Glossary

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.

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AAMA (American Architectural Manufacturers Association) – The former name of the window and door industry association that wrote the AAMA standards still cited in many specifications, such as AAMA 2604 and 2605 for aluminum coatings and AAMA 1503 for condensation resistance. In 2020 AAMA became the Fenestration and Glazing Industry Alliance. See FGIA.

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.

ACH50 – Air changes per hour at 50 pascals of pressure difference, the airtightness result of a blower door test. The Passive House Institute limit is 0.6 ACH50; Phius uses a limit per square foot of enclosure area instead.

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.

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.

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 Barrier – The continuous system of materials that stops air moving through the building enclosure. At each window the frame must be sealed to the wall air barrier on all sides, usually with tape, a fluid-applied membrane or sealant. See Installation.

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 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.

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

Ambient Temperature – The outdoor temperature.

Anchor Clip – A metal clip or strap fastened to the window frame and then to the structure, used where fasteners cannot go directly through the frame, or to hold the window out in the insulation layer. Anchor type and spacing in impact products must match the product approval.

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 – Ordinary float glass that has been cooled slowly and evenly in a long oven (a lehr) to relieve internal stress, emerging as a flat, “fire polished” product. When annealed glass breaks it forms long, sharp shards, so building codes require safety glazing instead in doors and other hazardous locations. Every GlasFëns vacuum insulating unit starts from tempered safety glass.

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) – The organization that accredits standards developers and approves American National Standards. Many window standards, including NAFS and the NFRC procedures, carry the ANSI designation.

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 – The American Society of Heating, Refrigerating and Air-Conditioning Engineers. It publishes ASHRAE 90.1, the energy standard most U.S. commercial and multifamily codes reference, and the climate and heat-transfer data used throughout the industry.

ASHRAE 90.1 – The ASHRAE energy standard for commercial buildings and multifamily buildings of four or more stories, adopted directly or as an alternative to the IECC commercial chapters in most U.S. jurisdictions.

ASTM International – Formerly the American Society for Testing and Materials. It publishes material standards (including glass) and test methods used for windows, such as ASTM E283 (air leakage), E330 (structural), E331 and E547 (water), E1886/E1996 (impact) and E2112 (installation).

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).

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Back Bedding – The process of adhering and sealing.

Back Dam – A raised lip at the inside edge of a sill pan that stops water from running into the building and directs it back out to the exterior.

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.

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

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

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

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.

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.

Blower Door Test – An airtightness test in which a fan pressurizes or depressurizes a building and measures the air flow needed to hold the pressure. Results are reported as ACH50 or air flow per unit of enclosure area.

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.

Building Performance Standard (BPS) – A local law that limits the energy use or carbon emissions of existing buildings, with limits that tighten over time, such as New York City’s Local Law 97 or Boston’s BERDO. Better windows are often part of meeting them.

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.

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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 (COG) – All the glass area of a window except the band within 2.5 in (63.5 mm) of the frame sightline. Center-of-glass values are often quoted for glass alone; whole-window values, which include the frame and edge of glass, are what codes and labels use.

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.

CMA (Component Modeling Approach) – NFRC’s rating method for nonresidential projects. Frame, glass and spacer components are approved separately, and whole-window ratings are calculated for each project and issued as a CMA label certificate in place of a sticker label.

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.

Condensation Index (CI) – NFRC’s current condensation rating under ANSI/NFRC 500-2020, from 1 to 100, based on the coldest inside temperatures of the frame, edge of glass and center of glass at 0°F (−18°C) outside and 70°F (21°C) inside. It replaces the older Condensation Resistance (CR) rating.

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

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.

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

CRF (Condensation Resistance Factor) – A rating of a window’s ability to resist condensation, tested under FGIA (formerly AAMA) 1503. The higher the CRF, the colder it can get outside before condensation forms. See also Condensation Index and Temperature Index.

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.

CSA A440.2 – The Canadian standard for calculating window and door energy performance, including U-factor, SHGC, the Energy Rating (ER) and the Temperature Index (I-value). Its companion is the Canadian supplement to NAFS, CSA A440S1.

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.

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Dade County – See Miami-Dade County.

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 a glazing transmits. A clear double-pane unit with no coatings transmits about 80 percent; low-e coatings and tints lower it. See Visible Transmittance (VT).

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.

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.

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

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 Set – An installation method in which the window frame is fastened directly to the rough opening through the frame, with no nailing fin. GlasFëns recommends direct set, with clips where needed, for most projects. See Installation.

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.

DOE-2 – An early whole-building energy simulation program developed for the U.S. Department of Energy. EnergyPlus has since replaced it for most code-compliance and LEED energy models.

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

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

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

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

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

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

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.

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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 2.5 in (63.5 mm) of the frame sightline, where the spacer makes the glass colder in winter than at the center. A warm-edge spacer raises its temperature.

Edge Seal – The seal around the perimeter of an insulating glass unit that keeps gas in and moisture out. GlasFëns vacuum insulating units use a proprietary non-leaded metallic edge seal that stays flexible and is formed below the temperature that would weaken tempered glass.

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 Coating – A painting or powder-coating process in which the paint or powder particles are given an electric charge and the metal part is grounded, so the particles are drawn to the part and wrap evenly around edges and recesses.

Embodied Carbon – The greenhouse gas emissions from making, transporting and installing building materials, as opposed to the emissions from operating the building. It is reported in an Environmental Product Declaration (EPD).

Emissivity – The relative ability of a surface to emit (radiate) heat, on a scale from 0 to 1. Clear glass is about 0.84; a low-e coating can be below 0.05. The lower the emissivity of the glass surfaces facing the gap, the less heat crosses an insulating glass unit by radiation, which lowers the U-factor.

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.

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.

ENERGY STAR® – The U.S. EPA program that labels energy-efficient products. For windows, doors and skylights it sets maximum U-factor and SHGC limits by climate zone, verified through NFRC ratings. ENERGY STAR Canada is a parallel program run by Natural Resources Canada.

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 (Energy Rating) – A Canadian window rating calculated under CSA A440.2 that combines solar heat gain, heat loss through the frame and glass, and air leakage into one unitless number for a typical house. The higher the ER, the better. ENERGY STAR Canada accepts either a maximum U-factor or a minimum ER for windows.

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 – See Vacuum Insulating Glass (VIG).

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

Exposure Category – In ASCE 7, the classification of the terrain around a building used to calculate wind loads: B for urban and suburban areas, C for open terrain, and D for flat, unobstructed areas facing large bodies of water. Higher exposure means higher design pressure.

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.

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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."

FGIA (Fenestration and Glazing Industry Alliance) – The North American industry association formed in 2020 when AAMA and IGMA merged. It publishes the AAMA standards (now FGIA standards), co-publishes NAFS and runs certification programs for windows, doors and insulating glass.

Fiberglass – A composite of glass fibers in a polymer resin. Window frames are made by pultrusion, which gives a frame that is strong and stiff, expands with temperature at nearly the same rate as glass, and conducts very little heat. GlasFëns frames are pultruded with polyurethane resin. See Pultrusion.

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 – Sheet metal or flexible membrane used to direct water out of a wall, especially at roof-wall intersections and around window and door openings. Self-adhered and fluid-applied flashings are now common around 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.

Florida Product Approval – The state approval required for windows, doors and other exterior products used in Florida under the Florida Building Code, showing design pressures and impact resistance and including installation instructions that must be followed.

Fluid-Applied Flashing – A liquid membrane, often silyl-terminated polyether (STPE) or silicone, brushed or troweled onto the rough opening to form a seamless air and water barrier that wraps corners and laps onto the wall barrier.

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 structure that carries a building’s load to the ground. Can consist of poured concrete, concrete block, stone or brick.

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.

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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 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).

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.)

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 – Switchable glazing in which a dilute hydrogen gas is passed through the cavity over a coated pane (typically tungsten oxide with a catalyst layer), darkening the glass and lowering its solar and visible transmittance; oxygen reverses it. See also Switchable Glazing.

Getter – A small reactive material placed inside a vacuum insulating unit that absorbs stray gas molecules over time, helping the unit hold its vacuum for decades.

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

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

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

Glass – A hard, brittle, usually transparent material made by melting silica sand with soda ash, limestone and small amounts of other oxides, then cooling it without crystallizing. Despite a popular myth, glass at room temperature is a solid and does not flow; old windows that are thicker at the bottom were made unevenly and often installed thick edge down.

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.

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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.

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

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.

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 low-e coated polyester film suspended between the panes of an insulating glass unit. It transmits visible light but reflects long-wave heat, creating an extra insulating space without the weight of another pane. Heat Mirror™ is a trademark of Eastman Chemical Company (formerly Southwall Technologies).

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.

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.

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 sealed around its edges against moisture and gas loss. In a conventional unit the panes are held apart by a perimeter spacer that contains desiccant; in a vacuum insulating unit they are held apart by small support pillars and sealed with a flexible metallic edge seal.

HERS Index – The Home Energy Rating System score for a home, calculated by a certified HERS rater. 100 represents a home built to the 2006 IECC; lower is better, and 0 is a net-zero home.

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 – See Relative Humidity (RH).

Hurricane Impact Resistant Glazing – Glazing for coastal regions exposed to hurricane winds and wind-borne debris. Laminated glass is used so that, if the glass breaks on impact, the interlayer and the bond to the frame keep the panel in place and the building closed. Testing (ASTM E1886/E1996, or TAS 201, 202 and 203 in the High Velocity Hurricane Zone) strikes the unit with a missile and then cycles it through thousands of positive and negative pressures.

HVHZ (High Velocity Hurricane Zone) – Miami-Dade and Broward Counties in Florida, where the Florida Building Code requires the strictest wind and impact testing (TAS 201, 202 and 203) and specific product approval.

Hybrid Insulating Glass Unit – An insulating glass unit that combines a vacuum insulating glass unit with one or more conventional panes and gas-filled spaces. The GlasFëns standard glass package is a hybrid unit reaching R-23 at the center of the glass.

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.

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ICC (International Code Council) – A U.S. organization that publishes model codes for adoption by states and cities, including the International Building Code (IBC), International Residential Code (IRC) and International Energy Conservation Code (IECC).

IECC (International Energy Conservation Code) – The model energy code published by the ICC and adopted, often with amendments, by most U.S. states and cities. Its residential chapters set maximum window U-factors and SHGC by climate zone; its commercial chapters do the same or allow ASHRAE 90.1 as an alternative. It succeeded the Model Energy Code.

IGCC (Insulating Glass Certification Council) – A certification program for sealed insulating glass, now run jointly with IGMA as the IGCC/IGMA program and administered by FGIA. Certified units pass accelerated durability testing to ASTM E2190 and unannounced plant inspections.

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.

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-filled Glass.

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.

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).

Insulating Value – See U-factor.

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.

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.

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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.

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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.

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.)

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.

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

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Laminated Glass – Two or more panes bonded by a plastic interlayer, commonly PVB or a stiffer ionoplast. If the glass breaks, the pieces stay bonded to the interlayer and the panel stays in the frame. Laminated glass qualifies as safety glazing, is required for hurricane impact resistance and many overhead applications, reduces sound transmission and blocks most UV.

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

LEED – Leadership in Energy and Environmental Design, the green building rating system of the U.S. Green Building Council. Windows contribute mainly through energy performance, daylight, views, acoustics and material disclosures.

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.

Lift-and-Slide Door – A large sliding door whose panel lifts off its seals when the handle is turned, rolls easily, then lowers onto the seals when closed, giving a tight seal and very large panels.

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.

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 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-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.

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

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.

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 transmittance may also be reduced, depending on whether solar heat is to be rejected or admitted. Low-e glass increases a window’s insulating value, controls solar heat gain and reduces 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.

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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 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.

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

Miami-Dade County – The Florida county, including Miami, whose product control office sets some of the strictest hurricane requirements for windows and doors. See Miami-Dade NOA and HVHZ.

Miami-Dade NOA (Notice of Acceptance) – Product approval issued by Miami-Dade County for use in the High Velocity Hurricane Zone. It lists the tested sizes, design pressures, glass and anchorage that must be followed.

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) – A model residential energy code cited in the 1992 U.S. Energy Policy Act (EPAct). It was replaced by the International Energy Conservation Code (IECC) in 1998.

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.

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NAFS (North American Fenestration Standard) – AAMA/WDMA/CSA 101/I.S.2/A440, the standard that rates windows, doors and skylights for structural, air and water performance, giving ratings such as CW-PG45. It is referenced by U.S. and Canadian building codes. See Performance Class and Performance Grade.

Nailing Fin – A flange integrated into or attached to the perimeter of a window frame and fastened to the face of the wall framing at the head, jambs and sill. It holds the window in the opening and laps with the flashing and weather-resistive barrier. GlasFëns recommends direct set through the frame for most projects, with nailing fins available on A-Series. See Installation.

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.

NECB (National Energy Code of Canada for Buildings) – Canada’s model energy code for buildings other than houses and small residential buildings, adopted with changes by most provinces.

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.

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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.

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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 House – A building standard for very low energy use, high comfort and airtightness. The Passive House Institute (PHI) in Darmstadt, Germany certifies buildings and components worldwide; Phius certifies buildings and windows in North America.

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 Class – Under NAFS (AAMA/WDMA/CSA 101/I.S.2/A440) there are four performance classes: R (Residential), LC (Light Commercial), CW (Commercial Window) and AW (Architectural Window). Each class sets minimum test sizes, minimum design pressures and stricter tests as it rises, so the specifier can match the product to the building type, height and exposure. Together with the Performance Grade it forms the rating, for example CW-PG45. See NAFS.

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 Grade (PG) – The number in a NAFS rating, such as the 45 in CW-PG45. It is the design pressure in psf the product has been tested to for structural, water and air performance; the structural test is run at 1.5 times that pressure and the water test at a percentage of it. See Performance Class.

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."

Phius – The Passive House Institute US, which publishes the Phius CORE and Phius ZERO building standards with climate-specific targets and runs a window performance verification program.

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.

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

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

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).

Psi-install (Ψ-install) – The extra heat lost through the joint between a window frame and the wall, measured in W/m·K. It is lowest when the window sits in the insulation layer and the insulation laps over the frame.

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

Pultrusion – A continuous manufacturing process that pulls glass fibers through a resin bath and a heated die to form a constant profile, such as a window frame. GlasFëns frames are pultruded with polyurethane resin, which is tougher and more crack-resistant than the polyester used in most fiberglass windows.

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

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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.

Rainscreen – A wall design with a drained and ventilated cavity behind the cladding, so most water stops at the cladding and any that gets past drains away before reaching the water-resistive barrier.

RAL – A European color reference system used by paint and coating makers. GlasFëns finishes can be ordered by RAL number.

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.

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

Relative Humidity (RH) – The amount of moisture in the air as a percentage of the most it could hold at that temperature. At 100 percent relative humidity, moisture condenses and water droplets form. The higher the indoor RH in winter, the warmer a window’s inside surfaces must be to stay dry.

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 – A measure of resistance to heat flow of a material or assembly. The higher the R-value, the better it insulates. R-value is the inverse of U-factor (R = 1/U), so an R-5 window has a U-factor of 0.20.

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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 – A synthetic polymer with a backbone of silicon and oxygen atoms and carbon-based side groups. Silicone sealants, gaskets and foams stay flexible from very low temperatures up to about 230°C (446°F) and resist UV and weathering far better than most organic rubbers, which is why silicone is the sealant of choice for structural glazing and for sealing fiberglass frames.

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.

Sill Pan – A preformed or site-built tray under a window that collects water that gets past the window and drains it to the exterior. It usually has end dams and a back dam. See Installation.

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-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.

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

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 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.

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.

Sound Transmission Class (STC) – A single-number rating calculated under ASTM E413 from laboratory sound transmission loss, weighted toward the frequencies of indoor speech. The higher the STC, the better the sound insulation. A 10 decibel reduction is heard as roughly halving the loudness. For traffic, rail and aircraft noise, OITC is the more useful rating.

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

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

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.

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.

Stretch Code – An optional, more efficient energy code that cities or states can adopt in place of the base code, such as the Massachusetts Stretch and Specialized codes or NYStretch.

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.

Support Pillars – Tiny spacers set in a grid between the panes of a vacuum insulating unit that resist atmospheric pressure and keep the glass surfaces apart.

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(s), Photochromic Glazing and Gasochromic Glazing.

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

TEDI (Thermal Energy Demand Intensity) – The annual heating energy a building needs per square meter of floor area, used as a target in the BC Energy Step Code and Toronto Green Standard. Window performance has a large effect on it.

Temperature Factor (fRsi) – The Passive House Institute’s condensation and comfort measure: the coldest inside surface temperature as a fraction of the difference between outdoor and indoor temperature. Certified components must reach at least 0.70 in cool-temperate climates and 0.75 in cold climates.

Temperature Index (I-value) – The optional Canadian condensation rating under CSA A440.2, expressing the coldest relevant inside surface temperature as a percentage of the distance between outdoor and indoor temperature. The higher the I-value, the more humidity a room can hold without condensation.

Tempered Glass – Glass that is heated to about 620°C (1,150°F) and rapidly cooled with air, putting its surfaces in compression. It is about four times stronger than annealed glass and breaks into small, blunt pieces, so it qualifies as safety glazing. ASTM E1300, the standard used to size window glass for wind load, rates fully tempered glass at four times the load resistance of annealed glass of the same thickness. All cutting, edge work and drilling must be done before tempering. Also known as toughened glass. For overhead glazing and skylights, laminated glass may be required; check the applicable building code.

Thermal Barrier – A low-conductance material, such as polyamide or polyurethane, placed between the inside and outside parts of a metal frame so the inside surface stays warmer and less heat is lost. The terms thermal barrier and thermal break are used interchangeably. See Thermal Break.

Thermal Break – A low-conductance material placed between the inside and outside parts of a metal frame to reduce heat flow and keep the inside surface warmer. Aluminum frames need one; fiberglass frames conduct so little heat that they do not.

Thermal Bridge – A part of the building enclosure that conducts heat much more readily than its surroundings, such as a metal frame, a slab edge or a poorly placed window. See Psi-install.

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.

Tilt-and-Turn Window – An inswing window whose single handle lets the sash tilt in at the top for secure ventilation or swing in fully like a casement for cleaning and egress.

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.

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UBC (Uniform Building Code) – A model building code formerly published by ICBO and used mainly in the western United States. It was replaced by the International Building Code (IBC) in 2000.

U-Factor (U-value) – A measure of the rate of non-solar heat loss or gain through a material or assembly; the lower the U-factor, the better it insulates. It is expressed in Btu/hr·ft²·°F (imperial) or W/m²·K (metric); multiply the imperial value by 5.678 to convert, so U-0.20 imperial equals about U-1.14 metric. NFRC rates windows at 0°F (−18°C) outside, 70°F (21°C) inside and a 12.3 mph (5.5 m/s) wind, with no sun. A U-factor may be quoted for the center of glass or for the whole window including frame and spacer; codes use whole-window values. U-factor is the inverse of R-value.

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 Light (UV) – The invisible short-wavelength part of sunlight beyond violet. It causes fading of fabrics, carpets and finishes and deterioration of some materials. Laminated glass and low-e coatings block most of it. See Ultraviolet Radiation (UV).

Ultraviolet Radiation (UV) – Invisible solar radiation with wavelengths shorter than violet light. UV causes fading of fabrics, carpets and finishes, chalking of paints and degradation of some plastics. Laminated glass and low-e coatings block most of it.

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) – Another name for U-factor: the rate of heat flow through a material or assembly per unit area and per degree of temperature difference. "U-value" is more common in Europe and in metric (W/m²·K) work; "U-factor" is the term NFRC and U.S. codes use. The lower the U-value, the better it insulates. U-value is the inverse of R-value. See U-Factor.

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Vacuum Glazing – See Vacuum Insulating Glass (VIG).

Vacuum Insulating Glass (VIG) – Two panes of glass separated by a thin vacuum gap, held apart by tiny support pillars and sealed around the edge. With almost no gas to carry heat, a VIG unit insulates like a much thicker triple-pane unit in a fraction of the thickness. See Hybrid Insulating Glass Unit.

Vapor Barrier (vapor retarder) – A membrane or coating that resists the passage of water vapor, more accurately called a vapor retarder. Its placement depends on climate and must be coordinated with the air barrier.

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 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.

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).

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.

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Warm Edge – Spacer technology that uses low-conductance materials in place of aluminum to keep the edge of an insulating glass unit warmer, reducing heat loss and edge condensation. GlasFëns uses Quanex Super Spacer®, a structural silicone foam spacer.

Weather-Resistive Barrier (WRB) – The layer in a wall, such as housewrap, building paper or a fluid-applied membrane, that sheds water that gets past the cladding. Window flashings lap with it so water drains out, not in. Also called a water-resistive barrier.

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).

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

Wind-Borne Debris Region – Areas defined by the building code, mainly along the Atlantic and Gulf coasts, where wind speeds are high enough that glazing must resist impact from flying debris or be protected by shutters.

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 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.

Window-To-Wall Ratio (WWR) – The ratio of the total area of a building facade which is occupied by windows (glass area and frame).

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.

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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.

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

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