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by the NFCC



Window glazing can consist of single or multiple (most commonly two) panes of glass. It can also be made from plain, Georgian wired, fire-resisting or security glass.

See also: External finish: glazing and Structural materials: glass

General considerations

The various types of glazing all carry their own specific properties.

Plain glass will fracture and shatter when exposed to sudden temperature rises and so offers little protection against the passage of fire. Increasing the number of layers (i.e. double or triple glazing) may increase the amount of time for which the glass provides a barrier to fire and smoke. However, note that clear glass provides no barrier to radiant heat and its time to failure is known to be extremely unpredictable in fire.

Fire-resisting windows are often used to protect external escape routes or form part of a compartment boundary and therefore are likely to be part of the building's fire safety measures. It is important to identify fire-resisting glazing and its purpose. Fire-resisting glazing may also be designed to become opaque when it is heated so that the transfer of radiant heat is limited or eliminated.

It is difficult to break fire-resisting glazing (which can be 20mm thick), Georgian wired glazing or security glazing. Layers of plastic built into security glazing may burn when exposed to fire.

Inherent benefits

  • Can be used to ventilate fire or gain access for rescue
  • Window failure can give clues as to rate of fire build-up

Inherent hazards

  • Window panes can fall out, crack or shatter, particularly as a result of thermal shock (e.g. sudden cooling from firefighting jets); this is a particular problem with toughened glass
  • Glazing does not have to be directly involved in a fire for the fire to spread through it by radiative transfer