Glass glossary

A polyvinyl butyral interlayer formulated with a soft inner layer that damps sound transmission. Compared to standard PVB, acoustic PVB provides 3–5 additional STC points in a laminated glass assembly.

The space between the two lites of an IGU. May be filled with air, argon, or krypton. Also called the "cavity" or "gas space." Optimal width is typically ½" (12 mm) for argon-filled units.

The surface of a float glass lite that faced upward (away from the tin bath) during manufacturing. Free of tin contamination. The preferred surface for most coating applications.

An NFRC-rated metric expressing how much air infiltrates through a fenestration product per square foot at a standard wind pressure (25 mph). Reported in cfm/ft². Lower is better.

Iridescent streaks or patterns visible in tempered glass when viewed in polarized light (such as sunlight reflected off a building). Caused by periodic stress variations from furnace rollers during the tempering process. Not a defect, an inherent characteristic of heat-treated glass.

Float glass that has been cooled slowly in the annealing lehr to relieve internal stress without introducing compressive surface stress. The baseline state of flat glass, not heat-treated, not laminated.

A thin-film coating that reduces surface reflectance. Standard clear glass reflects about 8% of light; AR coatings can reduce that to 1–2%. Used in display cases, museums, and high-daylight applications.

An inert gas used to fill the cavity of IGUs. Denser than air, it reduces convective heat transfer across the gap, lowering the unit's U-factor. Typical fill concentration is ≥90%. Less expensive than krypton but less effective per unit of gap width.

An edge finish in which the sharp corner left by cutting is lightly ground off at approximately 45°. Reduces chipping and handling injury without providing a formal finished appearance. The minimum acceptable edge finish for most applications.

American Society of Heating, Refrigerating and Air-Conditioning Engineers energy standard for commercial buildings. Establishes fenestration U-factor and SHGC requirements by climate zone. Referenced by the IECC as a compliance path for commercial buildings.

ASTM standard for flat glass. Defines quality grades (q1, q2, q3), nominal thickness designations, permitted defects, and minimum thickness tolerances for annealed float glass.

ASTM standard for heat-treated flat glass. Covers both heat-strengthened (Kind HS) and fully tempered (Kind FT) glass, including surface compression requirements and fragmentation testing.

ASTM standard specifying windborne debris impact resistance requirements for glazing in hurricane-prone regions. Defines Missile Level A (large lumber) and Missile Level B (small steel ball) tests combined with cyclic pressure loading.

ASTM standard for insulating glass unit durability. Evaluates seal performance through humidity cycling, UV exposure, and temperature cycling, followed by dew point and fogging acceptance tests. Basis for IGMA certification.

Glass curved during the heating phase of tempering or heat strengthening by allowing it to sag over a mold. Used for curved facades, canopies, and specialty architectural features.

An edge finish in which the perimeter of the lite is ground to a wide flat angled face. Decorative; provides a prismatic effect at the perimeter. Common in mirrors and display glass.

The distance a glass lite is captured (held) within a glazing frame rebate. Minimum bite requirements ensure the glass cannot fall out under load. Typically ⅜" to ½" for most commercial applications.

An overall curvature of a glass lite in one direction, the lite bends along its length or width. ASTM C1036 and C1048 set maximum bow limits as a function of lite length. Heat treatment typically increases bow compared to annealed glass.

A warm amber-brown body tint produced by iron oxide and selenium additions to the glass batch. Reduces visible transmittance to approximately 50–70% depending on thickness. Common in commercial facades for glare control.

Polyisobutylene, the primary seal in an IGU. A hot-applied thermoplastic extruded onto the spacer faces. Provides the moisture vapor barrier that protects the desiccant and interior glass surfaces. Sometimes called "butyl" or "PIB."

A small tube installed through the spacer of an IGU used for high-altitude applications. Allows pressure equalization between the sealed gas space and the outside atmosphere, preventing seal stress caused by barometric pressure differences at elevation.

The sealed space between the glass lites of an IGU. May contain air, argon, or krypton. Also called the "air gap" or "gas space."

Thermal performance value (U-factor, SHGC, VT) calculated or measured at the center of the glass lite, away from the edge-of-glass zone and frame. COG values are always better than whole-window values. Energy code compliance requires whole-window (NFRC) values, not COG.

A colored ceramic material applied to glass by screen printing and fused permanently into the surface during heat treatment. Used for spandrel opacification, decorative patterns, and bird-friendly fritting. Once fused, the frit cannot be removed.

Glass strengthened by an ion-exchange process (immersion in molten potassium salt) rather than thermal quenching. Produces very high surface compression in a thin layer. Used in mobile device screens and specialty applications; rarely used in architectural flat glass.

Standard untinted, uncoated flat glass produced by the float process. Has a slight natural green tint from iron oxide in the sand feedstock. Visible transmittance approximately 87–89% at ¼" thickness.

A thin-film material applied to a glass surface to modify its optical or thermal properties. Coatings may be applied on the float line (pyrolytic/hard coat) or at a secondary facility (sputtered/soft coat). Common coating types include Low-E, solar control, anti-reflective, and decorative.

An NFRC index (0–100) indicating how well a fenestration product resists condensation on its indoor-facing surface. Higher CR = more resistant. Not a mandatory code requirement, but relevant in cold climates and high-humidity occupancies (pools, hospitals).

US Consumer Product Safety Commission safety standard for architectural glazing. Defines impact resistance requirements for safety glazing materials in hazardous locations. Fully tempered glass and laminated glass are common compliant products.

The process used to apply pyrolytic (hard-coat) Low-E coatings on the float line. Coating precursors are introduced into the atmosphere above the hot glass ribbon; they decompose and deposit as a fused oxide layer.

A moisture-absorbing material packed inside the hollow spacer of an IGU. Absorbs residual moisture from the sealed cavity to prevent condensation on the interior glass surfaces. Common desiccants include molecular sieve and silica gel. Desiccant capacity determines how long the unit can tolerate minor seal degradation before fogging occurs.

The temperature at which moisture begins to condense inside an IGU gas cavity. A well-sealed unit should have an internal dew point of ≤ −40°F (−40°C). ASTM E2190 uses dew point as the primary acceptance criterion for seal durability testing.

The tool used to drill holes in glass. A hollow cylindrical drill bit with diamond-impregnated cutting edges on the face. Requires continuous water cooling during use. Must be used on annealed glass before heat treatment.

An insulating glass unit consisting of two glass lites separated by a spacer and sealed perimeter. The most common IGU configuration in commercial and residential construction. Also called "double-glazed" or "IG unit."

An IGU with two distinct sealant systems: a primary seal (PIB/butyl) that provides the moisture vapor barrier, and a secondary seal (silicone or polysulfide) that provides structural bond and secondary moisture protection. Standard construction for quality IGUs.

A switchable glazing technology in which a thin electrochromic film changes from clear to tinted when a small voltage is applied. Color change is controlled electrically, either manually or by a building automation system. Reversible; no degradation over switching cycles.

The process of removing the soft-coat Low-E or solar control coating from the perimeter band of a glass lite before IGU assembly. Required because PIB (the primary seal) must bond directly to bare glass; it will not adhere to most sputtered coatings.

The distance from the edge of a glass lite to the center of a drilled hole. Minimum edge distance is typically twice the hole diameter to prevent stress concentration from causing breakout at the glass edge.

A measure of a surface's ability to radiate thermal energy, on a scale of 0 to 1. Clear glass has emissivity of approximately 0.84. Low-E coatings reduce emissivity to 0.02–0.15, dramatically cutting radiant heat transfer across the glass cavity.

Float glass formulated with low iron content to minimize the natural green tint. Also called "low-iron," "ultra-clear," or "starphire." Visible transmittance exceeds 90% vs ~88% for standard clear. Used where color neutrality matters, retail displays, solar panels, museum glazing.

Glazing that provides a rated barrier against the passage of flame and hot gases for a specified time (20, 45, 60, or 90 minutes), but does not limit radiant heat transmission. Products include wired glass, ceramic glass, and intumescent-filled IGUs.

Glazing that provides both a flame/smoke barrier and limits radiant heat transmission to 12,500 Btu/hr·ft² on the unexposed face. More demanding than fire-protection glazing. Typically achieved with intumescent-filled laminated IGUs or specialized borosilicate glass.

The manufacturing process used for virtually all flat glass. Molten glass is poured onto a bath of molten tin, where it floats, spreads to uniform thickness, and cools. The tin bath surface produces the characteristic "tin side" on the bottom face of every float glass lite.

Visible haze or deposit on the interior surfaces of an IGU cavity, caused by seal failure allowing moisture or volatile organic compounds to enter. Fogging cannot be cleaned because it is on sealed interior surfaces. Indicates the unit requires replacement.

A destructive test for fully tempered glass per ASTM C1048. A sample lite is fractured and the number of fragments in a 50mm × 50mm area is counted. Fully tempered glass must produce ≥40 fragments, confirming the fracture pattern is safe.

Dots, lines, or patterns of ceramic frit applied to glass for solar control, privacy, or bird deterrence. A "fritted" pattern reduces solar heat gain proportional to the coverage percentage while maintaining some view-through. Common in high-performance facades.

Heat-treated glass with surface compression ≥10,000 psi (69 MPa). Approximately 4× stronger than annealed glass. Breaks into small, blunt fragments, meeting safety glazing requirements per CPSC 16 CFR 1201 and ANSI Z97.1.

An inert gas (argon or krypton) injected into an IGU cavity to improve thermal performance over air-filled units. Gas fill reduces convective heat transfer. Typically specified at ≥90% concentration to achieve rated performance.

Excessive brightness contrast that causes visual discomfort or disability. In glazing, glare comes from high visible transmittance or low exterior reflectance. High VT glass admits more daylight but may require interior shading to control glare on work surfaces.

A sealant used to seal the perimeter of a glass lite within a frame, the interface between glass edge and frame. Must be compatible with the IGU secondary seal and the frame material. Incompatible glazing compounds can attack IGU seals.

A body tint produced by iron and chromium oxide additions. Reduces solar heat gain while maintaining relatively high visible transmittance, better VLT/SHGC ratio than bronze or grey tints. Common in commercial applications seeking solar control with good daylighting.

A neutral-colored body tint that reduces both visible transmittance and solar heat gain proportionally. Less favorable light-to-solar-gain ratio than green or blue tints. Produces a visually neutral appearance that doesn't alter perceived color rendition significantly.

A Low-E coating applied on the float line by chemical vapor deposition (CVD) at high temperature. Fused into the glass surface, durable, can be placed on exposed surfaces, can be heat-treated after coating. Lower performance than soft coat but more versatile placement options.

A quality control process for fully tempered glass in which lites are held at ~290°C (554°F) for several hours to trigger spontaneous breakage of any lites containing nickel sulfide (NiS) inclusions. Surviving lites have significantly reduced risk of in-service spontaneous breakage.

Glass heat-treated to surface compression of 3,500–7,500 psi, approximately 2× the strength of annealed glass. Fractures in large irregular pieces (not safety glazing). Preferred for spandrel and overhead applications where fragment control is required but safety glazing classification is not needed.

Miami-Dade and Broward Counties in Florida, the highest wind-impact-resistance requirement zone in the US. Products used in HVHZ require a Notice of Acceptance (NOA) from Miami-Dade County in addition to meeting ASTM E1996.

Model building code that establishes construction requirements for most building types in the US. Contains glazing requirements for hazardous locations, fire-rated assemblies, and structural glass. Adopted (with amendments) by most US jurisdictions.

Model energy code published by the ICC. Sets minimum U-factor, SHGC, and visible transmittance requirements for fenestration by climate zone. Most US states have adopted a version of the IECC for residential and commercial buildings.

Two or more glass lites sealed together with a perimeter spacer to create a sealed gas cavity. The primary strategy for reducing U-factor in architectural glazing. Also called "double-pane" (two lites) or "triple-pane" (three lites).

A trade association that operates a third-party certification program for IGU manufacturers. IGMA certification requires passing ASTM E2190 testing and periodic unannounced production audits. The IGMA mark on an IGU label indicates third-party verified quality.

Foreign particles trapped inside the glass during melting, stones (undissolved raw material), seeds (gas bubbles), or debris. ASTM C1036 limits inclusions by size and number per quality grade. Large inclusions are stress concentration points and may initiate fracture.

The bonding material between the glass lites of laminated glass. Most common are PVB (polyvinyl butyral), SentryGlas Plus (SGP/ionoplast), and cast-in-place (CIP) resin. Interlayer choice affects post-breakage structural behavior, sound attenuation, and UV blocking.

An insulating glass unit in which the cavity is filled with a transparent gel that expands and opacifies when exposed to fire heat. Limits radiant heat transmission to achieve OH (fire-resistance) ratings. More expensive than standard fire-protection glazing.

A chemical strengthening process in which sodium ions in the glass surface are replaced by larger potassium ions from a molten salt bath. Creates high surface compression without the fragmentation behavior of thermal tempering. Used in specialty and mobile device glass.

Designation under ASTM C1048 for fully tempered glass. Surface compression ≥10,000 psi. Required for safety glazing locations. Breaks into small fragments.

Designation under ASTM C1048 for heat-strengthened glass. Surface compression 3,500–7,500 psi. Not classified as safety glazing. Breaks in large pieces similar to annealed glass.

A noble gas used as a premium fill for IGU cavities. More effective than argon per unit of gap width, allows thinner cavities to achieve the same U-factor. Significantly more expensive. Preferred in very-thin IGUs where cavity width is constrained by frame depth.

Glass made by bonding two or more lites together with an interlayer (PVB, SGP, or resin). When broken, the interlayer holds fragments in place. Used for safety glazing, hurricane resistance, overhead glazing, and sound control.

The annealing oven through which glass travels after leaving the float bath. The lehr slowly and uniformly cools the glass to relieve internal stresses from the manufacturing process. Without annealing, glass would be too brittle to cut or use.

Visible light transmittance divided by SHGC (LSG = VLT ÷ SHGC). A measure of coating "selectivity", how effectively it admits light while rejecting solar heat. Values above 2.0 indicate a high-performance spectrally selective coating.

An individual piece of flat glass, cut to a specific size. In an IGU, each pane of glass is called a lite. A double-pane IGU has two lites.

Glass with a thin metallic or metal oxide coating that reduces the emissivity of the coated surface from ~0.84 to 0.02–0.15. Dramatically reduces radiant heat transfer across the gas cavity of an IGU, lowering the U-factor. The primary technology for energy-efficient glazing.

Float glass manufactured with reduced iron content to minimize the natural green tint. Achieves visible transmittance above 90% and high color neutrality. Also called "extra-clear," "ultra-white," or by brand names like Starphire or Diamante.

The vacuum deposition process used to apply soft-coat Low-E and solar control coatings. In a vacuum chamber, magnetic fields confine a plasma that bombards a target material (silver, tin, etc.), ejecting atoms that deposit on the glass surface in precise layers.

Glass thickness expressed in millimeters, used internationally and increasingly in US commercial specs. Common architectural thicknesses: 6mm (¼"), 8mm (5/16"), 10mm (3/8"), 12mm (½"), 19mm (¾").

A defined impact projectile used to test windborne debris resistance. Level A uses a 9 lb 2×4 at 34 mph (severe zone). Level B uses a 2g steel ball at 80 mph (moderate zone). Products must survive the applicable missile impact plus 9,000 pressure cycles.

A single lite of glass, not laminated, not an IGU. The simplest glazing configuration. Used in interior applications, in IGUs as the individual lites, or in applications where breakage pattern is acceptable.

Another term for the magnetron sputtering process used to apply soft-coat Low-E coatings. See "Magnetron sputtering."

The portion of the solar spectrum (780–2500 nm) that carries significant solar energy but is invisible to the human eye. High-performance Low-E coatings selectively block near-IR while transmitting visible light, the basis of the light-to-solar-gain (LSG) ratio.

Non-profit that administers the US certification and labeling system for fenestration thermal performance. NFRC ratings (U-factor, SHGC, VT, AL, CR) are third-party certified whole-window values required for energy code compliance.

A small crystalline inclusion that can form in float glass during melting. NiS inclusions undergo a phase transformation at room temperature over months or years, expanding slightly and initiating spontaneous fracture in tempered glass. Heat soak testing eliminates most at-risk lites before installation.

Miami-Dade County's product approval for glazing and construction products used in the High Velocity Hurricane Zone (HVHZ). Required in addition to ASTM E1996 testing. Products without a valid NOA cannot be installed in HVHZ.

A concave cut into the edge of a glass lite, used for hardware clearance or fitting access. Every notch must terminate in a drilled hole at its inside corner to avoid a sharp stress-concentrating re-entrant corner.

A qualitative scale indicating how much view-through is blocked by patterned glass texture. Ranges from light diffusion (silhouettes visible) to fully opaque (no view-through). Pattern depth and surface area coverage determine obscurity.

Glass made opaque for use in the spandrel zone (floor-to-floor area behind the facade where structure, insulation, and MEP systems are hidden). Methods include ceramic frit, painted backing, or spandrel glass with factory-applied opacifier coating.

Glass installed at an angle greater than 15° from vertical, skylights, sloped roofs, canopies. Requires laminated glass or laminated IGU to retain fragments if broken. Heat-strengthened outer lite is often preferred over tempered for overhead applications to maintain large-fragment retention.

Glass with a surface texture rolled into it during manufacturing. The pattern scatters light (providing privacy or diffuse daylighting) while maintaining light transmittance. Common patterns include linen, flemish, granite, arctic, and cathedral.

A switchable glazing technology using a film of liquid crystals in a polymer matrix laminated between glass lites. When no voltage is applied, the crystals are randomly oriented and the glass appears milky white. When voltage is applied, crystals align and the glass becomes transparent. Switches between opaque and clear instantly.

The primary seal material in an IGU. A hot-applied thermoplastic with very low moisture vapor transmission rate, the key property that protects the IGU cavity. Must bond directly to bare glass; cannot adhere to most sputtered coatings (hence edge deletion requirement).

A structural glazing system in which glass is supported by fittings (spiders, bolts, or clamps) through drilled holes or clamp attachments at discrete points rather than in a continuous frame. Requires tempered or laminated tempered glass. All holes must be drilled before tempering.

An edge finish in which the edge is ground flat and then progressively polished to a smooth, optically clear surface. The highest-quality edge finish. Required for exposed edges in frameless glass systems, doors, and architectural glass furniture.

A sealant used as the secondary seal in IGUs. Good adhesion and weather resistance. Less UV-stable than silicone, typically used where the secondary seal will be covered by a frame (not exposed to direct UV).

The most common interlayer for laminated glass. A tough, transparent thermoplastic film that bonds glass lites together under heat and pressure in an autoclave. Standard PVB provides post-breakage fragment retention; acoustic PVB additionally damps sound.

Thermal resistance, the inverse of U-factor (R = 1/U). Sometimes used informally for glass but rarely seen on formal glazing specifications. U-factor is the standard metric for fenestration thermal performance in the US.

A localized optical distortion in glass caused by compositional variation, areas with slightly different refractive index that bend light rays. Visible as wavy distortion in reflected or transmitted views. ASTM C1036 limits ream by quality grade.

The fraction of incident light or solar radiation that is reflected from the glass surface. Expressed as a percentage. Exterior reflectance affects building aesthetics and glare to neighbors; interior reflectance affects view-out quality at night.

Periodic surface undulation in heat-treated glass caused by the furnace rollers during the heating phase. Visible in reflections at low angles. A characteristic of thermal tempering; minimized by advanced furnace control and roller configuration.

Glass that meets specific impact resistance requirements defined by CPSC 16 CFR 1201 or ANSI Z97.1. Includes fully tempered glass, laminated glass, and wired glass. Required by IBC in hazardous locations (doors, sidelights, wet areas, floors, overhead, and other defined locations).

An edge finish slightly more refined than an arris, the raw cut edge is lightly abraded with a belt or wheel to remove the sharpest micro-cracks and reduce chipping risk. Common for edges that will be fully concealed in a frame.

The outer sealant in a dual-seal IGU (silicone or polysulfide). Applied over the PIB primary seal to fill the perimeter channel and provide structural bond and secondary moisture protection. The secondary seal provides structural integrity; the primary seal provides the vapor barrier.

Small air or gas bubbles trapped in the glass during melting. A type of defect regulated by ASTM C1036. Very small seeds (under 1mm) may be acceptable in standard quality grades; larger seeds are limited or prohibited.

An ionoplast interlayer for laminated glass (brand name by Kuraray, formerly DuPont). Much stiffer and stronger than standard PVB, 100× the stiffness and 5× the tear strength. Allows thinner glass to achieve the same structural performance and enables longer post-breakage retention. Used in hurricane-rated, overhead, and structural glass applications.

The fraction of total incident solar energy (UV + visible + near-IR) that passes through a glazing assembly and enters the building as heat. Ranges from 0 to 1. Lower SHGC = less cooling load. A primary energy code parameter for most US climate zones.

A flexible, UV-stable sealant used as the secondary seal in IGUs where the edge will be exposed (structural glazing, point-fixed systems). High UV resistance and wide temperature range. More expensive than polysulfide.

Glazing that can switch between transparent and tinted or opaque states on demand. Technologies include electrochromic (EC), polymer dispersed liquid crystal (PDLC), and suspended particle device (SPD). Controlled electrically or by building automation systems.

A Low-E or solar control coating applied by magnetron sputtering at room temperature. Multiple thin metallic layers (typically including silver) achieve very low emissivity. Must be protected inside an IGU cavity (cannot be placed on exposed surfaces). Higher performance than hard coat but more delicate.

A coating designed primarily to reduce solar heat gain (low SHGC) rather than to reduce radiant heat transfer (low emissivity). Often combined with Low-E properties. Typically has higher reflectance and a more tinted appearance than low-solar-gain Low-E coatings.

The component that separates the two lites of an IGU at the perimeter and maintains the cavity width. Hollow (to hold desiccant) and sealed with PIB on both faces. Materials include aluminum, stainless steel, foam composite (warm-edge), and structural foam.

Opaque or translucent glass used in the spandrel zone of a curtainwall, the non-vision area between the top of one floor's windows and the bottom of the next floor's windows, concealing structure and mechanical systems.

A switchable glazing film containing needle-like particles that align when voltage is applied, allowing light to pass. Without voltage, particles are randomly oriented and the glass appears dark. Faster switching than EC; typically achieves darker tint states. Available as laminated glass.

A single-number rating for the sound isolation of a partition or glazing assembly, per ASTM E413. Higher STC = better sound isolation. Standard double-pane IGU: ~STC 30. Laminated IGU with acoustic PVB: ~STC 40–50. See also OITC.

A facade system in which the glass is bonded to the frame using structural silicone sealant, with no exposed mechanical frame members on the exterior face. The silicone carries wind load and dead load. Requires fully tempered or heat-strengthened glass.

The compressive stress locked into the surface layer of heat-treated glass during quenching. Compressive surface stress makes the glass harder to fracture because surface tension must first overcome the compression before a crack can propagate. ASTM C1048 specifies minimum values by glass kind.

Fully heat-treated glass (ASTM C1048 Kind FT) with surface compression ≥10,000 psi. Approximately 4× stronger than annealed glass. Breaks into small blunt fragments, classified as safety glazing per CPSC 16 CFR 1201. Cannot be cut, drilled, or ground after tempering.

The industrial oven that heats glass to approximately 620–680°C (1150–1250°F) for tempering or heat strengthening. Horizontal roller furnaces are standard; the rollers that transport the glass create the characteristic roller wave in finished product.

An insulating material inserted into an aluminum frame or spacer to interrupt the direct metal-to-metal heat conduction path. Thermal breaks dramatically improve the edge-of-glass and frame U-factor. Without a thermal break, aluminum frames are a dominant source of heat loss in glazing systems.

Stress induced in glass by non-uniform temperature distribution. When one area is significantly hotter than adjacent areas (e.g., direct sun on a partially shaded lite), differential thermal expansion creates tension at the cooler edge. Can cause spontaneous breakage in annealed glass. Tinted glass has higher thermal stress risk than clear glass.

The surface of a float glass lite that contacted the molten tin bath during manufacturing. Contains residual tin oxide. Fluoresces blue-white under UV light, the standard field identification method. Some coating adhesion requirements are specific to tin side vs. air side.

Float glass with colorants (metal oxides) added to the batch to reduce solar transmittance. Common tints: green, blue, bronze, grey. Tinting reduces solar heat gain and visible transmittance proportionally, unlike coatings, which can be spectrally selective.

An insulating glass unit with three glass lites and two sealed gas cavities. Provides lower U-factor than double-pane (typically 0.15–0.22 vs 0.25–0.35). Higher cost and weight. Standard in very cold climates (IECC Zones 6–8); increasingly used in high-performance buildings in Zones 4–5.

The rate of heat flow through a glazing assembly, expressed in Btu/hr·ft²·°F. Lower U-factor = better insulation. Includes conduction, convection, and radiation components. NFRC whole-window U-factor is the standard for energy code compliance in the US.

See "Extra-clear glass" and "Low-iron glass."

The ability of glass or an interlayer to block ultraviolet radiation (280–380 nm). Standard float glass blocks most UV-B but transmits some UV-A. PVB interlayers in laminated glass block >99% of UV. Important for protecting artwork, merchandise, and furnishings from fading.

An IGU in which the cavity is evacuated to near-vacuum rather than filled with gas. Eliminates convective heat transfer across the cavity, allowing very thin units with extremely low U-factors. Requires a support pillar array to maintain lite separation under atmospheric pressure. Emerging technology, not yet widely used in commercial construction.

The fraction of visible-spectrum light (380–780 nm) that passes through a glazing assembly. Expressed as a decimal (0–1) or percentage. Higher VLT = brighter interior. ASHRAE 90.1 sets a minimum VLT of 0.27 to prevent overly dark glass from eliminating daylighting benefits.

An IGU spacer made from materials with lower thermal conductivity than standard aluminum, typically stainless steel, foam composite, or structural foam. Reduces the "cold edge" thermal bridging effect of conventional aluminum spacers, improving edge-of-glass U-factor and reducing condensation risk at the perimeter.

Twisting or compound curvature of a glass lite, distortion in more than one plane. Distinguished from "bow" (simple curvature in one direction). Both are limited by ASTM C1036 and C1048 as a function of lite dimensions.

Glass with a wire mesh embedded during rolling. Originally used in fire-rated applications because the wire holds fragments together after breakage. Now largely replaced by ceramic glass and intumescent products in hazardous locations because wired glass does not meet current safety glazing impact standards.

An NFRC-certified performance value (U-factor, SHGC, VT) for the complete fenestration product, glass plus frame, edge-of-glass, and spacer. Always worse than center-of-glass values because the frame conducts heat and the edge zone is degraded by the spacer. Required for energy code compliance.

The percentage of a building's exterior wall area that is glazed. The IECC and ASHRAE 90.1 set prescriptive WWR limits (typically 40% maximum). Buildings exceeding the limit must use an energy model to demonstrate whole-building compliance.

See "Wired glass."