Cavity barrier

Approved document B, Fire Safety, Volume 2, Buildings other than dwellinghouses (2019 edition), defines a cavity as:

A space enclosed by elements of a building (including a suspended ceiling) or contained within an element, but that is not a room, cupboard, circulation space, protected shaft, or space within a flue, chute, duct, pipe or conduit.

And a cavity barrier as:

A construction within a cavity, other than a smoke curtain, to perform either of the following functions.

Close a cavity to stop smoke or flame entering.
Restrict the movement of smoke or flame within a cavity

As most buildings contain a multitude of concealed cavities and voids within walls, floors, ceilings and roofs, effective cavity barriers are essential to restrict the spread of smoke or flames. If there is a fire, the intumescent material which makes up the barrier will expand, sealing off the gaps.

Cavity barriers are typically pieces of fire-stopping material which are fitted within building cavities - horizontally at each floor, and vertically at each party wall. This assists with fire compartmentation.

The location of each cavity will determine the appropriate maximum distance between the barriers, as well as the class of surface which is exposed within the cavity. The performance of cavity barriers much be maintained throughout the lifecycle of a building, with consideration given to any building movement due to subsidence, shrinkage, and so on.

Cavity barriers must be fitted tightly to rigid construction, or suitably fire-stopped at a junction between itself and construction to which it cannot be fitted so tightly (e.g. slates, tiles, sheeting, etc.). The likelihood of the fixings failing in the event of a fire should be carefully considered. Services that may run through a barrier should be fire-stopped using appropriate materials.

PAS 9980:2022, Fire risk appraisal of external wall construction and cladding of existing blocks of flats – Code of practice, published by BSI Standards Limited in 2022, defines a cavity barrier as a: ‘product used to close or separate a concealed space, the purpose of which is to restrict the spread of smoke and/or fire.’

It defines a closed state cavity barrier as a: ‘nonloadbearing element designed to provide fire separation within or at the edges of a concealed space (cavity) by forming a tight seal (possibly under compression) between the inner and outer surfaces of the cavity.’

And an open state cavity barrier as a: ‘non-loadbearing element designed to provide fire separation in a concealed space (cavity), which is open to allow ventilation and drainage in the cold state, but which closes when exposed to a developing fire.’