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GLOSSARY OF TERMS

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

We’ve provided a glossary of commonly used terms in the glazing and construction industry that you may come across throughout the duration of your project or while browsing our website.

A

[AAMA(American Architectural Manufacturers Association)] – A national trade association that establishes voluntary standards for the window, door, storefront, curtain wall, and skylight industries.

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.

Adjustment Clip – Hardware on hung window jambs to align jamb after window installation.

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

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 (air infiltration) - The amount of air leaking in and out of a building through cracks in walls, windows and doors. Specifically to fenestration, it is 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). 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 Temperature – The outdoor temperature.

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 – A glass which is created in molten form. The process occurs in a long oven, where the glass is heated and then slowly cooled under controlled conditions, emerging as a flat and “fire polished” glass product. When annealed glass is broken, long, jagged, razor-sharp shards are produced, which can be dangerous. Because of this, building codes in many parts of the world restrict the use of annealed glass. GlasFëns does not incorporate annealed glass in any of our products because of the danger if the glass breaks.

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.

[ANSI (American National Standards Institute)] – A clearinghouse organization for all types of standards and product specifications.

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.

Apron – Interior flat trim piece, used under the stool at the bottom of the window.

Argon – An inert, nontoxic gas placed between glass panes in insulated glass units in order to improve the insulating value of sealed glass units. Argon has a thermal conductivity 67% that of air.

[ASHRAE] – American Society of Heating, Air-conditioning, and Refrigeration Engineers

[ASTM International] – (American Society for Testing and Material) – an organization that establishes material standards (including glass) and test methods. It has also produced a window installation standard.

Astragal – The center member of a double door that is attached to the fixed or inactive door panel.

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 the sash swinging outward from the bottom.

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).

B

Back Bedding – The process of adhering and sealing.

Balance – A mechanical device, normally spring loaded, used in hung windows to counterbalance the weight of the sash during operation.

Balance Shoe – Nylon hardware in a hung window jamb that connects the balance with the sash.

Bay – The section of a structure from one post to the next, extending from the sill to the ridge.

Bay Window – A composite of three windows, usually made up of a large center unit and two flanking units at 30 or 45 degree angles to the wall.

Bay Width – The distance from one centerline of a rafter or post to the center of another.

Base Wall – A short wall below the glazing on exterior walls. Also see Knee Wall. (Base wall is typically completed by another contractor.)

Bead – A molding or stop placed around a window frame to hold the glass in place by pressure.

Billet – The cylindrical form of aluminum just prior to the extruding process.

Bite – A glazing term referring to the dimension of the glazing leg overlapping the edge of the glass.

Blackbody - The ideal, perfect emitter and absorber of thermal radiation. It emits radiant energy at each wavelength at the maximum rate possible as a consequence of its temperature, and absorbs all incident radiance.

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

Bow Window – A composite of four or more window units in a radial or bow formation and a gently curved contour. Bow windows project from the walls of the structure.

Brickmold – A type of external casing which frames windows and doors. The brickmold covers jambs and provides means for nailing during installation.

BTU – The heat required to increase the temperature of one pound of water one degree Fahrenheit.

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

Butyl – A synthetic rubber that can be used as a sealant and architectural glazing tape. Butyl is prepared by co-polymerization of isobutylene with a small amount of isoprene.

C

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, in-swinging are French in origin; out-swinging are from England.

Casing – Molding of various widths, shapes, and thicknesses applied to the framework of window and door units. Interior casing is a flat, decorative molding which covers the inside edge of the jambs and the rough opening between the window unit and the wall. Exterior casing (or brick mold) serves the same purpose, while it also is an installation device through which nails are driven to install the window unit into the wall.

Caulking – A compound for filling joints and sealing cracks to prevent leakage of water and air.

Center of Glass – All glass area of window except that within 2.5” (10 cm) from the edge of the glass; used in measuring and calculating glazing performance, such as R-values and U-values.

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.

CFM (Cubic Feet per Minute) – A unit of measurement used in air infiltration testing. (E.g., “maximum 0.10 cfm per foot of sash perimeter”).

Check Rail – On a double-hung window, the bottom rail of the upper sash and the upper rail of the lower sash where the lock is mounted. The top rail of the lower sash and the bottom rail of the upper sash meet when a double-hung window is closed. Both pieces should be weather stripped for maximum weather ability.

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

Circlehead – A generic term referring to any of a variety of window units with one or more curved frame members, often used over another window or door opening.

Cladding – Material placed on the exterior of a frame and sash components.

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 – A glazed section located near the top of a high wall, often found in churches and cathedrals.

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

CMR (Centerline of Meeting Rail) – A reference line used to locate integral mullions and/or size oriel (unequal) sash, e.g., “the height of the lower sash should be 22 inches from the frame sill to CMR.”

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.

Collection – The act of trapping solar radiation and converting it to heat.

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.

Comfort Engineering – Optimizing glazing selections on a window-by-window basis to fulfill a prioritized set of objectives; e.g., comfort, energy efficiency (both summer and winter), UV protection, natural lighting, desired aesthetics, views, and budget.

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 – 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.

Cottage Double-Hung – A double-hung window in which the upper sash is shorter than the lower sash.

Countersink – When the top of a screw or nail is flush with or below the surrounding material

Crack Length – Total outside perimeter of window sash/vent; used when defining the AAMA air infiltration rate.

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. The higher the CRF, the less likely condensation is to occur. Based on AAMA standard.

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.

Curb – Watertight wall or frame used to raise slope glazing above the surface of the roof as a preventative measure against water leakage from melting snow or rain runoff.

Curb Appeal – The visual attractiveness of a home as seen from the street; often the first impression. Realtors see curb appeal as an influential factor in evaluating and selling homes.

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.

Custodial Lock – Window hardware only operable with a tool or key.

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

D

Dade County – Florida county, including Miami, that has set numerous standards and requirements for hurricane-resistant windows and doors.

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).

Daylight Transmittance – The percentage of visible light that glazing transmits through a window; a standard clear dual pane has a daylight transmittance of 82%.

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 an aluminum 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.

Degree Day - A unit that represents a one-degree Fahrenheit deviation from some fixed reference point (usually 65° F) in the mean, daily outdoor temperature. See also heating degree day.

Desiccant – A porous, crystalline substance used to absorb moisture and/or sealant solvents from within the sealed air space of an insulating glass unit.

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 project wind load to be determined by the architect and expressed in psf, e.g., “the project design load shall be 38.7 psf, both positive and negative.”

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

Designation Number – Prescribed by AAMA; one for each window style. It provides a code for architectural selection, e.g., TR-5000=DH-C45=Double Hung-Commercial Grade-45 psf Design Pressure.

DewPoint – The temperature at which water vapor will condense as warm, moist air is cooled.

Die – A perforated steel block through which aluminum or vinyl is extruded.

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 Gain – A direct gain passive solar system utilizes south-facing windows to give a structure the most sun exposure; a large window area accepts direct sunlight while thermal mass serves as storage.

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.

Dormer – An area that protrudes from the roof of a structure.

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 Panel (DGP) – A removable interior glass panel which creates an air space between the exterior glazing and itself. It provides improved insulation and condensation control and allows for between-glass shading options, such as muntins, blinds, and pleated shades.

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.

Double Strength Glass (DSG) – 1/8” thickness. Sheet glass between 0.115" and 0.133" (2.92–3.39 mm) thick.

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.

Drip Cap – A molding placed on the top of the head brickmold or casing of a window frame.

Dry Glazing – A method of securing glass in a window frame with a dry, preformed, resilient gasket, without the use of a glazing compound.

Dual Durometer – A material that has two or more levels of flexibility.

Dual Durometer Bead – A vinyl bead with a softer flap against the glass and a harder section inserted into a sash member.

Dual Glazed – Two single lites glazed into a split sash with an airspace, not hermetically sealed, between the two single lites.

Dual Window – Two windows joined together, one in front of the other, to provide superior sound control.

Dual Pane – Two panes of glass with a single airspace, held together by an edge spacer; the most economical Insulated glazing unit (IGU).

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.

E

Eave – The area of a structure where the front wall meets the roof.

Eave Height – The vertical dimension that spans from finished floor to the eave.

Edge Effects - Two-dimensional heat transfer at the edge of a glazing unit due to the thermal properties of spacers and sealants.

Edge of Glass – The glass area within 2 ½ inches (10 cm) of the edge of a 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.

Egress Window – A window with specific release hardware and minimum clear opening size to allow occupants to escape through the window in case of a fire.

Electrochromic(s) - 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.

Electrostatic – A painting process by which the aluminum is grounded and the paint carries a positive electric current. This creates a magnetic attraction between the paint and the aluminum, allowing for uniform paint coverage on all exposed extrusion surfaces.

Emittance - The ratio of the radiant flux emitted by a specimen to that emitted by a blackbody at the same temperature and under the same conditions.

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).

End Dam – Used to close the ends of a subsill and prevent water leakage. It makes the subsill a complete water trough, allowing it to collect excess water and drain it to the exterior.

EPDM (Ethylene Propylene Diene Monomer) – EPDM stands for Ethylene Propylene Diene Monomer, a synthetic rubber used in a range of applications. EPDM is used most commonly used in the automotive and construction industries for various seals due to its excellent resistance to environmental factors such as Ozone, UV and general weathering. Typical applications of EPDM are window and door seals (glazing), waterproofing sheets and electrical gaskets.

ER Rating – Energy Rating number developed by CSA (Canadian Standards Association) to compare the thermal performance of windows. Measured in watts per square meter (W/m2).

Escutcheon Plate – Hardware for handles, etc.

Etch – To alter the surface of glass with hydrofluoric acid or other caustic agents. Unintentional permanent etching of glass may occur from alkali and other runoff from surrounding building materials.

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.

Expansion Mullion – Self-mulling window frame jambs that, when slipped together, permit expansion/contraction while preserving strength and water tightness.

Extension Jambs – Flat wood parts which are nailed to the interior edges of the window jamb to extend it in width and adapt to a thicker wall. The inside edge of extension jambs should be flush with the finished wall surface. Interior casing is then nailed into it.

Exterior Glazed – Glass glazed from the exterior of the building.

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.

Extrude – The process of shaping aluminum or vinyl by forcing it through a die.

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.

F

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.

Fin Seal – A form of pile weatherstrip with a plastic mylar fin centered in the pile. This fin reduces air infiltration and ensures weatherstrip contact throughout the window’s life.

Finger-Jointing – A means of forming a single longer piece of wood by joining smaller individual pieces together. The ends of the pieces are machined to form a set of interlocking fingers, which are then coated with adhesive and meshed together under pressure.

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 Lite – A pane of glass installed directly into non-operating framing members; also, the opening or space for a pane of glass in a non-operating frame.

Fixed Panel – Non-operable door, usually combined with operable door units.

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.

Flange Frame – A window frame with the head, jamb, and sill exterior perimeter leg longer than the interior perimeter leg.

Flanges – Aluminum edges used to attach fenestrations to a structure.

Flashing – Metal used to prevent water penetration and/or provide water drainage, especially between a roof and wall, and over exterior door openings and windows.

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.

Foundation – The wall that supports the structure of a building. Can consist of poured concrete, block, stone, or break.

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.

French Door – Generally refers to a pair of hinged doors that open from the middle.

French Sliding Door – A sliding door which has wider panel members around the glass, giving the appearance of a French hinged door.

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.

Front Wall – Wall of a structure running the length of a building between the two gable ends.

G

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).

Gable – A vertical surface commonly situated at the end of a building, usually adjoining a pitched roof.

Gable End – A wall of a structure with a gable at its end.

Gas-filled Glass – A gas, other than air, placed between insulated glass to reduce the U-factor by suppressing conduction and convection. (Usually Argon, Krypton, or Carbon Dioxide.)

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).

Gasket – A rubber or plastic pliable material used to separate glazed glass and aluminum or vinyl. 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.

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.

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 – The glass or polycarbonate panes in a structure. 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 molding or stop around a window frame that applies pressure to the glass to hold it in place. A vinyl or wood strip, applied around the perimeter of the glass on the exterior of the window sash, for holding the glass in place.

Glazing Stop – The part of the sash or door panel which holds the glass in place.

Green Building – A movement in architectural and building circles aimed at creating occupant and environmentally friendly structures. Criteria such as sustainability, energy efficiency, and healthfulness are considered.

Greenhouse Effect – The property of glass that permits the transmission of short-wave solar radiation, but is opaque to long-wave thermal radiation. The interior of a car heating up from direct sun illustrates the greenhouse effect.

Grille – A term referring to windowpane dividers or muntins; typically detachable for cleaning.

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

Gusset – A concealed, strong, right-angled shape used to reinforce mitered corners in tubular aluminum extrusions.

H

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.

Head Board – A flat board, cut to fit the contour of a bow or bay window that is installed between the head jambs and the wall to finish off the area. Finishes the area that would normally be ceiling.

Head Expander – A u-shaped extrusion slipped over the frame head that closes the gap above the window when pushed up after window installation.

Header – A heavy beam extended across the top of the rough opening to prevent the weight of a wall or roof from resting on the frame.

Head Track - The track provided at the head of a sliding glass door. Also, the head member incorporating the track.

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 Fusion – Welding method to join PVC frame and/or sash members by heating the cut-ends, squeezing them together, and allowing the assembly to cool.

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.

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 – Produced by heating the glass to temperatures approaching 1300º F, then rapidly cooling it with air, similar to tempered glazing. The process results in a thermally-strengthened glass that is approximately two times stronger than a piece of annealed glass. Heat-strengthened glass benefits from reduced breakage from thermal and bending stress, which allows the use of larger pieces that do not require increased thickness.

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

Heat Treating – The process where glass or aluminum extrusions are heated and cooled to become harder and stronger.

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.

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.

Hermetically Sealed Unit – An insulating glass unit that is sealed against moisture. The unit is comprised of two lites of glass that are separated by a roll-formed metal spacer tube (at the full perimeter), which contains a moisture and/or solvent absorbing material. The unit is then completely sealed, creating a moisture free air space.

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

Hollow Extrusion – An extrusion having an enclosed cavity within it.

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.

Hurricane Impact Resistant Glazing – Used in coastal regions that are subjected to hurricane winds and flying debris. Specially designed, laminated glass products can fulfill these requirements – the multiple layers of glass may break from an impact, but the plastic interlayer and the structural bonding of the glass to the window frame allows the panel to remain in place, protecting the interior of the building. The test method for compliance involves impacting the window in multiple locations and then subjecting the assembly to cycles of positive and negative pressures to replicate the exposure seen during a hurricane.

Hybrid Tubular Daylighting Device (HTDD) - A tubular daylighting device (TDD) whose light transmitting tube consists of more than one material and/or has more than one geometry throughout its length. Typically used with suspended ceilings or to illuminate spaces without ceilings.

I

[ICC (International Code Council)] – A national organization that publishes model codes for adoption by states and other agencies. Codes include the International Building Code (IBC) and the International Energy Conservation Code (IECC).

IECC - International Energy Conservation Code published by the ICC. The successor to the Model Energy Code, which is cited in the 1992 U.S. Energy Policy Act (EPAct) as the baseline for residential Energy Codes in the United States.

[IGCC (Insulating Glass Certification Council)] – Directs a certifications program of periodic, accelerated laboratory testing and unannounced plant inspections to ensure sealed, insulating glass performance is in conformance with ASTM E 774-88.

Impact-resistant – Term used to describe window and door products that have passed established tests for resistance to wind-borne debris. Such products are typically used in coastal areas that are prone to hurricanes. The ability to withstand mechanical blows or shock without damage seriously affecting the effectiveness of the material or system.

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

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.

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.

Inside Snap Trim – Used in retrofit work to cover the inside gap between the new window and the existing opening.

Insolation – Incident solar radiation; the total radiation striking an exposed surface.

Insulated 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. The added insulation will isolate the room from the outdoor temperatures. Insulating values can be improved by adding a Low-E coating or inert gas fills, such as argon. Variations, such as heat-treated and laminated glass, can be incorporated where additional strength and security may be desired. The thickness of the glass used will depend on window size and building design requirements (e.g. wind load).

Insulation – Construction materials used for protection from noise, heat, cold or fire. A material with high resistance (high R-value or low U-value) that is used to retard heat flow. Air, Argon, or Krypton gas spaces between panes of glass provide insulation in IGUs.

Insulating Value – See U-factor.

Integral Mullion – A frame member trapped within the master frame to separate vents or fixed glass.

Interior Glazed – Glass glazed from the interior of the building.

Interior Venetian Blinds - A venetian blind installed between two panes of glass and remotely controlled.

Interlock – A design feature which enables sashes to engage with one another when closed. 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.

Internal Gain – The heat produced in the interior of a dwelling from the operation of lights, appliances, etc.

J

Jack Stud – Vertical framing members, generally 2×4’s, which form the inside of the window or door rough opening. They support the header and run down to the sole plate.

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 – 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.

Jamb Liner – The tracker installed inside the jambs of a modern double hung window, on which the window sashes slide. Vinyl or metal covering applied to the side jambs of double hung and single hung windows. They are generally formed to include an integral balance system and stops for the exterior and interior surfaces of the sash.

K

K.D. (Knocked Down) – Product components that are shipped unassembled, e.g., the frame for a sliding glass door.

Keeper – A hardware device into which a window locking latch engages for security, also known as the receiving portion of a lock that interlocks with the lock arm. It’s mounted to the upper sash check rail on a double hung or single hung window and the inside surface of the sash stile on a casement window.

Kitchen Bay Window – A small bay window used above counter-height in kitchens; allows for a wider view and an inside sill for herbs and flowers.

Krypton - An inert, nontoxic gas placed between glass panes in insulated glass units in order to improve the insulating value of sealed glass units. Krypton has about half the conductivity of argon. Krypton is denser than argon.

Knee Wall – A short wall below the glazing on all or some of the exterior walls. Also see Base Wall. (A Knee wall is typically completed by another contractor.)

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

L

Laminated Glass – Consists of a plastic interlayer material, polyvinyl butyral, that keeps the glass in place should it break. The pieces are retained within the frame, even after suffering an impact. Laminated glass can qualify as a safety glazing with any combination of annealed, heat strengthened, or tempered glass as layers.

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

Left or Right – Location information, always outside looking in, that can be used to specify direction, e.g., “the operating sash slides to the right.”

Lift – A handle or grip installed or routed into the bottom rail of the lower sash of a double hung or single hung window.

Light (Lite) - 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.

Lintel - A horizontal member above a window or door opening that supports the structure above.

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.

Lite (Light)– A pane of glass, a window, or a compartment of a window. Sometimes spelled “light.”

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.

M

Marine Glazing – A u-channel of soft PVC, which is wrapped around the edge of the glass, and cushions it against the aluminum or vinyl. This process allows for unrestricted expansion and contraction while providing water tightness.

Masonry Opening – The space in a masonry wall that is left open for windows or a door; the same as a rough opening in a frame wall. The header (or lintel) in a masonry opening is usually a steel beam.

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.

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.

Mill – Unfinished and unpainted aluminum material.

Miter Joint – A joint formed by fitting together two pieces cut at an angle to form a corner.

Model Energy Code (MEC) - The Model Energy Code is cited in the 1992 U.S. Energy Policy Act (EPAct) as the baseline for residential Energy Codes in the United States. It has been succeeded by the International Energy Conservation Code (IECC) published by the International Code Council (ICC).

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.

Monolithic Glazing – Consists of a single sheet of glass that is formed using the float glass manufacturing process. From this development, the monolithic glass is modified for increased strength, improved insulating capability, and safety glazing requirements. The glass can be annealed, heat-treated, or bonded to another piece of glass with a plastic interlayer to form laminated glass and it is installed as a single pane. Monolithic glass allows the heat and cold from the outside to penetrate the structure and greatly affect the inside temperature.

Monomer – A substance or simple chemical compound that can be polymerized to yield a much larger polymer molecule.

Mortise Lock – A lock fitting a rectangular-shaped cavity in the edge of a door.

Mullion – A horizontal or vertical member that supports and holds panels, glass, sash, or sections of a curtain wall.

Muntin – Horizontal or vertical bars that divide the sash frame into smaller lites of glass. Muntins are smaller in dimensions and weight than mullions.

Muntin Grilles - Wood, plastic, or metal grids designed for a single-light sash to give the appearance of muntins in a multilight sash, but removable for ease in cleaning the window.

N

Nailing Fin – A vinyl or metal flange integrated into (or attached to) the perimeter of a window frame for installation on the rough opening’s header, jack studs, and rough sill. A continuous nailing fin may surround the frame as with an all-vinyl window, or a nailing fin may be affixed to the head and side jambs as with a vinyl ultraviolet stability.

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.

Neoprene – A synthetic rubber with physical properties that closely resemble those of natural rubber. Neoprene has extremely good weather and temperature resistance, for both heat and cold, with ultraviolet stability.

NFRC - National Fenestration Rating Council.

Nite Latch – Hardware which, when extended, restricts the sash opening to a predetermined dimension.

Non-Tempered Glass – See annealed glass.

O

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

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

Operator – A metal arm and gear device which allows for easy opening and closing of projecting windows; e.g. casement and awning windows.

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

Orientation – The compass direction that a structure faces. To optimize heat-gain in the northern hemisphere, the major facade of a building should face south. (A variation as much as 30 degrees east or west of south should not seriously affect performance.)

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.

P

Palladian Window – A large, arch-top window flanked by smaller windows on each side.

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.

Panel – Framework holding the glass option in an operable door or fixed window frame.

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.

Parting Stop - A narrow strip, either integral or applied, that holds a sash or panel in position in a frame. A strip of wood applied to the jamb to separate the sash in a double-hung window.

Passive Solar – A solar heating system which operates on natural thermal processes and does not use external mechanical power to move the collected heat. Generally, the building’s structure itself forms the solar system.

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.

Patio Doors – Sliding glass doors, often used for access to a deck or terrace.

Patterned Glass - Glass that has a pattern impressed on one or both sides. Used extensively for light control, bath enclosures, and decorative glazing. This is sometimes called “rolled”, “figured,” or “obscure” glass.

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).

Picture Frame Casing – The use of casing on all four sides of the interior of a window. The stool and apron at the sill are replaced with casing.

Picture Window – A large stationary (non-ventilating) window which is designed for a maximum view without obstruction.

Pitch – The degree of the inclination upward from horizontal or flat. It may be expressed in degrees or as the ratio of the number of inches it rises in each 12″ of horizontal span (e.g. 3/12 means the roof rises 3″ in every foot of horizontal span).

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.

Pleated Shade – A shade of folded fabric which can be installed like regular shades or between the panes of glass of a window.

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.

Polycarbonate – Strong, semi-transparent plastic used in place of glass that is available in varying thicknesses. Polycarbonate is a less expensive infill selection when compared to glass.

Polymerization – A reaction occurring when two or more molecules of a compound are united to form a larger, more complex compound.

Polyvinylchloride - See PVC.

Prime Frame – A window frame which has an extended perimeter leg 1” back from the window exterior plane, which becomes a nailing fin for new construction installation.

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.

Projection – The distance from a structure’s point of attachment to an existing structure to the farthest point outward of the structure.

PSF (Pounds Per Square Foot) – A measurement of air pressure used in window testing, e.g., 1.56 psf (25 mph) or 6.24 psf (50 mph).

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

Purlins – The horizontal components of a roof structure that are used to support and attach roof panels.

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 coating which typically uses tin oxide with some additives deposited directly onto a glass surface while it is still hot. The result is a baked-on surface layer that is hard and durable and thus sometimes referred to as a "hard coat." Pyrolytic coatings are typically used in insulated glass units with the low-E surface inside the sealed air space, but can also be applied to single-pane glass and separate storm windows

R

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.

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, or particles, 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.

Rafter – Structural members of a roof that support the roof load and run from the ridge to the eaves.

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.

Receptor – Framing system consisting of two snap-together extrusions that are used to contain a window frame head and jambs in a masonry-type opening. It allows for deflection and inconsistencies in the openings.

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

Reflectance Back – A measurement in percentage of the visible light reflected back into a room. A lower rating equals better external visibility.

Reflected Radiation – Solar radiation that strikes an exposed surface (like a window) after being reflected from the ground, trees, buildings, snow, etc. Reflected radiation can provide a significant amount of heat when vertical windows are used.

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.

Relative Heat Gain – A measurement of the total heat gain through glazing for a specific set of conditions.

Relative Humidity – Humidity expressed as a percentage of the maximum possible humidity at a given temperature.

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.

Removable Grilles – Ornamental or simulated muntins and bars designed to provide a divided light appearance. Generally made of wood, they are applied to the inside of the sash against the glass surface for easy removal.

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

Ridge Height – The vertical dimension from finished floor to the ridge.

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 – An opening in a wall or the framework of a building where a door frame, window frame, or sub frame is fitted.

Rough Sill – The horizontal framing piece, usually a 2×4, which forms the bottom of the rough opening. It is toe-nailed into the jack studs and supported by cripples.

R-value – The measure of resistance to heat gain or loss (insulated ability). R-values, rather than thickness, can be compared for different materials. The higher the R-value, the greater the glass’s resistance to heat flow and the higher the insulating value. The R-value is the inverse of the U-value.

S

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.

Sash Balance – A coiled spring or spiral system integrated into the jamb liners to allow double-hung or single-hung sashes to open and close. They also allow the sashes to remain open in varied positions.

Sash Cord – In double-hung windows, the rope or chain which attaches the sash to the counterbalance.

Sash Lift – A protruding handle screwed to the inside bottom rail of the lower sash on a double-hung window.

Screw Channel – The threaded channel centered in the glazing track.

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.

Seismic Load – The force produced on a structural system due to the action of an earthquake.

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.

Self-cleaning Glass – Glass treated with a special coating that uses the sun’s UV rays to break down organic dirt through what is called a photocatalytic effect. The coating also provides a hydrophilic effect, which reduces the surface tension of water, causing it to sheet down the surface easily and wash away dirt.

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.

Shade Screen - A specially fabricated screen of sheet material with small narrow louvers formed in place to intercept solar radiation striking a window; the louvers are so small that only extremely small insects can pass through. Also called sun screen. Also, an awning with fixed louvers of metal or wood.

Shading Coefficient (SC) – The total amount of solar energy that passes through a glass relative to a 1/8″ (3 mm) thick clear glass under the same design conditions; includes solar energy transmitted directly plus any absorbed solar energy subsequently re-radiated or convected into a room; lower values indicate better performance in reducing summer heat gain and therefore air-conditioning loads. 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.

Sidelites – A fixed frame of glass beside a window or door.

Silicone - Silicone is an inorganic polymer comprising of carbon, hydrogen and oxygen (along with Silicone). Silicone rubber has a number of advantages but it is most commonly known for and used due to its resistance to extreme temperatures. Unlike EPDM, silicone stays stable and keeps its physical properties up to 230°C (446°F) and high temperature silicone grades can withstand up to 270°C.

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.

Simulated Divided Lites (SDLs) – Glass that has the appearance of a number of smaller panes of glass separated by muntins, but is actually a larger glazing unit with the muntins placed between or on the surfaces of the glass layers.

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-Strength Glass - Glass with thickness between 0.085"and 0.100" (2.16–2.54 mm).

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 Door – Door in which a vent panel moves horizontally on a track system past a fixed panel.

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). The lower the coefficient, the better the window is at blocking the sun’s heat. Windows in hot or temperate climates should have a low SHGC; south-facing windows in cold climates should have a high SHGC.

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

Solar Spectrum - The intensity variation of sunlight across its spectral range.

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. Measures the amount of noise reduction that can be achieved with a given product. A noise reduction of 10 decibels represents cutting the noise level in half, as interpreted by the human ear. So, a rating of 25 means the product reduces the outside noise by approximately 25 decibels, cutting the noise in half 2-1/2 times, or cutting it by over 80%.

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

Span – The distance between any two consecutive structural supports.

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.

Sputtered Coating - A multi-layered low-E coating deposited on glass or plastic film. Typically consists of three primary layers with at least one layer of metal. Sputtered coatings often use one, two or three silver layers and must be protected from humidity and contact. For this reason, they are often referred to as "soft coat." Sputter coatings are typically used in insulated glass units with the low-E surface inside the sealed air space, but can also be applied to plastic and used in suspended films or retrofit window films.

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

Stool - The shelf-like board of the interior part of the window sill, against which the bottom rail of the sash closes.

Stop - The molding on the inside of a window frame against which the window sash closes; in the case of a double-hung window, the sash slides against the stop. Also called bead, side stop, window stop, and parting stop.

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.

T

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. The pieces of broken glass are sufficiently smaller in size, allowing it to be classified as a “break safe” glass. Note: For skylight applications in commercial situations, laminated glass may be required. Please review the local and state building codes in your area.

Thermal Barrier, Thermal Break - A stop of non-conducting material, such as wood, vinyl, or foam rubber, that is used to separate the inside and outside surfaces of a metal frame. The thermal barrier minimizes the conduction of heat to the outside which results in a cold inside surface.

Thermal Break – An element of low conductance placed between elements of higher conductance to reduce the flow of heat. 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 Expansion - Change in dimension of a material as a result of temperature change.

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.

Threshold - The member that lies at the bottom of a sliding glass door or swinging door; the sill of a doorway.

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 – A horizontal transverse beam or bar in a frame; a crosspiece separating a door or the like from a window or fanlight above it. Also, a window above a door or other window built on and commonly hinged to a transom.

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.

Tubular Daylighting Device (TDD) - A non-operable device primarily designed to transmit daylight from a roof surface to an interior ceiling surface via a tubular conduit. The device consists of an exterior glazed weathering surface, a light transmitting tube with a reflective inside surface and an interior sealing device, such as a translucent ceiling panel. See also Hybrid Tubular Daylighting Device.

U

UBC - Uniform Building Code.

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 (US) or W/sq m-°K (European metric). 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.

To convert the U-factor from US (imperial/IP) to European (metric/SI), multiply the imperial number by 5.678. For example, If U=0.35 Btu/hr-sq ft-°F in imperial units, then 0.35*5.678 = 1.9873. The U-factor in metric units will be 1.9873 W/sq m-°K.

U-Factor (whole-window) - 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.

uPVC – Unplasticized polyvinyl chloride, a rigid, chemically resistant form of PVC used for piping, window frames, and other structures.

U-value (U-Factor) – The measure of different structural components’ ability to conduct heat. The U-value of glass is measured by the number of BTUs that will pass through each square foot area, per degree of temperature difference, from one side of the glass to the other. U-values indicate how well the glass will hold the heated or cooled air. The lower the U-value, the greater the glass’s resistance to heat flow, and the higher the insulating value. The U-value is the inverse of the R-Value.

Ultraviolet Light (UV) – Invisible rays of solar radiation at the short-wavelength violet end of the spectrum. Ultraviolet rays can cause fading of paint finishes, carpets, and fabrics, as well as deterioration of some materials.

V

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.

Vent Panel – The panel that moves horizontally on a sliding door.

Vent Unit – A window unit that opens and closes.

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.

W

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.

Weep Hole - A small opening in a wall or window sill member through which water may drain to the building exterior.

Wind Load – The force of wind against the exposed surfaces of the structure. Expressed in pounds per square foot (psf).

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 – Rolled glass with a layer of meshed or stranded wire completely imbedded. Available as polished glass and patterned glass. Approved polished wired glass is used as a transparent or translucent fire protection-rated glazing option. The wire keeps the fragments from falling out of the frame when broken.

X

Xenon - An inert, nontoxic gas placed between glass panes in insulated glass units in order to improve the insulating value of sealed glass units. Xenon is denser than krypton.

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.

Z

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

GLOSSARY OF TERMS

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A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

R

S

T

U

V

W

X

Z