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Glossary of terms

Your complete guide to window and door industry terminology

Do you know your mullions from your main frames? Check out our helpful A-to-Z guide to the terms and phrases commonly used in the building and plastics industry.

A-G ratings

British Fenestration Ratings Council (BFRC) energy ratings are a simple, clear and easily understandable method of informing customers about the relative energy efficiency of particular products. Windows are rated on an A to G scale, in the same way as white goods, light bulbs and other consumer products. An A-rating indicates the most energy efficient products, a G-rating the least efficient.


Another name for a soffit.


British Board of Agrement. The BBA is the UK’s major approval body for new construction products and installers. BBA Agrément Certificates are recognised by specifiers and other industry decision-makers as proof that the products covered by them have been rigorously assessed, will comply with Building Regulations and will last for a defined period.

Bifold door

A contemporary door system comprising three or more large glass panes which fold back fully upon themselves to create a large opening.


British Plastics Federation. The BPF is the leading trade association of the UK plastics industry, representing 80 per cent of industry turnover.

Building Regulations

Building Regulations are Government codes which set out the criteria that new building projects and refurbishment work must conform to. The parts of the Building Regulations that apply to windows and doors include Part M, Part L and Part F. They help to ensure that buildings are safe, secure and energy efficient.

Butt hinge

A hinge that allows the full closing of a door or window. Butt hinges stand proud of the door or window surface.


The term used to describe a conventional window, with a sash that is side or top hinged. Casement windows open outwards.


Window cill. The protruding lip that directs water away from brickwork at the bottom of a window opening.

Composite door

A robust, secure style of door that looks and feels like a traditional timber door. It’s made from an outer skin of GRP (Glass Reinforced Plastic) over a wooden frame infilled with foam insulation.


The process of extruding a PVC-U profile and its gasket simultaneously.

Curtain walling

Glazed walling used in large buildings, often several storeys high.


A secure locking system used in modern doors.

Double glazing

Glazing made up of two layers of glass with a space between them to improve insulation and reduce noise. Double glazing comes in several widths, although 24mm and 28mm are the most common.


Most of the profiles used in PVC-U windows, doors, conservatories and roofline are extruded. Extrusion is the process of pushing PVC-U pellets through a die to form a solid shape.


Part of the roofline system that seals a house from the elements, between the tiles and the brickwork. Fascia is the front vertical board on the complete roofline system. Fascia is also sometimes used as the collective word for roofline.

French doors

Double outward-opening doors that meet in the middle without a mullion, giving a clear unobstructed opening.

French windows

Double outward opening windows that meet in the middle without a mullion, giving a clear unobstructed opening.


The black rubber strip that runs around the pane of glass in a window to create a weatherproof join.


Glass Reinforced Plastic, also known as fibreglass. GRP is a composite material made of a plastic reinforced by fine fibres of glass. GRP is an extremely tough material used in the marine industry for its hardwearing and weatherproof characteristics. GRP is also used as the outer skin on composite doors.

Head drip

Head drips appear at the top of a window and are a means of channelling water away from the front face of the window or door.


A type of door lock that throws a hook or multiple hooks into a keep when the door is locked.

Inline patio door

A sliding patio door system where the moving door pane slides in front of the fixed pane.

Injection moulding

The manufacturing technique used to make parts from both thermoplastic and thermosetting plastic materials in production. Molten plastic is injected at high pressure into a mould, which is the inverse of the product’s shape. Moulds are made from either steel or aluminium and are precision-machined to create the features of the desired part.


The International Organisation for Standardisation (ISO) is an international standard-setting body composed of representatives from various national standards organisations. ISO sets standards for manufacturing and environmental processes.

LAPFAG approval

The Local Authority PVC-U Frame Advisory Group (LAPFAG) is made up of a number of Local Authorities, Housing Associations and other public bodies engaged in the production or specification of PVC-U windows. LAPFAG approval helps to satisfy Building Regulations requirements.

Low ‘e’ glass

Low ‘e’ stands for low emissivity glass. This glass varies from normal clear glass in that one side has a special metal coating, technically known as a low emissivity coating. Low ‘e’ glass is a type of insulating glass which works by letting in energy from the sun while blocking heat loss from the home.

Main frame

The section of profile used to construct the outer frame of a window.


A mullion is the vertical structural element which divides adjacent window units.

Outer frame

An alternative term for the main frame of a window system.

Part M

Access to and use of Buildings – Building Regulations. Part M stipulates parameters for the width of front doors and the height of door thresholds, ensuring that people with reduced mobility can easily access buildings. Eurocell Part M compliant thresholds have been designed to offer easy wheelchair access.

Part L

Conservation of Fuel and Power – Building Regulations. Part L sets the thermal efficiency standards that buildings must comply with. All windows must now meet the standards set out in Part L.

Part F

Ventilation – Building Regulations. Part F sets out how much ventilation a building must have. Its impact on windows is often the addition of trickle vents.

PAS 23/24

PAS 23 – General performance of doors / PAS 24 – Enhanced security. PAS23/24 are accreditations that test the performance and security of doors.

Pilkington Activ

Pilkington Activ™ is self-cleaning glass. In a dual-action process, organic dirt is broken down by daylight and is then washed away by rain, thus making it easier to keep clean.


Polyvinyl Chloride – Unplasticised. The rigid plastic used to manufacture Eurocell windows and doors. PVC-U provides low maintenance coupled with long life. The term PVC-U is often misunderstood. Mis-spellings include PVC, PVC-U, U-PVC, UPVC, PVCu and uPVC. PVC-U is sometimes also referred to as just plastic, e.g. plastic windows.


Polyvinyl Chloride – Unplasticised expanded. Often referred to as foam products. PVC-Ue is used to manufacture many Eurocell roofline products. PVC-Ue has a hard external finish and a slightly less dense core than PVC-U.


The opening portion of a window.

Self-cleaning glass

Glass that uses a dual-action process in which organic dirt is broken down by daylight and is then washed away by rain, thus making it easier to keep clean. Pilkington Activ and St Gobain Bio-clean are brand names for this type of glass.


An alternative spelling of cill.


Part of the roofline system on a house that seals the house from the elements, between the tiles and the brickwork. Soffit is the horizontal part that runs from the fascia back to the house wall.

St Gobain Bio-clean

St Gobain’s brand of self-cleaning glass. In a dual-action process organic dirt is broken down by daylight and is then washed away by rain, making windows easier to keep clean.



A transom is the horizontal structural element that divides adjacent window units.

Thermal break

A way of stopping heat or cold transfer between two surfaces. In windows, thermal breaks are often used within the double glazed unit – see warm edge spacer.

Tilt and turn

A window system that opens inwardly in two ways. Tilt and turn windows swing completely inwards allowing for easy cleaning. They also tilt at the top to provide good ventilation without leaving the window fully open.

Trickle vent

A ventilation system built into the top of the window frame.


A measure of heat loss. The lower the U value, the more energy efficient the window.

Vertical slider

A window in the style of a traditional vertical-sliding sash window. Eurocell PVC-U sliding sash windows allow both the top and bottom frames to slide freely. The frames also tilt inwards for ease of cleaning.

Warm edge spacer

A new innovation that replaces the aluminium spacer. Used to separate the two panes of glass in a double-glazed unit with a spacer made of a different material (often plastic) that transmits less heat. This makes the window more energy efficient.

Secured by Design

Secured by Design (SBD) is the UK Police flagship initiative supporting the principles of “designing out crime”. Windows and doors that carry SBD approval have been assessed to provide a high degree of resistance to break-ins.

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