Knowledge centre / Glazing terminology
A practical guide to the language used across aluminium doors, windows and architectural glazing.
Search a term or browse by category. Definitions are written in plain English, with the detail that matters when you are comparing systems.
Glossary finder
A measure of how effectively a glazing system reduces sound passing through it.
Usually expressed as a weighted sound reduction value in decibels. The glass build-up, seals, frame and installation all influence the result.
dB (decibel) IGUThe amount of air that passes through a closed window or door when it is exposed to a pressure difference.
A higher tested classification indicates better resistance to unwanted air leakage. Site conditions and installation quality still matter.
Weather performance Wind resistanceA continuous aluminium profile formed by pushing heated aluminium through a shaped die.
Extrusions create the frame, sash, track and other profiles that make up an aluminium glazing system.
Outer frame Thermal breakAn electrochemically formed protective oxide layer on aluminium.
Anodising creates a durable metallic finish. Colour and appearance differ from polyester powder coating.
Powder coating Aluminium extrusionA component that helps prevent a sliding panel being lifted out of its track from the outside.
It forms one part of a system's security design and should not be considered in isolation.
Multipoint lock PAS 24An inert gas commonly used between panes in an insulated glass unit to reduce heat transfer.
Argon performs better thermally than ordinary air when the cavity and spacer are correctly designed.
IGU U-valueA method where removable profiles, called glazing beads, mechanically retain the glass in the frame or sash.
Beads allow the glass unit to be replaced without replacing the complete sash. Bead position varies by system.
Glazing bead Bonded glazingA door made from hinged panels that fold and stack to one or both sides of the opening.
Panel sizes, stack direction, threshold choice and hardware determine how the door operates and looks.
Traffic door ThresholdA construction method where the glass is structurally bonded to the sash with an approved adhesive.
Bonding can make the glass part of the sash's structural performance and can support slimmer visible profiles.
Beaded glazing Structural siliconeA glass-to-glass junction with little or no visible corner post, usually sealed with silicone.
Often used at fixed corners to create a more open view. Structural requirements and movement must be considered.
Glass-to-glass corner Structural siliconeA window sash that opens on hinges, typically at the side, top or bottom.
Casements can be configured for different ventilation and escape requirements.
Sash Friction stayA lower frame profile that projects outward and helps direct water away from the building.
In UK window terminology, cill is commonly used for the external profile; sill often refers more broadly to the base or internal ledge.
Threshold DrainageA frame designed to be hidden or substantially covered by the surrounding construction or finishes.
The effect depends on correct interface detailing, tolerances, drainage and access for future maintenance.
Frameless glazing Outer frameWater that forms when moist air meets a surface cold enough for the vapour to become liquid.
Condensation on room-side or external glass can be environmental. Moisture inside an IGU cavity usually indicates seal failure.
IGU Thermal breakA configuration in which panels open away from a corner without a permanent fixed corner post.
It can create a wide, uninterrupted connection between two elevations. Structural support must be resolved separately.
Open corner Sliding doorA non-loadbearing external wall system, commonly formed from aluminium framing and glass or opaque panels.
Curtain walling transfers wind and its own weight back to the building structure rather than carrying floor or roof loads.
Mullion TransomThe unit used to express sound level and sound reduction performance.
A decibel figure needs its full test descriptor and context; it is not a simple percentage reduction.
Acoustic insulationA formal document declaring a construction product's performance against relevant essential characteristics.
The declared values relate to the product and standard stated in the document; check that it matches the system and configuration being specified.
UKCA marking PAS 24The amount a profile, panel or glass pane bends under load.
Allowable deflection depends on the component, glass type, span, support and applicable design criteria.
Wind resistance Maximum sash weightAn insulated glass unit made from two panes separated by a sealed cavity.
Glass thicknesses, coatings, spacer and cavity gas are selected to meet safety, thermal, solar and acoustic requirements.
IGU Triple glazingThe designed routes that collect and discharge water from glazing rebates, tracks and frame chambers.
Drainage paths and outlets must remain clear, and external interfaces must be installed to the system detail.
Weep holes ThresholdA glazed area that does not open.
Fixed lights can be standalone or combined with opening windows and doors, often with slimmer profiles because no operating hardware is required.
Outer frame SashA sliding arrangement in which open panels align neatly within the same stack or pocket zone.
The exact stack position and visible frame dimensions depend on the number of tracks and system configuration.
Pocket door TrackA threshold designed to align closely with the adjacent finished floor levels.
It can improve visual continuity and accessibility, but drainage, exposure and weather performance require careful project-specific detailing.
Threshold DrainageArchitectural glazing designed to minimise visible framing.
The term describes the visual effect; glass still needs suitable support, fixings and weather sealing somewhere within the construction.
Concealed frame Butt jointA hinged mechanism that supports and controls the movement of a casement window sash.
The stay type and size are selected for sash dimensions, weight, opening angle and any safety requirements.
Casement window RestrictorThe proportion of solar energy that passes through glazing, directly and as re-radiated heat.
A lower g-value reduces solar heat gain; a higher value admits more solar energy. It should be balanced with daylight and thermal goals.
Solar control glass U-valueA resilient seal fitted between components to help control air and water leakage and cushion the glass.
Gaskets should remain continuous, correctly seated and compatible with adjacent materials.
Weather seal Glazing beadA corner formed by two glass panes meeting with minimal visible framing.
The junction may use a silicone butt joint or a specialised detail. Loads, glass movement and edge quality must be designed correctly.
Butt joint Structural siliconeA removable profile used to retain a glass unit in a frame or sash.
Beads may be internal or external depending on the tested system design and security strategy.
Beaded glazing GasketThe horizontal top member of a window, door or glazed frame.
Loads, fixings, tolerances and drainage above the head need to be coordinated with the building structure.
Jamb Outer frameA controlled heating process used after toughening to reduce the risk of breakage from nickel sulphide inclusions.
It reduces risk but cannot eliminate every possible cause of spontaneous glass breakage.
Toughened glass Laminated glassInsulated glass unit: two or more panes sealed around a spacer to create one or more cavities.
The complete build-up may combine coatings, gas, laminated panes and different thicknesses to achieve required performance.
Double glazing Spacer barThe meeting profiles where adjacent sliding panels overlap when the door is closed.
Interlock width is often a key part of the central sightline on a sliding door.
Sightline Sliding doorA vertical side member of a window or door frame.
The jamb connects the head and threshold or cill and carries fixings, seals or locking keeps depending on the configuration.
Head Outer frameThe frame-mounted component that receives a lock's hook, bolt or latch.
Keeps must align correctly so the locking points engage without excessive force.
Multipoint lock Lock cylinderTwo or more glass plies bonded with an interlayer that helps retain fragments if the glass breaks.
Different laminated build-ups can contribute to safety, security, acoustic or structural requirements.
Toughened glass Heat soak testingThe keyed component that operates a door lock mechanism.
Cylinder type, protection and compatibility contribute to the complete door's security performance.
Multipoint lock KeepA reduced-height door threshold intended to ease passage compared with a standard rebated threshold.
Low and flush thresholds are not automatically identical; confirm the actual upstand, drainage and weather rating for the chosen detail.
Flush threshold ThresholdA microscopically thin, low-emissivity coating that reduces radiant heat transfer through glass.
Low-E coatings help improve thermal insulation and are usually positioned on a protected surface within an IGU.
U-value IGUThe greatest opening panel weight permitted by a system and its hardware.
It is a technical limit, not a recommended target. Dimensions, aspect ratio, glass and operating forces can impose tighter constraints.
Sash RollerThe vertical edges of two door leaves or window sashes that meet when closed.
On sliding systems the equivalent meeting zone is commonly described as the interlock.
Interlock SashA sliding door operated by an electric drive rather than solely by hand.
Power, controls, safety sensors, access, manual release and maintenance must be coordinated during design.
Sliding door Maximum sash weightA vertical framing member dividing adjacent areas of glazing or separate window or door units.
Mullions may carry wind load and support adjacent glass or opening elements.
Transom Curtain wallingA locking mechanism that secures a sash or door at several points along its edge.
The complete result depends on the lock, keeps, cylinder, hardware, frame, glazing and installation.
Keep PAS 24A door arrangement that removes the usual fixed post when both sides are opened.
The building structure above must be independently supported and the meeting detail must be designed for the intended exposure.
Corner opening Meeting stileA window that projects outward from the face of a building without extending to ground level.
Modern oriel windows may use structural glass and minimal framing, but support and movement remain key design considerations.
Frameless glazing Structural glazingThe fixed perimeter frame secured to the building opening.
It receives opening sashes, fixed glazing or tracks and interfaces with membranes, sealants, insulation and finishes.
Sash JambA UK test specification for enhanced security performance of doorsets and windowsets.
A PAS 24 claim must relate to the tested product scope and configuration; it does not mean a product is burglar-proof.
Multipoint lock UKCA markingA sliding panel designed to disappear into a purpose-built pocket beside the opening.
The pocket must accommodate the panel, tracks, drainage and tolerances while allowing suitable inspection and maintenance access.
Sliding door Flush stackingA low-conductivity polyamide section connecting the inner and outer aluminium profiles.
It reduces direct heat flow through the metal while maintaining a structural connection.
Thermal break Aluminium extrusionA factory-applied dry powder finish that is electrostatically deposited and cured onto aluminium.
Correct pretreatment, powder specification and maintenance are important for appearance and durability, particularly in exposed locations.
RAL colour Anodised finishA colour reference from the widely used RAL colour matching system.
The same RAL reference can appear different with gloss level, texture, substrate and lighting, so use a physical sample when colour matching is critical.
Powder coatingA device that limits how far a window or door can open.
Restrictors may be used for safety, ventilation or protection, but the required type and release method depend on the application.
Casement window Friction stayA wheeled carriage that supports and guides a sliding panel along its track.
Roller capacity, adjustment, cleanliness and correct track installation influence operating effort and service life.
Track Maximum sash weightA raised glazed roof structure, often formed with pitched sides around a ridge or central apex.
It differs from a flat rooflight and requires coordinated weathering, support, drainage and thermal detailing.
Rooflight UpstandA glazed unit installed in a roof to bring daylight into the space below.
The required product and detail depend on roof type, pitch, upstand, loads, drainage and whether the unit opens.
Roof lantern UpstandThe framed part of a window or door that holds the glass and may open or slide within the outer frame.
In sliding doors, each moving glazed panel can be described as a sash.
Outer frame Maximum sash weightAn opening window arrangement designed to create a counter-level pass-through between spaces.
Sliding, bifold or hinged configurations may be used, with project-specific consideration for support, guarding and weathering.
Sliding door ThresholdThe visible width of framing when viewing a closed glazing system face-on.
Sightlines are often quoted at the interlock, jamb, head or threshold; compare like-for-like positions.
Interlock System depthA glazed door panel that moves horizontally along a track.
Systems may use one or more tracks and combine sliding, fixed, pocket or open-corner panels.
Track InterlockGlass designed to limit the amount of solar heat entering a building while admitting useful daylight.
Its performance is commonly described using g-value and light transmission alongside colour and reflectance.
g-value Low-E coatingThe perimeter component that separates panes and contains or works with desiccant within an IGU.
Spacer material and design affect edge-of-glass thermal performance, appearance and unit construction.
Warm-edge spacer IGUA glazing approach in which glass or bonded joints perform a structural role within the façade or assembly.
It requires engineered glass, compatible adhesives or fixings, controlled manufacture and project-specific design.
Structural silicone Bonded glazingA specialist silicone used in designed joints to transfer loads as part of a structural glazing assembly.
It must be selected, sized and applied under controlled conditions with verified adhesion and compatibility.
Butt joint Structural glazingThe overall front-to-back dimension of a frame or profile system.
Greater depth does not by itself guarantee better performance; profile design, thermal break, seals and glass are also important.
Sightline Thermal breakA low-conductivity section separating the internal and external aluminium parts of a frame.
It reduces heat flow and the risk of cold internal surfaces compared with an unbroken aluminium profile.
Polyamide thermal break U-valueThe rate of heat flow through a building element for a given temperature difference.
It is expressed as a U-value in W/m²K; lower values indicate less heat transfer.
U-value W/m²KThe lower frame or track detail across a door opening.
Threshold choice affects access, drainage, weather resistance, visible upstand and interface with floor finishes.
Flush threshold Low thresholdA window that can tilt inward for ventilation or swing inward like a side-hung casement.
One handle controls the two operating modes through multipoint hardware.
Casement window RestrictorHeat-treated safety glass with increased strength that breaks into many relatively small pieces when fractured.
It behaves differently from laminated glass and does not normally remain in the opening after breakage.
Laminated glass Heat soak testingThe channel or rail along which a sliding panel travels.
The number of tracks determines possible panel arrangements; cleanliness and drainage are important to operation.
Roller Sliding doorThe first-opening leaf in a bifold set, used for everyday access without folding the complete system.
Its handing and position should be agreed when the door configuration is specified.
Bifold door Multipoint lockA horizontal framing member dividing areas of glazing or separate window elements.
It may support glass edges, hardware or adjacent frame sections and transfer loads to the surrounding structure.
Mullion Curtain wallingAn insulated glass unit made from three panes and two sealed cavities.
It can improve thermal and acoustic performance, but adds thickness and weight that must suit the selected system.
Double glazing IGUA measure of how readily heat passes through a building element, expressed in W/m²K.
Lower is better for thermal insulation. Always check whether a figure is for the glass alone or the complete window or door.
Ug / Uw / Ud Thermal transmittanceU-value notation distinguishing glass, complete windows and complete doors.
Ug is centre-of-glass or glazing performance, Uw is the whole window and Ud is the whole door. They are not interchangeable.
U-value IGUA conformity marking used for applicable products placed on the market in Great Britain under relevant legislation.
Marking requirements and transition arrangements can change; confirm the current rules and product scope for the intended market.
Declaration of Performance (DoP) PAS 24A raised kerb or supporting construction above the roof surface on which a rooflight is installed.
Height, insulation, waterproofing, structural support and dimensions must match the rooflight and roof build-up.
Rooflight Roof lanternWatts per square metre kelvin: the unit used for thermal transmittance or U-value.
It states heat flow per square metre for each degree of temperature difference; a lower U-value means better insulation.
U-value Thermal transmittanceAn IGU spacer made from material with lower thermal conductivity than a conventional aluminium spacer.
It reduces heat loss around the glass edge and can improve internal edge temperatures.
Spacer bar IGUThe resistance of a closed glazing system to water penetration under a specified pressure test.
A laboratory classification relates to the tested specimen; real performance also depends on size, exposure, drainage and installation.
Drainage Air permeabilityA gasket, brush or flexible seal that helps resist air and water at a joint between moving or fixed profiles.
Seals must be continuous, correctly compressed and maintained without incompatible chemicals.
Gasket Air permeabilitySmall drainage outlets that allow collected water to leave a frame, track or glazing cavity.
They must remain open and discharge to a suitable external location.
Drainage ThresholdA system's ability to withstand wind pressure without unacceptable damage or deformation.
Classification is based on defined testing. Building height, location, opening size and local exposure determine project wind loads.
Deflection Air permeabilityTry a broader search or reset the filters.
01Commonly confused
Similar phrases can describe very different things. These are the distinctions worth knowing before you specify.
Glass is adhered to the sash and may contribute structurally.
Glass is mechanically retained by removable glazing beads.
How readily heat passes through a product. Lower means better insulation.
How much solar energy enters through glass. Lower means less solar gain.
Breaks into many relatively small pieces and usually leaves the opening.
The interlayer retains broken fragments and helps the pane stay together.
The width of frame you see face-on.
The front-to-back dimension of the frame profile.
A base profile or projection primarily used to shed water.
The bottom detail people cross at a door opening.
Performance of a defined specimen under controlled test conditions.
Also affected by size, location, interfaces, workmanship and maintenance.