In the event of a fire, cavity barriers are crucial to reducing the spread of flames and smoke. Therefore, they play an integral role in a passive fire protection project. Through this article,...Continue Reading
In the event of a fire, cavity barriers are crucial to reducing the spread of flames and smoke. Therefore, they play an integral role in a passive fire protection project. Through this article, construction professionals can not only begin to understand the purpose of cavity barriers, but also the regulations that ensure they comply with industry-wide standards.
What is a cavity barrier?
A cavity can be defined as an enclosed space created by the design and positioning of a building’s structural elements. Common examples include gaps between walls, floors and ceiling voids. These cavities are often hidden, but they must be properly sealed as part of a structural fire protection strategy.
In the event of a fire, gaps between walls and floors can create ‘air buoyancy’, a push-and-pull effect caused by differences in air temperature. This can enable flames and smoke to easily move between different areas of a building, otherwise known as the ‘chimney effect.’
The upward flow of air, heat and flames also creates major challenges for fire safety in tall buildings. A fire that starts on the ground floor and moves up through empty cavities can accumulate on higher levels, with no place for hot air and smoke to escape. Therefore, cavities in a building must be appropriately sealed.
Cavity barriers work by blocking the passage of fire between cavities. They are made from either fire-resistant or intumescent materials (which expand in high temperatures to create a protective seal). By containing the spread of fire to a single area, they can reduce large-scale damage whilst also creating a window of time for a building to be safely evacuated.
Fire barrier building regulations
According to BS 9991, buildings must be constructed with due consideration as to how fire can spread through compartment floors, walls and ceilings. This requirement can be met by installing vertical and horizontal cavity barriers.
Cavity barriers should be installed near the edges of internal cavities, such as at window and door openings or extract vents. They should also be placed at junctions whenever the wall cavity is aligned with a building compartment wall or floor. There are some exceptions to this requirement, depending on how fire-resistant the building material is. Cavity walls of a certain material or thickness may not require additional cavity barriers placed at junctions.
Fire Safety Approved Document B2 requires that, as a minimum, cavity barriers provide 15 minutes of fire resistance and be able to hold their structural integrity for 30 minutes. Approved Document B2 also specifies that the performance of cavity barriers cannot be undermined by the following factors:
A building’s movement as a result of subsidence or environmental factors such as weather or temperature change.
Damage to any cavity barrier fixings, or structural elements with which the cavity barriers share a common boundary. For example, cavity barriers cannot fail prematurely due to the wall or ceiling not having the same minimum level of fire resistance.
The degradation or destruction of any services that are penetrating the cavity barriers, including ducts, wires and pipes. (To learn more about service penetrations, consult our guide to firestopping methods.)
Cavity barrier inspection
When a new building project is initiated, the installation of cavity barriers will be incorporated into the process by construction managers and site teams. However, when carrying out works on an older building, it’s important to remember that it may have been built before current requirements were put into effect. That’s why it’s vital to perform a thorough assessment of a building’s compartmentation and fireproofing features.
Unfortunately, a recent report following a tower block fire in Worcester Park, South West London, found that there were ‘defective’ cavity barriers. Following an inspection, it was revealed that the barriers were too small to effectively stop the fire from spreading through a 16cm timber-framed cavity. This reinforces the need for regular inspections of cavity barriers. The tower block, which was almost entirely destroyed in the fire, was owned by the housing association Metropolitan Thames Valley Housing (MTVH). If you’d like to learn about CLM Fireproofing’s work with housing associations, you can visit our dedicated local authorities page.
When cavity barriers are installed, it’s important to record all locations and maintenance information to include in the building’s fire safety manual. Building regulations require records of fire barrier installations, including drawings of the position and specifications of cavity barriers located in all compartment walls and doors. These can be used to inform cavity barrier inspection and maintenance with a fire safety company.
At CLM Fireproofing, we include the installation and maintenance of cavity barriers as part of our industry-leading fire compartmentation services. We work with project managers and site teams across a range of sectors, helping to ensure that all passive fire protection systems are fully compliant with the latest industry standards.
Contact our team of passive fire protection experts to find out more.