Glazing Terms Explained

Don’t know your mullion from your transom? Our secondary glazing jargon buster.

Mullion Fixed vertical dividing bar of wood, metal or stone, in a frame or window opening separating glass panes or opening casements.

Transom Fixed horizontal dividing bar of wood, metal or stone, in a frame or window opening separating glass panes or opening casements.

Window Reveal Part of the side of a window at 90° to the glass forming the window sill area or door opening, between the outer surface of a wall and the window or door frame.

Splayed Reveal Where the angle of the reveal widens from the glass/window frame, to the inner wall of the room.

Face Fix (or direct fix) Where the unit is attached directly to the face of the existing window frame. The simpler of the 2 methods to attach the units, ideal for thermal insulation requirements.

Reveal Fix (or side fix) Where the units are attached to the side wall of the reveal e.g. at 90 degrees to the glass. Normally be selected for acoustic insulation requirements, enabling units to be positioned away from the primary glass, increasing the air-gap and increasing the sound proofing efficiency. Can also assist where there is limited of no frame available to attach to round the primary window.

U-Value Measure of heat transmission through a building part or a given thickness of insulating material, expressed as the number of British Thermal Units that will flow in 1 hour through 1 square foot of the structure. The lower the number the more efficient the window. e.g UK government target for replacement windows is a u-value of 2 W/m2K.

Window Stay Type of hinge which prevents/limits the ingress or egress of the window.

Handing Applies to windows to determine direction of swing. A casement window may be left-handed, right-handed, or double. A horizontal slider can be left or right handed and will determine which panel is nearest the room.

RAL European colour matching system. Usually a four digit reference, mainly used for varnish and powder coating.

Safety Glass Glass that resists shattering; toughened or tempered glass, which is glass that has been processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. It can also be a composite of two sheets of glass with an intermediate layer of transparent plastic. Also called laminated glass.

Gasket Used to seal a glass type into the frame and to prevent fluid or gas leaks by providing a barrier between the two adjoining surfaces.

Trim Decorative colour coded molding made of uPVC or hardwood to give the installed windows a neat finish.

Interlocker/ Coupler Formed piece of aluminium section designed to lock two windows together, providing a neat and tidy unobtrusive join. A heavyduty interlocker is used to provide vertical stability to larger height windows, usually when they are over 2 metres tall.

Stile Vertical member of a panel or frame, as in a door or window sash.

Casement Opening/moving part of the window (also, see sash), casements tend to hinge outwards, occasionally inward.

Sash Opening/moving part of the window (also, see casement).Sashes usually slide up and down. However, note that ‘casement’ and ‘sash’ are terms often used to describe the same thing.

For more information on choosing the best secondary glazing for your property, visit find your window type

Help CentreGet a QuoteSavings CalculatorContact Us