Glazing terms explained

A B C D E F G H I K L M O P R S T U W

A

Acoustic laminate

Acoustic laminate is laminated glass made with a special interlayer designed to damp sound vibrations. Used in a sealed unit or secondary glazing, often with panes of different thicknesses, it reduces noise from traffic, trains and aircraft more than standard laminated glass of similar thickness.

Anti-bandit glass

Anti-bandit glass is a thick laminated glass with multiple interlayers, designed to resist sustained manual attack with tools such as hammers or axes. It is used for shopfronts, counters and other high-risk glazing where the glass needs to delay entry.

Approved Document K

Approved Document K is the government guidance for England on protection from falling, collision and impact. Section 5 covers safety glazing in critical locations. Wales uses Approved Document N for glazing safety, Scotland its technical handbooks and Northern Ireland Technical Booklet V.

Approved Document L

Approved Document L is the guidance on conservation of fuel and power. In England, Volume 1 sets limiting U-values for replacement windows and doors in existing dwellings. Wales publishes its own Approved Document L, while Scotland and Northern Ireland use their own handbooks and technical booklets.

Approved Document Q

Approved Document Q is the guidance for England on security in dwellings. It sets standards for doors and windows to resist physical attack by burglars, and requirement Q1 applies only to new dwellings, not to replacement windows and doors in existing homes.

Argon fill

Argon fill means the cavity of a sealed unit is filled with argon, an inert gas that conducts heat less readily than air. It improves the unit's insulation without changing its appearance. Some gas gradually escapes over the life of the unit, more quickly if the seal fails.

B

Back-painted glass

Back-painted glass has a coloured paint or coating applied to its rear face, so the colour is seen through the glass with a glossy finish. It is used for splashbacks and wall cladding, and is often toughened where it sits behind a hob.

Balustrade

A balustrade is a barrier that protects people from falling at the edge of a stair, landing, balcony or terrace. Glass balustrades use toughened or laminated toughened panels, framed, posted or frameless, and must be designed to resist the loads expected for the building's use.

Bevel

A bevel is a sloping edge ground and polished around the face of a pane of glass or mirror, giving a light-catching framed border. The bevel width is specified when ordering. Bevelled edges are common on decorative mirrors and traditional door glass.

Bifold door

A bifold door is a set of glazed panels hinged together that fold back on each other and stack at one or both sides of the opening, clearing almost the full width. Most sets include a single traffic door that opens on its own for everyday use.

BS 6262-4

BS 6262-4 is the British Standard code of practice for glazing safety related to human impact. It recommends glass types for locations where people may collide with glazing, including sloping overhead glazing. Scotland's technical handbooks refer to it. The current edition was published in 2018.

BS EN 12150

BS EN 12150 is the European standard, published in the UK by BSI, for thermally toughened soda lime silicate safety glass used in buildings. Part 1 defines the product, including how it must fragment when broken. Toughened panes are often marked with this standard number.

BS EN 12600

BS EN 12600 is the pendulum impact test for flat glass. It classifies glass into classes 1, 2 and 3 by drop height and records how it breaks: Type A like ordinary glass, Type B like laminated glass and Type C like toughened glass. Codes such as 1(C)1 summarise the result.

BS EN 14449

BS EN 14449 is the product standard for laminated glass and laminated safety glass used in buildings. It sets out how the product's conformity is evaluated and how manufacturers control production. Laminated safety glass is often marked with this standard number and its impact classification.

Building notice

A building notice is a type of building regulations application in England and Wales suited to smaller projects. Work can start two days after the notice is given to the building control body, which then inspects the work. It is an alternative to using a competent person scheme installer.

Building warrant

A building warrant is the approval needed before many types of building work start in Scotland. Replacing a window, door or rooflight including its frame does not need one, but the work must still meet the building standards. Altering the size of an opening generally does need one.

C

Casement window

A casement window has one or more sashes hinged at the side or top, opening outwards or inwards like a door. Casements are made in uPVC, aluminium, timber and composite frames, usually with fixed panes alongside the opening sashes.

Certass

Certass is one of the competent person schemes authorised for installing replacement windows and doors in existing homes in England and Wales. Registered installers can self-certify that the work complies with building regulations, instead of the homeowner applying to building control, and a compliance certificate is issued.

Cill

A cill, also spelt sill, is the horizontal section at the bottom of a window frame or opening that sheds rainwater away from the wall. Window cills can be uPVC, aluminium, timber, stone or brick, and an internal board often sits on the room side.

Competent person scheme

A competent person scheme lets registered installers self-certify that certain building work, such as replacement windows and doors, complies with building regulations in England and Wales. It replaces the need for the homeowner to apply to building control. Scotland and Northern Ireland use different arrangements.

Condensation

Condensation is water that forms when warm, moist air meets a cold surface such as glass. On the room-side face it points to humidity and ventilation; on the outside face it can occur with well-insulated glass. Condensation between the panes of a sealed unit means the seal has failed.

Conservation area

A conservation area is an area of special architectural or historic interest designated locally, where changes are managed to preserve or enhance its character. Replacement windows may need to match the existing style, and some permitted development rights can be removed.

Critical location

A critical location is where people are likely to collide with glass, so safety glazing is required. Guidance in England, Wales and Scotland covers glass in doors and within 300mm of a door edge up to 1500mm above the floor, and other glass within 800mm of floor level. Northern Ireland has its own guidance.

Curtain walling

Curtain walling is a non-loadbearing external wall system, usually aluminium framing with glass or panels, hung from the building's structure. It keeps out the weather and transfers its own weight and wind loads back to the frame. It is common on commercial and multi-storey buildings.

Cylinder

A cylinder is the replaceable lock barrel that the key turns in most uPVC and composite door locks. Anti-snap, or snap-secure, cylinders are designed to resist attacks that break the cylinder to open the lock. When replacing one, ask which recognised security standard it has been tested to.

D

Desiccant

Desiccant is the drying agent inside the spacer bar of a sealed unit. It absorbs moisture trapped when the unit is made and any that enters slowly over its life. Once it is saturated, usually because the seal has failed, the unit begins to mist between the panes.

E

Espagnolette

An espagnolette is a window locking mechanism in which turning one handle drives a bar that engages several locking points around the sash. It is common on modern casement and tilt and turn windows and gives more even security than a single latch.

External beading

External beading means the glazing beads are on the outside face of the window. It was common on older uPVC windows. Because beads can potentially be removed from outside, security tape or clips, or replacement with internally beaded windows, are often recommended.

F

Fanlight

A fanlight is a window above a door, originally semicircular with radiating glazing bars like an open fan. The term is now used for any glazed panel above a door or above the transom of a window. Fanlights may be fixed or opening.

FENSA

FENSA is one of the competent person schemes authorised for installing replacement windows and doors in existing homes in England and Wales. A registered installer self-certifies that the work meets building regulations, the local authority is notified, and the homeowner receives a certificate.

Friction stay

A friction stay is the hinge mechanism on many uPVC and aluminium casement windows, fitted at the top and bottom of the sash. It holds the window open at any angle by friction. Some types allow the sash to open wide for escape or slide across for cleaning.

G

g-value

The g-value, or solar factor, is the proportion of the sun's heat that passes through glass into a room, given as a figure between 0 and 1. A lower g-value means less solar heat gain, which helps prevent overheating in sun-facing rooms, conservatories and roof glazing.

Georgian bar

A Georgian bar is a glazing bar that divides a window into smaller panes for a traditional look. In double glazing it is usually fitted inside the sealed unit between the panes, or applied to the outside faces of the glass, rather than holding separate pieces of glass.

Glazing bead

A glazing bead is the strip of uPVC, aluminium or timber that holds a sealed unit or pane of glass in its frame. Removing the beads allows the glass to be taken out and replaced without removing the frame from the wall.

Glazing packer

A glazing packer is a small plastic shim placed under and around a sealed unit within the frame. Packers carry the weight of the glass at the correct points, keep sashes square and stop the unit touching the frame. Missing or misplaced packers can cause dropped sashes and seal failure.

H

Handrail

A handrail is the rail you hold for support along a stair, ramp or balustrade. On glass balustrades it may sit on top of the glass as a capping rail or be fixed separately. In some designs it also ties the panels together if one breaks.

Heat-soak test

A heat-soak test is a factory process in which toughened glass is heated and held at temperature so that panes containing nickel sulphide inclusions break before installation rather than afterwards. It is defined in BS EN 14179-1. It reduces the risk of spontaneous breakage but does not remove it entirely.

I

IGU (insulating glass unit)

IGU stands for insulating glass unit, the technical name for a sealed double or triple glazed unit. The term appears on manufacturers' specifications and quotes, often with the overall thickness and the glass, cavity and spacer make-up listed pane by pane.

Internal beading

Internal beading means the glazing beads are on the room side of the window, so the glass can only be removed from inside. It is generally preferred for security on modern windows, because the glass cannot be taken out from outside without breaking it.

K

Kitemark

The Kitemark is a certification mark owned and operated by BSI. It shows that a product has been independently tested and certified by BSI against a specified standard. When you see it on glass, locks or door hardware, check which standard the certification covers.

L

Laminated glass

Laminated glass is two or more panes bonded together with a plastic interlayer. If it breaks, the interlayer holds the pieces in place, so the pane stays in its frame. It is used for safety, security, noise reduction and overhead glazing. The product standard is BS EN 14449.

Leaded light

A leaded light is a window made of small pieces of glass held together by lead strips, called cames. Modern versions often apply lead strip to the surface of a sealed unit to recreate the look. Original leaded lights may be worth repairing rather than replacing, especially in listed buildings.

Line load

A line load is the horizontal force, measured in kilonewtons per metre, that a balustrade or barrier must resist along its top edge. The required figure depends on how the space is used, so a domestic stair and a public balcony need different designs.

Listed building consent

Listed building consent is permission needed for works that would affect the character of a listed building, inside or out, which can include replacing windows. It applies across the UK and is decided by the local planning authority or council. Carrying out such works without consent is a criminal offence.

Low-E glass

Low-E, short for low-emissivity, glass has a microscopically thin coating that reflects heat back into the room while letting light through. It is standard in modern double glazing and is one of the main ways sealed units meet energy requirements for replacement windows.

M

Mullion

A mullion is a vertical bar that divides a window frame into separate sections, or lights. It is part of the frame structure, unlike a Georgian bar, which is decorative. Mullions are common in bay windows and in older stone or timber windows.

Multipoint lock

A multipoint lock secures a door at several points along its edge, typically a central latch and deadbolt plus hooks, rollers or bolts above and below. It is standard on uPVC and composite doors and is operated by lifting the handle and turning the key.

O

Obscure glass

Obscure glass is patterned or textured glass that lets light through while blurring the view, used in bathrooms, front doors and side panels. Manufacturers describe patterns by how much privacy they give, from lightly to heavily obscured. It can be toughened or laminated for use as safety glass.

P

Permitted development

Permitted development rights allow certain building work without a planning application. In England, replacement windows and doors of a similar appearance to those on the existing house usually fall within them. The rights do not remove the need for listed building consent and can be withdrawn by an Article 4 Direction.

Polished edge

A polished edge is a glass edge that has been ground smooth and polished to a clear, glossy finish. It is used where the edge is visible, such as on mirrors, tabletops, shelves, splashbacks and frameless glass, and is safer to handle than a raw cut edge.

Polysulphide seal

A polysulphide seal is a type of secondary sealant applied around the edge of a sealed unit, outside the spacer bar, to bond the panes together and keep moisture out. Other secondary sealants include polyurethane and silicone, chosen to suit how the unit will be glazed.

PVB interlayer

PVB, or polyvinyl butyral, is the plastic interlayer most commonly used to bond laminated glass. It holds broken glass together, adds resistance to break-in and reduces the ultraviolet light passing through. Acoustic grades of PVB are made to reduce noise transmission.

R

Reveal

A reveal is the side of a window or door opening, the part of the wall between the frame and the face of the building. Internal reveals are often plastered and may need making good after a new frame is fitted. Secondary glazing is usually fixed within the reveal.

S

Safety-backed mirror

A safety-backed mirror has a film applied to its back that holds the glass together if it breaks, reducing the risk of injury from falling pieces. It is used for wardrobe doors, gym and studio walls, and other places where people could knock into the mirror.

Sash

A sash is the opening part of a window that holds the glass and moves within the outer frame. In a traditional sliding sash window, two sashes slide vertically past each other, balanced by weights or spring balances. The term is also used generally for any opening vent.

Sealed unit

A sealed unit is the double or triple glazed panel fitted into a window or door frame: two or more panes held apart by a spacer bar and sealed around the edge to keep the cavity dry. When the seal fails, the unit mists between the panes and is replaced as one piece.

Soft coat

A soft coat is a low-emissivity coating applied to glass after it is made, using vacuum deposition, sometimes called sputtering. It generally gives lower heat loss than a hard coat but is delicate, so it must face into the sealed cavity of a double or triple glazed unit.

Solar control glass

Solar control glass has a coating or body tint that reduces the amount of the sun's heat passing through while still letting in daylight. It helps limit overheating in conservatories, roof lanterns and large south- or west-facing glazing, and its performance is described by its g-value.

Spacer bar

A spacer bar is the strip around the edge of a sealed unit that holds the panes a set distance apart. It is usually hollow and filled with desiccant, and it is bonded to the glass with sealants. Spacers are made in aluminium or lower-conductivity warm edge materials.

Stick system

A stick system is curtain walling assembled on site from separate mullions, transoms and glazing panels, piece by piece. It suits low- and mid-rise buildings and complex shapes and allows adjustment on site, but involves more site work than a unitised system.

Structural glazing

Structural glazing is glass that is bonded, bolted or clamped to a supporting structure with little or no visible frame, so the glass forms the face of the building. It requires engineering design, suitable glass such as laminated toughened, and specialist installation.

T

Thermal break

A thermal break is an insulating barrier, usually a polyamide or resin strip, built into an aluminium window or door frame between the inner and outer metal sections. It stops heat passing straight through the metal, improving insulation and reducing condensation on the frame.

Threshold

A threshold is the bottom part of a door frame that you step over. Low and flush thresholds make access easier but depend on good drainage to keep water out. In Wales, a replacement main entrance door in a dwelling built from 1999 onwards should keep its level threshold.

Toughened glass

Toughened glass, also called tempered glass, is heat treated to make it considerably stronger than ordinary glass. When broken it shatters into small, blunt pieces, so it is used as safety glass. It cannot be cut or drilled after toughening. The product standard is BS EN 12150-1.

Transom

A transom is a horizontal bar dividing a window or door frame into upper and lower sections. In a door frame, the glazed section above the transom is called a fanlight or toplight. Like a mullion, a transom is part of the frame structure.

Trickle vent

A trickle vent is a small controllable opening, usually in the window frame, that lets a background flow of fresh air into a room while the window is closed. In England, replacement windows should include them where the old windows had them, at least as large as before.

U

U-value

A U-value measures how readily heat passes through a window, door or pane of glass, in watts per square metre per degree of temperature difference, written W/(m²·K). The lower the figure, the better the insulation. Building regulations set maximum U-values for replacement windows and doors.

Unitised system

A unitised system is curtain walling made in a factory as large pre-glazed panels, which are lifted into place and interlocked on site. It reduces work at height and suits tall or repetitive facades, but needs more planning, lifting equipment and lead time than a stick system.

W

Warm edge spacer

A warm edge spacer is a spacer bar made from a material that conducts less heat than aluminium, such as plastic, composite or stainless steel. It keeps the edge of the glass warmer, which improves thermal performance and reduces condensation around the edge of the pane.

Window Energy Rating (WER)

A Window Energy Rating grades a whole window's energy performance on a scale from A++ to E, taking account of heat loss, solar gain and air leakage. The British Fenestration Rating Council runs the scheme. In England, replacement windows can comply by reaching Band B or better.