External masonry walls

6.1.4Fire resistance

External cavity walls shall adequately resist the passage of fire.

The cavity in the masonry cavity wall of a building can provide a route for the spread of smoke and flames. Cavities should be closed with cavity barriers, in accordance with Building Regulations.

Cavity barriers should be provided:

  • at the edges of cavities, including around openings e.g. window and doors,
  • where the cavity abuts compartment walls and floors,
  • where to break up extensive cavities which could act as a route for fire spread.

Cavity barriers need to achieve a minimum of 30-minute fire resistance (30/15 EI).

Where a cavity is fully filled (in full contact with both leaves) with non-combustible insulation (Euroclass A1 in accordance with BS EN 13501) a cavity barrier is not required.

Where cavity barriers are used, they should be sized appropriately for the dimensions of the cavity. Normally they are installed under compression and as such maintaining the design cavity width is critical.

Significant reduction in cavity width will mean that the barrier cannot be fitted without creating problems for the following leaf of masonry. Significant widening in the cavity width will mean that the barrier may not be fitted with the appropriate level of compression and its performance may be impaired.