Building standards technical handbook 2020: non-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.

Large air plenums present difficulties due to the very nature of their design and an alternative approach (see clause 2.0.7) may be the only viable alternative.

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 neighbouring 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.

Due to the special fire precautions within residential care buildings and hospitals, additional guidance is grouped in the annexes. The intention is to help designers and verifiers find the information they require quickly when designing or vetting such buildings. However it is important to remember that the guidance in the annexes is in addition and supplementary to the guidance to Standard 2.1 to 2.15. For additional guidance on:

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.D) installed around the edges of the cavity. This includes for example, around the head, jambs and cill 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 2 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.

Cavity barriers and ceilings provided as an alternative to cavity barriers generally do not need to be constructed of products that achieve European Classification A1 or A2 unless they perform more than one function such as a cavity barrier that is also a separating wall constructed from products which achieve European Classification A1 or A2.

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. However for roof space cavities above an undivided space the guidance in clause 2.4.3 should be followed. Additional guidance for buildings containing a sleeping risk (other than residential care buildings and hospitals) is also provided in clause 2.4.4.

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

The need to provide cavity barriers in a roof space above undivided (or open plan) spaces is less important than that in clause 2.4.2. This is because roof spaces are regarded as having a low fire risk and the occupants in an undivided (or open plan) space should be aware of any fire developing. In such cases, the occupants should be able to make their escape in the early stages of the fire growth. However where there is sleeping accommodation, the material exposed in the cavity and the size of a cavity, should be controlled due to the nature of the risk. In such cases the limits set in the table below should not be exceeded.

It is also possible to install cavity barriers in roof spaces constructed in accordance with a combination of the guidance for divided and undivided spaces. For example, rooms adjoining a large open plan room can be separated from the open plan room by installing cavity barriers in the roof space between the two spaces. This will allow the 2 roof spaces to be assessed independently.

A ceiling constructed in accordance with the guidance in clause 2.4.5 may be provided as an alternative to installing cavity barriers.

	Note
	Limit of 20m should be applied in residential care buildings and hospitals.

Divided spaces with a sleeping risk - where a roof space cavity or a ceiling void cavity extends over a room intended for sleeping, or over such a room and any other part of the building, cavity barriers should be installed on the same plane as the wall. The intention is to contain the fire within the room of fire origin allowing occupants in other parts of the building to make their escape once the fire alarm has activated (see guidance to Standard 2.11). However in cases where this is not the most practical solution, a fire resisting ceiling can be installed as an alternative to cavity barriers (see clause 2.4.5).

The risk of fire spread above ceilings in residential care buildings and hospitals is covered by the additional guidance for sub-compartmentation in annex 2.A and annex 2.B.

Where a fire resisting ceiling is provided as an alternative to cavity barriers as in clauses 2.4.2b, 2.4.2c, 2.4.3 and 2.4.4, the ceiling should have at least short fire resistance duration and be constructed in accordance with the guidance in clause 2.1.16.

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 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.E). However, this does not apply to insulation in a cavity that is between two leaves of masonry or concrete at least 75mm thick, and which has a cavity barrier around all openings in the wall and at the top of the wall-head (see also the guidance to Standard 2.6 and Standard 2.7).

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 2.1.14.

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 2.1.15 for additional guidance on junctions and clause 2.1.14 for additional guidance on fire-stopping materials.