Cavity Barrier
/ ˈkævɪti ˈbærɪə /
Also known as: cavity fire stop, fire stopping, cavity closer
Definition
A cavity barrier is a fire-resisting element installed within concealed wall cavities, floor voids, roof spaces, and cladding voids to prevent fire and smoke travelling through those voids and bypassing the building's fire compartmentation. Required by Building Regulations Approved Document B (Fire Safety) at: the edges of all cavities; junctions with compartment walls and floors; maximum intervals within large cavities (typically 8m x 8m for wall cavities above 5m building height); and around all wall openings. Minimum fire resistance: 30 minutes integrity. Materials: mineral wool (rock wool or glass fibre quilt), purpose-made steel/plastic proprietary systems with intumescent strips, or fire-rated boards. Distinct from firestopping (sealing penetrations through compartment boundaries); both are components of passive fire protection. The Grenfell Tower fire demonstrated the catastrophic consequences of absent or failed cavity barriers in external cladding systems.
In practice
In standard domestic cavity wall construction (two skins of masonry or a masonry outer with a timber frame inner), cavity barriers are required at all junctions with compartment walls and compartment floors (between dwellings in semi-detached and terraced housing, and between floors in flats). They are also required at the top of the cavity under the roof verge in certain configurations. The traditional method is mineral wool cavity barrier quilt compressed into the cavity at these positions, but purpose-made products are available that are easier to install accurately and less susceptible to the gaps that can occur when quilt is installed carelessly.
For extensions to existing buildings, cavity barriers must be provided where the extension wall cavity communicates with any existing cavity, to prevent fire spreading from the extension into the existing building's cavities (and vice versa). This is a frequently omitted detail in domestic extensions. Building Control surveyors should check cavity barrier installation at wall plate level and at compartment wall junctions, but it is easy to miss during inspections once the wall is closed up. On timber frame construction particularly, where combustible material is within the cavity, cavity barrier provision is critical and should be inspected before closing in.