Section 9: Cavities
9.1 Cavities in the construction of a building provide a ready route for the spread of smoke and flame, which can present a greater danger as any spread is concealed. For the purpose of this document, a cavity is considered to be any concealed space.
Provision of cavity barriers
9.2 To reduce the potential for fire spread, cavity barriers should be provided for both of the following.
a. To divide cavities.
b. To close the edges of cavities.
See Diagram 9.1. Cavity barriers should not be confused with fire-stopping details (Section 10).
Pathways around fire separating elements
Junctions and cavity closures
9.3 Cavity barriers should be provided at all of the following locations.
a. At the edges of cavities, including around openings (such as windows, doors and exit/entry points for services).
b. At the junction between an external cavity wall and every compartment floor and compartment wall.
c. At the junction between an internal cavity wall and every compartment floor, compartment wall or other wall or door assembly forming a fire resisting barrier.
This does not apply where a wall meets the conditions of Diagram 9.2.
9.4 It is not appropriate to complete a line of compartment walls by fitting cavity barriers above them. The compartment wall should extend to the underside of the floor or roof above.
Protected escape routes
9.5 If the fire resisting construction of a protected escape route is either of the following.
a. Not carried to full storey height.
b. At the top storey, not carried to the underside of the roof covering.
Then the cavity above or below the fire resisting construction should be either of the following.
i. Fitted with cavity barriers on the line of the enclosure.
ii. For cavities above the fire resisting construction, enclosed on the lower side by a fire resisting ceiling (minimum EI 30) that extends throughout the building, compartment or separated part (see Diagram 9.3).
Cavities affecting alternative escape routes
9.6 In divided corridors, cavity barriers may be needed to prevent alternative escape routes being affected by fire and/or smoke (see paragraph 2.27 and Diagram 2.9).
Separation of bedrooms in ‘residential (institutional)’ and ‘residential (other)’ buildings
9.7 Where a cavity exists above or below a partition between bedrooms because the enclosure is not carried to full storey height or to the underside of the roof covering, the guidance in paragraph 9.5 should be followed.
Double skinned corrugated or profiled roof sheeting
9.8 Cavity barriers are not required between double-skinned corrugated or profiled insulated roof sheeting if the sheeting complies with all of the following.
a. The sheeting is rated class A2-s3, d2 or better.
b. Both surfaces of the insulating layer are rated class C-s3, d2 or better.
c. Both surfaces of the insulating layer make contact with the inner and outer skins of cladding (Diagram 9.4).
Extensive cavities
Maximum dimensions of cavities
9.9 Cavity barriers should be used to divide any cavity (including roof spaces). Table 9.1 sets out maximum dimensions for undivided cavities.
9.10 Table 9.1 does not apply to any of the following cavities.
a. A cavity in a wall that is fire resisting only because it is loadbearing.
b. A cavity in a wall that meets the conditions of Diagram 9.2.
c. A floor or roof cavity above a fire resisting ceiling (Diagram 9.3) that extends throughout the building or compartment to a maximum of 30m.
d. In a building not put to residential or institutional use, a cavity that does not contain materials achieving class B-s3, d2 or worse and is formed either:
i. behind the external skin of an external cladding system with a masonry or concrete inner leaf a minimum of 75mm thick
ii. by overcladding an existing masonry (or concrete) external wall or an existing concrete roof.
e. A cavity that meets the conditions of paragraph 9.8.
f. A cavity below a floor next to the ground or next to oversite concrete, if either:
i. the cavity is less than 1000mm in height
ii. the cavity is not normally accessible by people, unless there are openings in the floor such that it is possible for materials to accumulate in the cavity (in which case cavity barriers should be provided and access should be provided to the cavity for cleaning).
9.11 If a single room with a ceiling cavity or underfloor cavity exceeds the dimensions in Table 9.1, cavity barriers need only be provided on the line of the enclosing walls/partitions of that room, if both of the following apply.
a. The cavity barriers are a maximum of 40m apart.
b. The surface of the material/product exposed in the cavity is class C-s3, d2 or better.
9.12 If the undivided area exceeds 40m in one or both directions, there is no limit to its size if all of the following conditions are met.
a. Together, the room and cavity form a compartment separated from the rest of the building.
b. Both of the following apply.
i. The building is fitted with an automatic fire detection and alarm system that conforms to BS 5839-1.
ii. Detectors are only required in the cavity to satisfy BS 5839-1.
c. If the cavity is used as a plenum then the recommendations for recirculating air distribution systems in Section 32 of BS 9999 are followed.
d. Both of the following apply.
i. The exposed surface of the material/product used in the construction of the cavity is class B-s3, d2 or better.
ii. The supports and fixings in the cavity are class A1.
e. Any pipe insulation system should achieve class C-s3, d2 rating or better.
f. Any electrical wiring in the cavity is laid in metal trays or metal conduit.
g. Other than those in (d)–(f), any materials are class A2-s3, d2 rated or better.
Construction and fixings for cavity barriers
9.13 Cavity barriers, tested from each side separately, should provide a minimum of both of the following:
a. 30 minutes’ integrity (E 30)
b. 15 minutes’ insulation (I 15).
They may be formed by a construction provided for another purpose if it achieves the same performance.
9.14 Cavity barriers in a stud wall or partition, or provided around openings, may be formed of any of the following.
a. Steel, a minimum of 0.5mm thick.
b. Timber, a minimum of 38mm thick.
c. Polythene-sleeved mineral wool, or mineral wool slab, under compression when installed in the cavity.
d. Calcium silicate, cement-based or gypsum-based boards, a minimum of 12mm thick. These do not necessarily achieve the performance specified in paragraph 9.13.
NOTE: Cavity barriers provided around openings may be formed by the window or door frame if the frame is constructed of steel or timber of the minimum thickness in (a) or (b), as appropriate.
9.15 Cavity barriers should be tightly fitted to a rigid construction and mechanically fixed in position. If this is not possible (e.g. where a cavity barrier joins to slates, tiles, corrugated sheeting or similar materials) the junction should be fire-stopped.
9.16 Cavity barriers should be fixed so their performance is unlikely to be made ineffective by any of the following.
a. Movement of the building due to subsidence, shrinkage or temperature change, and movement of the external envelope due to wind.
b. During a fire, collapse of services penetrating the cavity barriers, either by the failure of the supporting system or through degradation of the service itself (e.g. by melting or burning).
c. During a fire, failure of the cavity barrier fixings. (In roof spaces, where cavity barriers are fixed to roof members, there is no expectation of fire resistance from roof members provided for the purpose of support.)
d. During a fire, failure of any material or construction to which cavity barriers abut. (For example, a suspended ceiling that continues over a fire resisting wall or partition collapses, and the cavity barrier fails prematurely because the ceiling was not designed to provide a minimum fire resistance of EI 30.)
Openings in cavity barriers
9.17 Openings should be limited to the following.
a. Fire doorsets with a minimum rating of E 30, fitted in accordance with Appendix C.
b. The passage of pipes that follow the provisions in Section 10.
c. The passage of cables or conduits containing one or more cables.
d. Openings fitted with a suitably mounted and appropriate fire damper.
e. Ducts that are either of the following.
i. Fire resisting (minimum E 30).
ii. Fitted with a suitably mounted and appropriate fire damper where they pass through the cavity barrier.
9.18 If a cavity barrier is provided above or below a partition between bedrooms in ‘residential (institutional)’ and ‘residential (other)’ (purpose groups 2(a) and 2(b)) buildings, and the partition is not a fire resisting partition, then paragraph 9.17 does not apply. However, both of the following apply.
a. The number of openings in the barrier should be kept to a minimum.
b. Any penetrations should be sealed to restrict the passage of smoke with an appropriate fire-stopping material.
NOTE: For further guidance on openings in cavity barriers see Section 10.
