Triple Glazing Glossary

A-rated window: A window with a Window Energy Rating of band A: an energy index of 0 to under 10 kWh/m²/yr.
A+ and A++: Higher energy-rating bands: A+ is 10 to under 20 kWh/m²/yr and A++ is 20 or more. Solar gain helps windows reach them.
Acoustic laminated glass: Laminated glass with an interlayer designed to damp sound. The most effective single upgrade for noise.
Airtightness tape: Tape that seals the joint between a window frame and the wall so air cannot leak around it.
Alu-clad timber: A timber window frame with an aluminium cladding on the outside for weather protection.
Approved Document L: The Building Regulations guidance in England (and a separate edition in Wales) covering the conservation of fuel and power, including window U-value limits.
Argon: An inert gas used to fill the cavities of sealed units. It insulates better than air.
Article 4 direction: A council order removing permitted development rights in an area, often used in conservation areas to control window changes.
Astragal bar: A glazing bar that divides a pane into smaller sections, giving a traditional look.
Bay pole: A structural post fitted inside a bay window frame to carry the load of the wall or roof above.
BFRC: The British Fenestration Rating Council, which runs the Window Energy Rating scheme.
Bifold door: A door made of panels that fold against each other and slide to the side.
Building control: The council or approved inspector service that checks building work complies with the Building Regulations.
Casement window: A window with side- or top-hung sashes that open outwards.
Cavity: The gap between panes in a sealed unit, usually filled with argon. Triple units have two.
Certass: A competent person scheme for window and door installers in England and Wales.
Cill: The ledge at the bottom of a window frame that sheds rainwater away from the wall.
Competent person scheme: A government-authorised scheme letting registered installers self-certify their work meets the Building Regulations.
Composite door: A door with a solid insulated core and a glass-reinforced plastic (GRP) skin.
Condensation: Water that forms when moist air meets a cold surface.
Conservation area: An area of special architectural or historic interest where councils control changes more closely.
Dew point: The temperature at which moisture in the air condenses.
Door set energy rating (DSER): An energy rating scheme for external doors, similar to the Window Energy Rating.
Egress hinge: A hinge that lets a casement open wide enough to escape through in a fire.
EnerPHit: The Passive House Institute standard for retrofitting existing buildings.
Engineered timber: Timber laminated from several layers with alternating grain, which resists warping.
Espagnolette lock: A multi-point locking bar running along the sash edge, operated by the handle.
FENSA: A competent person scheme for window and door installers in England and Wales.
Fixed light: A pane or window that does not open.
Flush casement: A casement where the sash sits flush with the outer frame, like traditional timber windows.
Friction stay: The hinge mechanism on most casement windows that holds the sash where you leave it.
Future Homes Standard: England’s updated energy standard for new homes, in force from 24 March 2027.
g-value: The share of solar energy that passes through glass into the room, from 0 to 1.
Georgian bar: A glazing bar pattern dividing a sash into small panes, typical of Georgian windows.
Higher-risk building: In England, a building at least 18 m or 7 storeys tall with at least two residential units, subject to extra building safety rules.
Krypton: A denser inert gas than argon, used in some narrow-cavity triple units.
L-value: A measure of air leakage through a window, used in energy ratings.
Laminated glass: Two or more sheets of glass bonded with a plastic interlayer; it cracks but stays in place.
Lift-and-slide door: A sliding door whose panel lifts off its seals when the handle turns, making heavy panels easy to move.
Listed building consent: Permission needed to alter a listed building, including its windows.
Low-e coating: A microscopically thin metal oxide coating on glass that reflects heat back into the room.
Mullion: A vertical bar between window lights.
Multi-chamber profile: A uPVC frame section with several internal air chambers for insulation.
PAS 24: A British test standard for the security of doors and windows.
Passivhaus: A building standard for very low energy use, set by the Passive House Institute.
Permitted development: Building work allowed without a planning application, within set limits.
PHPP: The Passive House Planning Package, the energy model used for Passivhaus design.
Psi-value: Heat loss along a linear junction, such as the glass edge or the window-to-wall joint.
Rebate: The recess in a frame or sash that the glass unit sits in.
Reveal: The side of the wall opening around a window.
Rw: A single-figure rating of sound reduction in decibels.
Safety glass: Toughened or laminated glass required in critical locations such as doors and low-level panes.
Sash window: A window with sashes that slide vertically in a box frame.
Sealed unit: Two or three panes of glass sealed together around the edge, with gas-filled cavities between.
Secondary glazing: A separate inner window fitted behind an existing one, common in listed buildings.
Solar control glass: Glass with a coating that reduces solar heat gain.
Spacer bar: The strip around the edge of a sealed unit that holds the panes apart.
Tilt and turn: An inward-opening window that tilts at the top for ventilation or turns fully open.
Toughened glass: Heat-treated glass that breaks into small blunt pieces.
Transom: A horizontal bar between window lights.
Trickle vent: A small closable vent in a window frame providing background ventilation.
Triple glazing: Glazing with three panes of glass and two sealed cavities.
U-value: The rate of heat loss through a building element, in W/m²K. Lower is better.
Ug: The U-value at the centre of the glass only.
Uw: The U-value of the whole window, including frame and spacers.
uPVC: Unplasticised polyvinyl chloride, the rigid plastic used for most window frames.
Warm-edge spacer: A low-conductivity spacer bar that reduces heat loss and condensation at the glass edge.
Window Energy Rating (WER): A BFRC rating from A++ to E combining heat loss, solar gain and air leakage.

Cookies