Double Glazing Glossary of Terms

Making Window Industry Terminology Clear

Every industry has its own language. When it comes to windows and glazing you may well need a bit of interpretation to know what your window installer is talking about when he’s giving you a quote or working in your home.

In the interests of transparency, we’ve put together an A to Z glossary of terms to make things clear:


A-G energy ratings: Windows are rated on a scale of A to G, with A indicating the most energy efficient glass products. It’s a scale devised by the British Fenestration Ratings Council. Find out more here.

Acid Etching: A decorative process in which the glass surface is treated with hydrofluoric acid to make it smooth and opaque.

Acoustic Glass: Two or more sheets of glass bonded together to provide acoustic insulation for noise reduction.

Annealed Glass: A process in which hot float glass is gently cooled to release internal stresses so it can be further cut and processed after manufacture.

Argon Gas: The inert, non-toxic gas that fills the gap between the panes of double-glazed windows to improve their thermal efficiency.


Bead: A moulding or strip placed around a window frame to hold the glass in place by pressure.

Bevelled Edge: Glass edges that have been ground and polished to an angle, refracting light.

Butyl: A synthetic rubber used to seal insulated glass units to make them air and water-tight.


Cames: The narrow strips (usually lead, zinc or copper) which hold together the pieces of glass in stained glass windows.

Casement: The part of a window that opens, usually secured by a hinge fixed to the side, top or bottom of the window frame.

Cavity: The space between two panes of glass in a double-glazed unit, usually filled with air, or with argon gas for enhanced thermal performance.

Cullet: Waste broken glass that is recycled and used to make new glass.


Dessicant: A porous substance which is used to absorb moisture inside the sealed space of an insulating glass unit.

Double Glazing: Using two panes of glass, with a cavity inbetween, in a window to increase energy efficiency and improve sound-proofing.

Dry Glazing: A method of securing glass in a window frame with a gasket without using a glazing compound.


Electrochromic glass (smart glass): Electronically tintable window glass which can be controlled by a building’s occupants to clear and darken, enabling the management of light and glare without cutting out outdoor views.

Emissivity: A measure of how efficiently a surface – such as glass – radiates heat. The emissivity of glass can be regulated by coatings.


Fenestration: A term that describes the positioning and fitting of a window in a structure.

Fire resistant glass: A type of glass that provides a barrier against the transmission of lames, smoke and gases for a longer period than normal glass.

Float Glass: Sheets of glass made by floating molten glass on a bed of melted metal (usually tin). The glass is cooled very slowly in an oven called a lehr in a process called annealing, which relieves the internal stresses.


G Value: The G Value of glass is a measure of how well the glass transmits heat from the sun (solar energy). It is usually notated as a percentage. The more layers of glass there are, and/or the more coatings there are on the glass, the more there is to block the sun’s heat from penetrating inside a building, so the lower the G value will be.

Gaskets: Shaped extruded rubber parts used for securing and positioning glass into a frame.

Glazing: The act of installing glass into a frame.

Glazing bead: Metal or wood strips used to fix the glass into a frame.

Glazing tape: A double-sided tape used for installing single pane glass and double-glazed units into timber or steel windows and doors.


Inner pane: The pane of a double-glazed unit which faces to the inside.

Interlayer: The material used to bond leaves of glass together to make laminated glass.


Laminated Glass: Two or more glass sheets bonded together with interlayers of plastic or resin material. This provides glass that is held together and doesn’t fragment when broken.

Leaded Glass: Glass pieces joined together with lead strips (known as cames) which is soldered together to form a composite pane.

Low Emission Glass (Low-E glass): Clear glass with an extremely thin coating of metal oxide, which allows heat and light to pass through it into a building, but blocks the heat from radiating out of the room, reducing heat loss and saving energy.


Manifestation: Markings (a sticker, or an etching) applied to expanses of glass to make it visible and therefore avoid accidents.

Mullion: A vertical division between parts of a window – it could be decorative or supportive (as part of the frame).


Obscure Glass: Glass deliberately created with an uneven surface designed to diffuse light and offer privacy by obscuring the view inside and out.

Opacified Glass: Glass which has been enamelled or painted on one side to render it non-transparent.

Outer Pane: The pane of a double-glazed unit which is on the outside of a building.


Patterned Glass: Glass that is textured by having a pattern embossed on one side of the hot glass during the manufacturing process.

Photovoltaic (PV) glass: Using the same principle as solar panels (although it is transparent) this glass has integrated solar cells which convert energy into electricity.

Plate Glass: Largely overtaken by float glass (which is superior for most applications), plate glass is a soda-lime-silica glass created by rolling hot glass into a plate, subsequently being ground and polished smooth. This was the original way of producing window glass.

Polyvinyl Butryral (PVB): The commonly used plastic for the interlayer between glass leaves in laminated glass, preventing it from fragmenting when broken.


Rebate: The recess in a frame into which a glass pane is placed and secured.

Resin laminate: Laminated glass assembled by using a resin interlayer instead of PVB.


Safety Glass: Glass that is specifically designed to be less likely to break, and therefore less likely to cause an injury if it does break. There are two types of safety glass – laminated glass and tempered glass. Safety glass is mandatory in some situations.

Sandblasting: Firing fine sand at glass to create a frosted, translucent glass appearance.

Self-cleaning glass: Glass treated with an ultra-thin coating of titanium dioxide, resulting in the surface repelling water and reducing the adherence of dirt particles, which run off the surface without streaking.

Sill: The ledge that forms the bottom part of the window, inside and/or out.

Spacer Bar: A spacing bar along the edges of a double-glazed unit, filled with dessicant, designed to separate the panes of glass and create a cavity.


Tempered/Toughened Glass: This is a type of safety glass processed to increase its strength by controlled thermal or chemical treatments. It is much stronger than normal glass and resistant to breakage, but if it does break it shatters into small granular pieces rather than sharp shards.

Transom: A horizontal division in a window.

Trickle Vents: A small opening at the top of a window to allow for ventilation.


U Value: A measure of the rate of heat loss through a building component – such as a window or a solid wall. It is expressed in W/m2k (watts per square metre per degree Kelvin). The lower the U value of a window the better.


Warm Edge: This is a type of technology relating to the type of spacers used in a double glazed unit that conduct less heat or cold than standard units.

Contact us today for your free quotation

For general enquiries you may wish to fill in the contact form below and someone will get back to you.

We mostly operate across Gloucester, Wiltshire & Avon, but will gladly accept work in any area eg. Box, Enfield, Newcastle etc.

We have over 26 years of experience in glazing, call us today on 01249 651515 for a free quote on your glazing requirements or filling in our contact form.


Unit G, Ivy Road, Chippenham,
SN15 1SB,

01249 651515

Bath: 01225 632297

Malmesbury: 01666 460103

Hours of trade

8.30am-5pm Mon-Fri

Although we do offer a 24 hour call out service for emergency board ups.