space may or may not be filled with an inert gas, such as argon.

Jamb. The main vertical members forming the sides of a window or door frame.

Laminated glass. Two or more sheets of glass bonded together with one or more inner layers of transparent plastic (interlayer) to which the glass adheres if broken. The bonding is achieved by heating the glass/interlayer sandwich under pressure in an autoclave. The glass is used for overhead, safety glazing, and sound reduction.

Light. A window; or a pane of glass within a window. Double-hung windows are designated by the number of lights in the upper and lower sashes, as in six-over-six. Also, spelled informally, lite.

Light-to-solar-gain ratio (LSG). A measure of the ability of a glazing to provide light without excessive solar heat gain. It is the ratio between the visible transmittance of a glazing and its solar heat gain coefficient.

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-E (low-emittance) coating. A microscopically thin (less than 100 nm) metal, metal oxide, or multilayer coating deposited on a glazing surface to reduce its thermal infrared emittance and radiative heat transfer. Near-infrared emittance may also be reduced depending on whether solar heat is to be rejected or admitted. Low-emissivity glass is used to increase a window’s insulating value, block heat flow, and reduce fading

Mullion. A major structural vertical or horizontal member connecting windows, sliding glass doors, or frames.

Muntin, Muntin bars, Muntin grilles. Small, secondary horizontal or vertical framing members that divide glazing into separate vision areas within the basic framework of a door, window, sash, or ventilator. Sometimes referred to as: sash bar, window bar, or glazing bar.

Obscure glass. Any textured glass (frosted, etched, fluted, ground, etc.) used for privacy, light diffusion, or decorative effects. Also knows as vision-proof glass.

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, or a substitute for it, cut to size and shape and ready for glazing. Often called a square or a light.

Passive solar heat gain. The direct admittance of solar heat to a building (usually deliberately and in winter) through windows to reduce or eliminate the need for additional heating energy.

Patterned glass. One or both surfaces of glass with a rolled design; it is used for privacy and light diffusion.

Performance (energy). The thermal, solar, and visual properties of a product influence the building energy balance due to solar gains, heat loss, and daylight, and require auxiliary energy from artificial lighting, heating, and cooling; ventilation energy (fans) may also be affected. Therefore, a product has an impact on the overall primary energy use in a building.

Permeability. The ability of a porous material to permit transmission of water vapor.

PVC. A polymer known as polyvinylchloride made by combining several chemicals, fillers, plasticizers, and pigments. It is often used as an extruded or molded plastic material for window framing or as a thermal barrier for aluminum windows.

Pyrolytic coating. A low-E, thin-film coating applied at high temperature. See also Hard coating.

Radiant temperature. The temperature describes the infrared radiant field at a certain position and is the weighted average of surface temperatures surrounding the location; the weighting is dependent on surface emissivity and the view factors to the measurement point.

Rail. A horizontal member of a window sash or door panel. Also known as head rail, top rail, bottom rail, meeting rail.

Relative humidity. The percentage of moisture in the air in relation to the amount of moisture the air could hold at that given temperature. At 100 percent relative humidity, moisture condenses and water droplets are formed.

Rough opening. The framed opening in a wall into which a window or door unit is installed.

R-value. A measure of resistance to heat flow of a material or construction (insulating ability). The higher the R-value, the better the insulating effect and the lower the rate of heat flow.

Safety glass. Glass constructed, treated, or combined with other materials to reduce the likelihood of injury to persons in the broken or unbroken state. Types of safety glass include laminated safety glass, tempered glass, and wire glass.

Sandblasting. A method for creating a decorative effect on glass. Sandblasting consists of blasting an abrasive at the surface of the glass under pressure. Matte and peppered effects are achieved using different pressures and shading is achieved by changing the distance and pressure of blasting during application.

Sash. The portion of a window that includes the glass and framing sections which are directly attached to the glass. Not to be confused with the master frame into which the sash sections are fitted.

Sealant. A flexible material placed between two or more parts of a structure with adhesion to the joining surfaces to prevent the passage of certain elements such as air, moisture, water, dust, and other matter. Sealants are commonly made of silicone, butyl tape, or polysulfide.

Setting block. Small blocks made of neoprene, vinyl, etc., to distribute the weight of glass to the strong point of a sash or frame, to aid in centering the glass, and to prevent glass-to-metal contact.

Shading Coefficient (SC). The ratio of solar heat gain through a window to the solar heat gain through a single layer of 3mm clear glass under the same environmental conditions. This is meaningful for near-normal incidence only. This quantity has been replaced by the Solar Heat Gain Coefficient (SHGC).

Silk-screen process. A decorating process in which a design is printed on glass through a silk mesh, woven wire, or similar screen.

Sill. The lowest horizontal member in a door, window, sash, or ventilator frame. Also known as sill plate, inside sill, outside sill.

Single-glazed, single glazing. Glazing that is just one layer of glass or other glazing material (as opposed to sealed insulating glass which offers far superior insulating characteristics).

Single-hung window. A window consisting of two sashes, the top one stationary and the bottom movable. This window is similar to a double-hung window except that the top sash is stationary. See also Double-hung window.

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.

Soft coat(ing). Generally refers to silver-based, low-E coating; see above. So called due to its susceptibility to damage through abrasion. The coating generally consists of a multilayer structure of alternate dielectric and thin transparent metal layers which are deposited in a vacuum chamber. Also known as sputtered coating.

Solar heat gain. Heat from solar radiation that enters a building.

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. For near-normal incidence only, SHGC = 0.86 x SC. See also Shading Coefficient (SC).

Spacer. The linear object that separates and maintains the space between the glass surfaces of insulating glass.

Stile. The main vertical members of the framework of a sash or door panel.

Surface coating. The deposition of a thin-film coating on a surface.

Suspended film. Polymer-based, optically clear glazing layer mounted between glass layers in a multiple-glazed system.

Tempered glass. Treated glass that is strengthened by reheating it to just below the melting point and suddenly cooling it. When shattered, it breaks into small pieces. Since these particles do not have the sharp edges and dagger points of broken annealed glass, tempered glass is regarded as a safety glass and safety glazing material. Tempered glass is also approximately five times stronger than standard annealed glass. The glass must be cut to size and have any other processing (such as edge polishing and hole drilling) completed before being subjected to toughening, because attempts to work the glass after tempering will cause it to shatter.

Thermal barrier, thermal break. An element, made of a material with relatively low thermal conductivity, which is inserted between two members having high thermal conductivity in order to reduce the heat transfer. Such elements are often used in aluminum windows.

Thermal conductivity (k). The heat transfer property of materials, expressed in units of power per area and degree of temperature (e.g., Btu-per-hour per inch of thickness per square foot of surface per one degree F temperature difference).

Thermal resistance. A property of a substance or construction which retards the flow of heat; one measure of this property is R-value.

Tinted glass. Glass that is colored by incorporation of a mineral admixture, by surface coating, or by the application of retrofit film. Any tinting reduces both visual and radiant transmittance.

U-factor. The heat transmission in unit time through unit area of a material or construction and the boundary air films, induced by unit temperature difference between the environments on each side. A measure of the rate of non-solar heat loss or gain through a material or assembly. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value. Also known as U-value.

Vapor barrier. A membrane or coating which resists the passage of water vapor from a region of high vapor pressure to low pressure, more accurately called a vapor retarder.

Warm edge. Term used to describe technology that uses insulating spacers to achieve better thermal performance of an insulating glass unit, particularly evident in the increase of edge surface temperatures on the indoor side in the winter.

Weatherstrip, weatherstripping. A strip of resilient and flexible material for covering the joint between the window sash and frame in order to reduce air leaks and prevent water from entering the structure. Also, the process of applying such material.

Window. The frame, equipped with sash(es), and their fittings, which, when glazed with glass or substitute for it, closes an opening for the admission of air and/or light in the wall of a building. (From the old Norse word “vindauga,” which is formed from “vinder,” wind, and “auga,” eye. Therefore, a window is an “eye for the wind” or “wind-eye.”)