Building standards technical handbook 2019: domestic

Fire and smoke spread in cavities is particularly hazardous because fire can spread quickly throughout a building and remain undetected by the occupants of the building or by fire and rescue service personnel. Ventilated cavities generally promote more rapid fire spread around the building than unventilated cavities due to the plentiful supply of air. Buildings containing sleeping accommodation pose an even greater risk to life safety and demand a higher level of fire precautions. For these reasons, it is important to control the size of cavities and the surfaces exposed in the cavity.

The guidance for protection to cavities should not be assessed in isolation and reference should be made to the guidance to Standard 2.6 for spread to adjoining buildings and the guidance to Standard 2.7 for fire spread on external walls.

A cavity is a concealed space enclosed by elements of a building (including, for example, the void above a suspended ceiling) or contained within a building element, but not a room, cupboard, circulation space, stair enclosure, lift well, chimney or a space within a chute, duct, pipe or conduit. For the purposes of this guidance, a cavity includes a roof space, a service riser or any other space used to run services around the building.

Reference to surfaces in a cavity is intended to include the surface of the enclosing envelope of the cavity (including insulation material) but excludes timber roof trusses or lintols, joist ends, pipes, conduits or cables.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

A cavity barrier means any construction provided to seal a cavity against the penetration of fire and smoke or to restrict its movement within the cavity.

In order to inhibit fire spread in a cavity, every cavity within a building should have cavity barriers with at least a short fire resistance duration (see annex 2.A) installed around the edges of the cavity. This includes for example, around the head, jambs and sill of an external door or window opening. A cavity barrier should also be installed between a roof space and any other roof space or between a cavity and any other cavity such as at the wall-head between a wall cavity and a roof space cavity.

However cavity barriers are not necessary at a junction between two cavity walls each comprising two leaves of masonry or concrete at least 75mm thick.

Sealing cavities can sometimes create difficulties, especially where construction techniques rely on through ventilation of the cavity (see Section 3 Environment) or where the detailing should take into account the effect of thermal bridging (see Section 6 Energy).

In an external wall, open state intumescent cavity barriers may be used to inhibit fire and smoke spread and seal the cavity. It is recognised that smoke will spread beyond the cavity barrier at the incipient and early fire growth phases prior to the intumescent material reacting to heat but this is not considered to be a major concern as the cavity is ventilated to atmosphere.

Cavities should be measured either horizontally or vertically, as the case maybe, along the centre-line of the cavity and not diagonally.

Every cavity should be divided by cavity barriers so that the maximum distance between cavity barriers is not more than 20m where the cavity has exposed surfaces which achieve European Classification A1, A2 or B, or 10m where the cavity has exposed surfaces which achieve European Classification C, D or E.

Exclusions - cavity barriers are not necessary to divide a cavity:

Where a ceiling is provided as an alternative to cavity barriers as in clauses 2.4.2b and 2.4.2c, the ceiling should have a short fire resistance duration, and be constructed in accordance with the following recommendations:

Fire-fighters may not be able to apply a water jet from a fire-fighting hose directly onto a fire that has spread within an external wall or onto an external wall. This is because the external wall is either inaccessible or is too high above the ground to be within the reach capability of fire-fighting equipment such as hydraulic platforms or turntable ladders, therefore, the construction of external walls should not contribute to the development of fire or contribute to fire spread within cavities or vertical fire spread up the facade of the building. Fire-fighters may require to cut holes in the external envelope of a building in order to gain access to the cavity to extinguish or control fire spread in the cavity. This can be particularly challenging when the cavity is ventilated and the surfaces exposed in the cavity promote rapid fire spread.

Therefore, in a building with a storey at a height of more than 11m, thermal insulation material situated or exposed within an external wall cavity, or in a cavity formed by external wall cladding, should be constructed of products which achieve European Classification A1 or A2 (see annex 2.B). However, this does not apply to insulation in a cavity that is between two leaves of masonry or concrete at least 75mm thick and the external wall is provided with cavity barriers around all openings and at the top of the wall-head.

Alternative guidance - BR 135, ‘Fire Performance of external thermal insulation for walls of multi-storey buildings’ and BS 8414: Part 1: 2015+A1: 2017 or BS 8414: Part 2: 2015+A1: 2017 provides guidance on fire spread on external wall cladding systems. The guidance provided in these publications may be used as an alternative to European Classification A1 or A2 external wall cladding and for European Classification A1 and A2 products exposed in a cavity. BS 9414: 2019 (Draft June 2019) provides additional information on the application of results from BS 8414 tests.

A cavity barrier should be fixed so that its performance is not affected by:

However where a cavity barrier is installed in a roof space, there is no need to protect roof members that support the cavity barrier.

A cavity barrier and a ceiling provided as an alternative to a cavity barrier may contain a self-closing fire door (or a hatch in the case of a ceiling), or a service opening constructed in accordance with the guidance in clause 2.2.9.

All cavity barriers should be tightly fitted to rigid construction. Where this is not possible as in the case of a junction with slates, tiles, corrugated sheeting or similar materials, the junction should be fire stopped. See clause 2.2.10 for additional guidance on junctions and clause 2.2.9 for additional guidance on fire stopping materials.