Abrasion resistance. The ability to withstand scuffing, scratching, rubbing or wind-scouring.

Absorptance. The ratio of radiant energy absorbed to total incident radiant energy in a glazing system.

Absorption. Transformation of radiant energy to a different form of energy by interaction with matter.

Acoustic performance. The ability of a window to attenuate sound transmission in noisy environments, normally expressed as the sound transmission coefficient (STC) in decibels; the ratio of transmitted to incident sound intensity.

Acid embossing/etching. A process where the surface of flat or bent glass is obscured by treatment with hydrofluoric acid or its compounds.

Acid polishing. The polishing of a glass surface by acid treatment.

Acrylic. A noncrystalline thermoplastic with good weather resistance, shatter resistance, and optical clarity; sometimes used for glazing.

Aerogel. A microporous, transparent silicate foam which has a low thermal conductivity; used as a glazing cavity fill material. See also Transparent Insulation Material.

Accent lighting. Directional lighting designed to emphasize a particular object or to draw attention to a part of the field of view

Air change rate. The rate of replacement of air in a space, usually due to infiltration of outdoor air through cracks around windows and doors. Commonly expressed in air changes per hour.

Air film. The layer of air next to a surface, such as a glass pane, which offers some resistance to heat flow. The R-value of a still-air film is about 0.68, while that for the air film associated with a 15-mile-per-hour wind velocity is 0.17. See also R-value.

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.

Air leakage rating. A measure of a fenestration system’s rate of air leakage in the presence of a specific pressure difference. It is expressed in units of cubic-feet-per-minute per square foot of window area (cfm/sq ft) or cubic-feet-per-minute per foot of window perimeter length (cfm/ft). The lower a window’s air leakage rating, the better its airtightness.

Altitude (solar). The vertical angular distance of a point in the sky above the horizon. Altitude is measured positively from the horizon to the zenith, from 0 to 90 degrees.

Aluminum. A light, strong, noncorrosive metal that can either be extruded into shapes or used in sheet or coil form and bent into shapes (capping). It has a very high thermal conductivity.

Aluminum-clad window. Window consisting mainly of wood that is covered externally with aluminum sheet to deter the elements.

Aluminum spacer. A rectangular or contoured hollow aluminum bar filled with a desiccant (or moisture-absorbing material) that is traditionally used to separate the panes in double-pane glass units.

Ambient lighting. Lighting throughout an area for general illumination. See task lighting.

Angular-selective window. A glazed window whose visible and solar transmittance varies with angle of incidence. For example, high transmittance at near-normal incidence (to retain the view) and low transmittance for high angle of incidence (beam component of sunlight from near the zenith).

Annealed glass. Standard sheet of float glass, which is heat-treated to increase its impact resistance.

Annealing. Heat treatment that involves the heating of metal, glass, or other materials above the critical or recrystallization temperature, followed by controlled cooling to eliminate the effects of cold-working, relieve internal stresses, or improve strength, ductility, or other properties.

Anodized aluminum. Aluminum that is treated by electrolysis to develop a finished surface (an extremely hard, noncorrosive oxide film). The electrochemical process produces an anodic coating by converting aluminum into aluminum oxide by electrolytic action. The resulting finish may be either clear or colored, and is an integral part of the aluminum.

Anti-glare coating. A treatment applied to a glazing system to reduce the amount of unwanted diffuse visible transmittance.

Anti-reflective coating. A transparent coating, typically 150 nm thick, which reduces surface reflectance by using destructive interference between light reflected at the substrate surface and light reflected at the coating surface.

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.

Atrium. Traditionally, the central space of a building open to the sky; today, the atrium is usually multistory and glazed. An atrium differs from a court, also an outdoor area, but which is surrounded partially or entirely by buildings or walls.

Attenuation. The sound reduction process in which sound energy is absorbed or diminished in intensity as the result of energy conversion from sound to motion or heat.

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.

Azimuth (solar). Compass bearing, relative to true (geographic) north, of a point on the horizon. The horizon is defined as a huge, imaginary circle centered on the observer. Bearings are measured clockwise in degrees from north, ranging from 0 degrees (north) through 90 (east), 180 (south), 270 (west), and up to 360 (north again).

Blinding glare. Glare that is so intense that, for an appreciable length of time after it has been removed, no object can be seen.

Borate glass. A glass whose essential glass former is boron oxide rather than silica.

Brightness. The subjective perception of luminance.

Brise-soleil. An architectural device on a building (such as a projection, louvers, or a screen) that blocks unwanted sunlight.

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.

Casement. A window sash which swings open on side hinges.

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.

Casting. A process of shaping glass by pouring hot glass into or onto molds or tables.

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.

Caulking. Filling joints, cracks, voids, or crevices with a sealant (caulk) in order to prevent the passage of air or water.

Ceramic glass enamel (also ceramic enamel or glass enamel). A vitreous inorganic coating bonded to glass by fusion at a temperature generally above 500 degrees Celsius.

Chromogenic glazing. A broad class of switchable glazings including active materials (e.g. electrochromic) and passive materials (photochromic and thermochromic).

Clear glass. Architectural clear glass is mostly of the soda-lime-silica type, and composition varies between manufacturers, but is generally 70 – 74 percent silica, 5 – 12 percent lime, and 12 – 16 percent soda, with small amounts of magnesium, aluminum, iron, and other elements.

Clerestory. That part of a building rising clear of the roof or other parts, whose walls contain windows for lighting the interior.

Clerestory window. A venting or fixed window positioned above other windows or doors on an upper outside wall of a room.

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.

Commercial entrance system. Products used for ingress and egress in nonresidential buildings. Commercial entrance systems typically utilize panic hardware, automatic closers, and relatively large amounts of glass.

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.

CR. Condensation Resistance index; an indication of a window’s ability to resist condensation developed by NFRC. The higher the CR, the less likely condensation is to occur.

CRF. Condensation Resistance Factor; an indication of a window’s ability to resist condensation developed by AAMA. The higher the CRF, the less likely condensation is to occur.

Crown glass. Large panes that first became available in the seventeenth century and were incorporated in wooden sash windows. The glass was hand-blown through a pipe (pontil) into a circular disc, leaving a bubble or bullion where the pipe was inserted. Also known as bottle glass or bull’s eye glass.

Curtain wall. An external non-load bearing wall, applied in front of a frame structure, thereby bypassing floor slabs. The cladding is intended to separate the internal and external environments, and is distinct from the building structure. There are now many curtain walls systems manufactured from a variety of materials; the systems typically include both windows and spandrel sections.

Cylinder glass. A glass that is blown in the shape of a cylinder and flattened into a sheet.

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

Daylight factor. The ratio, in percent, of workplane 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).

Daylighting. A building energy conservation measure involving the deliberate displacement of artificial lighting by dispersed sunlight or diffuse sky light. Switching, dimming, or other light control strategies must be employed. The mere admission of natural light without a compensating reduction in electric lighting density will not result in a net energy or environmental benefit, although it might improve visual amenity.

Debridge. The process of cutting away the metal on the bottom of a thermal-break cavity once the two-part polyurethane has reached full strength, thus creating a thermally broken extrusion.

Decibel. A unit for expressing the relative intensity of sounds on a scale from 0 for average least perceptible sound to about 130 for the average pain level.

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.

Design heat loss. The calculated values, expressed in units of Btu per hour (abbreviated Btu/h), for the heat transmitted from a warm interior to a cold outdoor condition under prescribed extreme weather conditions. The values are useful for selecting heating equipment and estimating seasonal energy requirements. Infiltration heat loss is a part of the design heat loss.

Design life. The period of time during which a system or component is expected to perform its intended function, without significant degradation of performance and without requiring major maintenance or replacement.

Design Pressure (DP). The wind-load pressure to which a product is tested and rated to withstand.

Design wind load. The wind-load pressure a product is required by the specifier to withstand in its end use application.

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

Diffuse light. Lighting on a workplane or object that is not predominantly incident from any particular direction.

Diffusing glass. Glass with an irregular surface for scattering light; used for privacy or to reduce glare.

Diffuser. A translucent glazing layer or window accessory designed to intercept direct-beam radiation and transmit it diffusively (i.e. in many directions at the same time); also provides privacy.

Diffusivity, thermal. Thermal conductivity per unit of heat capacity.

Direct glare. Glare resulting from high illuminance or insufficiently shielded light sources in the field of view. Direct glare is usually associated with bright areas, such as the sky, that are outside the visual task or region being viewed.

Direct sunlight (beam sunlight). Daylight directly from the sun without any diffusion.

Disability glare. Glare resulting in reduced visual performance and visibility. Often accompanied by discomfort glare.

Discomfort glare. Glare producing discomfort. It does not necessarily interfere with visual performance or visibility.

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.

DOE2.1E. A building-simulation computer program used to calculate total annual energy use.

Double envelope. A facade comprised of a pair of “skins” separated by an air space, which acts as a buffer against temperature extremes, wind and sound. The cladding can be designed into a multiple permutations of solid and diaphanous members, operable or fixed. Sun-shading devices are often located within the cavity. The system goes by many names, including Double-Leaf Facade, Double-Skin Facade and Ventilated Facade.

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.

Drawn glass. Glass made by a continuous mechanical drawing operation.

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.

Dual-seal unit. A sealed multiple-pane glazing unit with two independent materials used in the edge seal for bonding the glass layers to the spacer. The dual seal reduces the possibility of mechanical failure (i.e., separation of glass from spacer and loss of dry air or other gas(es) used in the cavity).

Dynamic glazing. See Switchable glazing.

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.

Electrochromic(s) (glazing). Glazing with optical properties that can be varied continuously from clear to dark with a low-voltage signal. Ions are reversibly injected or removed from an electrochromic material.

Electromagnetic spectrum. Radiant energy over a broad range of wavelengths.

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.

Evacuated glazing. An insulating glazing composed of two glass layers, hermetically sealed at the edges, with a hard vacuum between (< 10-3 Pascals) to eliminate convection and conduction. A spacer system (commonly referred to as “pillars”) throughout the surface of glass (rather than just at the edges) is needed to keep the panes from touching.

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 aluminum 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 aluminum or 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.

Film conductance. The time rate of heat flow from a unit area of a surface to its surroundings, induced by a unit temperature difference between the surface and the environment.

Fire resistance. As applied to buildings, the property of a material or assembly to withstand fire or give protection from it, characterized by the ability to confine a fire or to continue serving a structural function, or both.

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. (1) Sheet metal or other material applied to seal and protect the joints formed by different materials or surfaces. (2) Applying a thin layer of opaque or colored glass to the surface of clear glass, or vice versa.

Flat glass. A general term covering sheet glass, plate glass, float glass, window glass, and various forms of rolled glass, and named according to the method used in its manufacture. See also Float glass, Plate glass, Rolled glass, Sheet glass.

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.

Fourcault process. The method of making sheet glass by drawing vertically upward from a slotted debiteuse block.

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.

Freeze-thaw resistance. Resistance to cycles of freezing and thawing that could affect application, appearance, or performance

Frit. Ceramic frit opacification is one or more coats of durable colored ceramic material fire-fused onto compatible base glass. The firing also produces a heat-treated product. Since the basic purpose is generally to render the glass opaque, the frit is typically applied to the second surface of monolithic glass or the fourth surface of an insulating unit (counting from the outside surface in). The opacity can be improved with thicker or multiple coats, which are available in a wide range of colors.

Gasochromic glazing. Glazing which uses the phenomenon of chromism due to tin injection/ejection to color the window. The application of gas flow transporting ions to the surface (catalyst), which changes solar and visible transmittance. See also Switchable glazing.

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).

Glare. High luminosity values from a point, line, or area source that may affect the visual amenity, depending on luminosity, background illumination, adaptation of the eye, and area size. There are upper limits for physiological glare (damage to the eyes) and psychological glare (feeling of discomfort).

Glare-veiling. Diffuse scattering from a glazing system, which obscures (masks) the visibility of objects beyond the glazing system.

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 bar. See Muntin.

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

Ground glass. A light-diffusing glass, usually sandblasted or ground.

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-absorbing glass. Glass having the property of absorbing a substantial percentage of radiant energy in the near-infrared range of the spectrum. See also Tinted glass.

Heat flow rate (Q). The quantity of heat transferred to or from a system in unit time.

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

Heat Mirror™. A thin, transparent-coated (low-e) polymer film that is inserted between double or triple glazing, which permits transmission of visible light but reflects far-infrared (and sometimes near-infrared) radiation. Heat Mirror&trad; is a commercial trademark of Southwall Technologies for their proprietary soft-coated, low-e polyester glazing films.

Heat-strengthened glass. Glass that has been subjected to a thermal treatment characterized by rapid cooling to produce a compressively stressed surface layer somewhat less stressed than that produced in tempered glass. Heat-strengthened glass is approximately twice as strong as annealed glass of the same thickness when exposed to uniform static pressure loads. Heat-strengthened glass is not considered safety glass and will not completely dice as with fully tempered glass.

Heat-treated (glass). A term sometimes used for both fully tempered and heat-strengthened glass.

Heliostat. A sun tracking device. Typically, an instrument consisting of a mirror or other reflective surface moved by clockwork, by which a sunbeam is made apparently stationary, by being steadily directed to one spot during the whole of its diurnal period. A heliostat, for instance, might be used with a skylight, reflecting direct sunlight through the aperture throughout the day to increasing illuminance.

High-transmission glass. Glass that transmits an exceptionally high percentage of visible light.

Holographic glazing. Glazing with a thin-film microstructure coating that refracts incident light in some advantageous way, e.g. as a light-redirecting glazing for daylighting applications.

Hopper window. A partially movable sash that is hinged at the bottom and opens inward.

Horizontal-pivoted window. A window fitted with a ventilator; it opens by rotating on centrally located pivots on upright frame members.

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.

Impact resistance. The ability to withstand mechanical blows or shock without damage seriously affecting the effectiveness of the material or system.

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.

Infiltration (air). The movement of outdoor air into the interior of a building through cracks around windows, doors, and the building envelope in general.

Infiltration heat loss. The heat loss due to infiltration. The loss depends upon the indoor and outdoor temperatures, the crack perimeter, and the rate of air leakage per foot of crack.

Infrared Radiation (IR). Invisible electromagnetic radiation, beyond red light on the spectrum, with wavelengths greater than 0.7 microns. Short-wave infrared radiation is from 770 nm to 2500 nm (0.77 to 2.5 microns), while long-wave infrared is from 2.5 microns and beyond.

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.

Interior venetian blinds. A venetian blind installed between two panes of glass and remotely controlled.

Jal-awning window. A window consisting of multiple top-hinged ventilators arranged in a vertical series and operated by one or more control devices that swing the bottom edges of the ventilator outward. The window does not contain a cross shaft or torque bar, but does have an individually operated locking mechanism.

Jalousie (window). Window made up of horizontally mounted, louvered glass slats that tightly abut each other when closed and rotate outward when cranked open. See also Louvered window.

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

Krypton. An inert, nontoxic gas used in insulating windows to reduce heat transfer.

kWh. Kilowatt-hour; a unit of energy equal to one thousand watt-hours.

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.

LCD switchable. Form of chromogenic (switchable) glazing that employs a liquid crystal device to modulate transmittance of solar radiation.

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.

Lightscoops. Clerestory roof monitors oriented away from the sun, utilized when and where indirect light is desired or solar heat gains are undesirable.

Light pipe. A generic term for a system employing bulk optics (lenses, mirrors, reflective ducts, or other optical waveguide technology) designed to transport light (natural or artificial) to parts of a building remote from the envelope. Also known as tubular daylighting devices (TDD).

Light-redirection system. A glazing unit or panel, possibly retrofitted, which intercepts incident sunlight and sky light and specularly reflects it in another direction, usually toward the ceiling. See also Prismatic glazing and Holographic glazing.

Light shaft. An insulated shaft built to direct the light from a roof window or skylight through the attic to the room below.

Light shelf. A daylight-enhancement device; an internal and/or external overhang with a reflecting upper surface normally above head height. Designed to reduce glare near the window and improve illuminance uniformity along an axis normal to the window wall.

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.

Lightwell. An open shaft in a building that provides air and light to windows opening onto the shaft.

Liquid crystal glazing. Glass in which the optical properties of a thin layer of liquid crystals are controlled by an electric current, changing from a clear state to a diffusing state.

Litrium (also Literium). An atrium designed to optimize daylighting in adjacent spaces. See also atrium.

Load. The amount of energy that must be added to or extracted from a space to thermal comfort. Sensible or latent cooling or heating loads are due to accumulated heat gains or losses through the building envelope, window, infiltration or ventilation, and occupancy.

Long-wave infrared radiation. Invisible radiation, beyond red light on the electromagnetic spectrum (above 2.5 micro meters or microns), emitted by warm surfaces such as a body at room temperature radiating to a cold window surface.

Louvered window. A window having louvers or slats that fill all or part of the opening.

Louvers. Slanted fins or slats in a window, ventilator, or venetian blind; the slats may be fixed or adjustable, and made of wood, metal, glass, or plastic.

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

Low-iron glass. Glass with a low concentration of ferrous compounds, which are absorbing in the near-infrared part of the solar spectrum. Particularly used for solar collector covers and equator-facing windows in cold-climate, passive-solar buildings where solar transmittance must be maximized.

Luminous efficacy (Ke). The ratio of the visible transmittance to the shading coefficient; it is a measure of the light-to-heat ratio of the transmitted energy. See also Light-to-solar-gain ratio.

Metal-clad window. Exterior wood parts covered with extruded aluminum, or other metal, with a factory-applied finish for protection.

Metal window. A window composed of a metal frame and sash; the metals are commonly aluminum, steel, stainless steel, and bronze, but the vast majority of metal frames are made of aluminum.

Micron. One millionth (10-6) of a metric meter.

Mil. One thousandth of an inch, or 0.0254 millimeters.

Moisture migration. The passage of moisture into or through a material or construction, in the form of water vapor, due to a difference in vapor pressure at the two faces.

Molded glass. Glass that is formed in a mold, as distinct from cast, rolled, drawn, or offhand ware.

Monitor. A raised section of roof that includes a vertical, or nearly vertical, glazed aperture for daylighting.

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.

Natural convection. A heat transfer process involving motion in a fluid (such as air) that is caused by a difference in the density of the fluid and the action of gravity. This is an important part of heat transfer from the glass surface to room air.

Natural ventilation. Air movement into and out of a building due to wind or differences in air pressure or temperature.

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

Operable window. A window that can be opened for ventilation.

Optical glass. High-quality glass with closely specified optical properties; it is used in the manufacture of optical systems.

Outdoor-Indoor Transmission Class (OITC). A single-number rating calculated in accordance with ASTM E 1332, using values of outdoor-indoor transmission loss. It provides an estimate of the sound insulation performance of a facade or building elements. The frequency range used is typical of outdoor traffic noises.

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.

Panning. In replacement window work, the outside aluminum trim that can extend around the perimeter of the window opening; it is used to cover up the old window material. Panning can be installed in the opening or attached directly to the window before installation.

Particle-dispersed glazing. Glazing in which the orientation of small particles between two sheets of glass is controlled electrically, thus changing its optical properties.

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.

Passive system. A solar heating or cooling system that uses no external mechanical power to move the collected solar heat.

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

Peak demand. The maximum hourly total building electricity use in the year. Electricity uses include space conditioning equipment such as chillers, fan coil units, electrical reheat coils, auxiliary equipment such as pumps and fans, electric lighting, and other office equipment (computers, copy machines, etc.).

Peak load. The maximum hourly total building heating or cooling load in the year.

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.

Performance class. There are five window performance classes; R – Residential, LC – Light Commercial, C – Commercial, HC – Heavy Commercial, and AW – Architectural. This classification system provides for several levels of performance so the purchaser or specifier may select the appropriate level of performance depending on climatic conditions, height of installation, type of building, etc.

Performance grade (design pressure). The minimum level of design pressure (air, water, wind) a product must be tested at to achieve a particular rating.

Perimeter heating. A system of heating in which radiators or registers are located along the exposed wall, usually below windows; heated air from the heating devices counteracts the cold convection flow from the windows.

Perm. Empirical unit of water-vapor permeance (mass flow rate), equal to one grain (avoirdupois) of water vapor per hour flowing through one square foot of material or construction induced by a vapor-pressure difference of one inch of mercury between the two surfaces.

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

Permeance. A measure of the transmission of water vapor through a material expressed in units of “perms.”

Photochromic glazing, photochromics. Glazing which changes its thermal, solar, and visible transmittance in response to outdoor illuminance or ultraviolet (UV) radiation. See also Switchable glazing.

Photopic response function. See V-lambda curve.

Photovoltaic. A device that produces electricity (voltage) directly from sunlight (photons).

Pivot (pivoted) window. A window with a sash that swings open or shut by revolving on pivots at either side of the sash, or at the top and bottom.

Plastic film. A thin, plastic substrate sometimes used as the inner layers in a triple- or quadruple-glazed window.

Plate glass. Flat glass with surfaces that are essentially plane and parallel; it is formed by a rolling process, ground, and polished on both sides. It is available in thicknesses varying from 1/8″ to 1-1/4″ (3.2 mm to 31.8 mm), but has been replaced by float glass.

Point fixings. In contrast to mullions or patch fittings, which project beyond the plane of the glazing, point fixings are interior. Typically, holes are drilled into the glass, and bolts or screws attach the glass in an interior frame structure.

Polished wire glass. Wire glass that is ground and polished on both sides.

Polyvinylchloride. See PVC.

Prismatic glazing. A daylighting device that consists of a light-redirecting glazing with a fine-structure, sawtooth cross-section, designed to refract incident sunlight and sky light toward the ceiling.

Projected window. A window fitted with one or more sashes opening on pivoted arms or hinges. The term refers to casements, awnings, and hoppers.

Protected opening. A window with a fire-resistance rating suitable for the wall in which it is located.

Psychrometric chart. A chart which shows various psychrometric quantities, like dry bulb and wet bulb temperatures, moisture content, partial pressure of water vapor, etc.

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.

Radiation. The transfer of heat, in the form of electromagnetic waves, from one surface to another. For example, 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. A horizontal member of a window sash or door panel. Also known as head rail, top rail, bottom rail, meeting rail.

Rebate. Part of a surround, the cross-section of which forms an angle into which the edge of the glass is received.

Reflectance. The fraction of incident radiation upon a surface that is reflected from that surface.

Reflection. The process by which incident flux leaves a surface or medium form the incident side, without change in frequency.

Reflective glass. Window glass that is coated to reflect radiation striking the surface of the glass.

Reflectivity. The reflectance of a microscopically homogeneous sample with a clean, optically smooth surface and of thickness sufficient to be completely opaque.

Refraction. The deflection of a light ray from a straight path when it passes at an oblique angle from one medium (such as air) to another (such as glass).

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.

Ribbon window (window band). A series of windows in a row across the face of a building.

Rolled glass. A flat glass with a patterned or irregular surface, produced by rolling, and having varying transparency. Types include flat wire glass, corrugated glass, patterned glass, obscured glass, cast glass, and figured glass.

Roll-up shade. A window shade installed on inside of building that rolls up around a cylindrical holder at the top. These shades serve to maintain privacy, reflect some solar radiation, and reduce convection flow when fully extended. Also known as a roller shade.

Roof window. A fixed or operable window similar to a skylight that is placed in the sloping surface of a roof. See also Skylight.

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.

Selective surface. A surface for which the spectral optical properties of reflectance, absorptance, emittance, or transmittance vary significantly with wavelength, enhancing the collection (or rejection) of radiant energy in a restricted portion of the spectrum.

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).

Sheet glass. Flat glass made by continuous drawing and whose surface has a characteristic waviness. Because of the long usage of the term, much thin float glass is still incorrectly referred to as sheet glass.

Short-wave Infrared Radiation. Short-wave infrared radiation is from 770 nm to 2500 nm (0.77 to 2.5 microns).

Sidelighting. Lighting from windows and translucent walls. Sidelighting historically was encouraged by the need for exterior views as well as light, and is best accomplished in buildings with narrow plans.

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

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.

Skylight. A sloped or horizontal application of a fenestration product which allows for daylighting. Skylights may be either fixed (non-operable) or venting (operating). Unlike roof windows, skylights need not provide provisions for the cleaning of exterior surfaces from the interior of the building. See also Sloped glazing, Roof 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.

Sloped glazing. A glass and framing assembly that is sloped more than 15 degrees from the vertical and essentially forms the entire roof of the structure. This is generally a single-slope construction. Also, any glazed opening in a sloped roof or wall, such as a stationary skylight or fully operable roof window.

Smart window. The generic term for windows with switchable coatings to control solar gain.

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 absorptance. The fraction of incident solar radiation absorbed by glazing.

Solar control coatings. Thin film coatings on glass or plastic that absorb or reflect solar energy, thereby reducing solar gain.

Solar-control glazing. Glazing modified to reduce its total solar energy transmittance by means of tinting, selective surface coating, or the application of a retrofit film.

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).

Solar radiation. The total radiation of energy from the sun, including ultraviolet and infrared wavelengths as well as visible light.

Sound-insulating glass. Glazing that is fixed on resilient mountings and separated so as to reduce sound transmission. Also known as sound-resistive glass.

Sound Transmission Class (STC). A single-number rating calculated in accordance with ASTM E 413 using sound transmission loss values. It provides an estimate of the sound insulation performance of an interior partition in common sound insulation situations. The frequency range used is typical of indoor office noises.

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

Spandrel. An exterior wall panel filling the space beneath a window sill that usually extends to the top of the window below in multistory construction.

Spandrel glass. Architectural glass that is used in spandrel panels.

Spectrally selective coating. A low-E coating 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.

Spectrally selective tint. A tinted glazing with optical properties that are transparent to some wavelengths of energy and reflective to others. Typical spectrally selective tints are transparent to visible light and reflect short-wave and long-wave infrared radiation.

Spectrally selective glazing. A specially engineered low-E coated or tinted glazing whose optical properties vary with wavelength. See Spectrally selective coating and Spectrally selective tint.

Specular surface. A mirrored surface which reflects light at the same angle as the light falling on the surface.

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

Storefront. A nonresidential system of doors and windows mulled as a composite structure. Typically designed for high use/abuse and strength. The storefront system is usually installed between floor and ceiling.

Structural glass. (1) Flat glass that is usually colored or opaque and frequently ground and polished, used for structural purposes. (2) Glass block, usually hollow, that is used for structural purposes.

Structural glazing. Glazing which is part of the structural design of a building.

Substrate. The underlying hard structure supporting a special purpose surface treatment (e.g. thin-film coating).

Sun-control film. A tinted or reflective film applied to the glazing surface to reduce visible, ultraviolet, or total transmission of solar radiation. It reduces solar heat gain and glare and, in some cases, can be removed and reapplied with changing seasons.

Sunscoops. Clerestory roof monitors oriented toward the sun, utilized when and where capturing direct light or solar gains are desired.

Superwindow. A window with a very low U-factor (typically less than 0.15) achieved through the use of multiple glazings, low-E coatings, and gas fills.

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.

Suspended glazing. Glazing system suspended from above. This innovation, first achieved in projects of the 1960s, made possible continuous glass facades, without mullions.

Switchable glazings. Glazings with optical properties that can be reversibly switched from clear to dark or reflective with the application of an external stimulus, e.g. heat, light, electric signal, etc. Also known as dynamic glazing. See also Electrochromic glazing, Photochromic glazing, and Gasochromic glazing.

Task lighting. Light used to illuminate visually demanding activities, such as reading.

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. Also known as toughened glass.

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 conductance (C). The same as thermal conductivity except that thickness is “as stated’ rather than one inch.

Thermal conduction. The mode of heat transfer through a material by molecular contact. Heat flows from a high-temperature area to one of lower temperature.

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 emissivity. Similar to thermal emittance, except that the suffix “-ivity” refers to a property of general material, while “-ance” refers to a specific material with a certain thickness, surface finish, etc.

Thermal emittance. The ability of a surface to emit long-wave radiation relative to that of a perfect black body. Also known as the long-wave infrared emittance. A perfect black body has an emittance equal to 1.0, while a perfect reflector has an emittance equal to zero.

Thermal mass. The mass in a building (furnishings or structure) that is used to absorb solar gain during the day and release the heat as the space cools in the evening.

Thermal radiation. The heat transfer by radiation from surfaces at or near the room temperature (i.e., wavelengths in the range 2.5 – 50 microns). It is often referred to as far IR radiation or long-wave IR radiation.

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

Thermal shock. A rapid change in temperature imposed on a glass body.

Thermal stress. Stress caused by the temperature differential across a glazing layer; e.g. for a tinted or switchable glazing in its darkened state, the sunlit side of the glazing will be hotter than the reverse side.

Thermochromic glazing, thermochromics. Glazing which changes its thermal, solar, and visible transmittance in response to its temperature. Because of absorption, the temperature of the glazing may differ from the ambient temperature.

Thermogram. An image of an object taken with an infrared camera that shows surface temperature variation.

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.

Toplighting. Lighting from skylights, roof monitors or clerestories. Toplighting historically has been utilized when floor areas are too large to be illuminated adequately by sidelighting.

Translucent. Permitting light to pass through, but with differing degrees of obscuration and diffusion.

Transmission. The quantity of heat flowing through a unit area due to all modes of heat transfer induced by the prevailing conditions.

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 window. The window sash located above a door. Also called transom light.

Transparent. Permitting light to pass through with clear vision.

Transparent Insulation Material (TIM). A generic name for a class of glazing materials having high visible transmittance and very low thermal transmittance. Includes so-called geometric media (honeycomb structures, aerogels, etc.). Some TIMs are translucent (diffusely transmitting) rather than transparent.

Triple glazing, Triple-pane glass. A window with three panes of glass or two outer panes of glass with a suspended plastic film in between. The layers are separated by two gas-filled spaces (usually Argon or Krypton) to increase energy efficiency and provide other performance benefits.

UBC. Uniform Building Code.

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.

U-Factor (total). The area-weighted average thermal transmittance of a complete window, including center-of-glass, edge-of-glass, and frame U-factors.

Ultraviolet Radiation (UV). Extremely short wavelength invisible radiation at the violet end of the visible spectrum. UV rays are found in everyday sunlight and can cause fading, chalking of dark paint finishes, or other damage. Extreme UV exposure can cause certain plastic materials to distort and can cause sunburn.

Ultraviolet transmittance-weighted. A measure of non-visible solar transmittance between 280 and 380 nanometers in wavelength.

Vacuum glazing (window). See Evacuated glazing.

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.

Vapor retarder. A material (usually in the form of a membrane or coating) that reduces the diffusion of water vapor across a building assembly, from a region of high vapor pressure to low vapor pressure.

Venetian blind. A light-controlling shading device consisting of overlapping thin, horizontal slats which can be raised or adjusted from wide open to closed positions by varying the tilt of the slats.

Vinyl. See PVC.

Vinyl-clad window. A window with exterior wood parts that are covered with extruded vinyl.

Visible light. The portion of the electromagnetic spectrum yielding light that can be seen. Wavelengths range from 380 to 720 nanometers.

Visible Transmittance (VT). The fraction of visible radiation transmitted by a glazing system between the limits of 380 and 770 nanometers (0.38 – 0.77 micrometers). It is weighted according to the photopic response of the human eye (V-lamdba curve) and is expressed as a number between 0 and 1. Also known as visible light transmittance (VLT).

Visible reflectance. The measured amount of energy in the visible wavelength range that is reflected by a window system; it is expressed as a percentage.

Visible spectrum. That portion of the total radiation that is visible to the human eye and which lies between the ultraviolet and infrared portions of the electromagnetic spectrum. The colors associated with the visible spectrum range from violet, indigo, blue, green, yellow, orange, through red.

Visual comfort. A set of qualities associated with the amenity of a window, such as freedom from glare and excessive contrast.

V-lambda curve. A bell-shaped function describing the relative response of the human eye to solar radiation as a function of wavelength under bright light conditions. Also known as photopic response function.

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), ventilator(s) or louvers, if any, 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.”)

Window-to-wall ratio (WWR). The ratio of the total area of a building facade which is occupied by windows (glass area and frame).

Window unit. A complete window with sash and frame.

Window wall. A metal curtain wall of the commercial type in which windows are the most prominent element.

Wind pressure. The pressure produced by stopping the wind velocity; the main cause of air infiltration.

Wire glass. A glass with inner wire mesh for strength and fire-retardant qualities.

Xerogel. A type of transparent insulation material similar to aerogel but simpler to manufacture. It has both a higher visible transmittance and higher thermal conductivity than aerogel.

Zenith. The point on the skydome directly overhead, a 90 degree solar altitude.