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 begin to understand the purpose of a cavity barrier and the regulations that ensure it complies with industry-wide standards.
What are the Risks of Unsealed Cavities?
A cavity is 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. However, they do not include ducts, conduits, chutes, pipes, and other cavities that serve a distinct purpose. These cavities are often hidden, but they must be adequately 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. That can enable flames and smoke to quickly move between different areas of a building, otherwise known as the ‘chimney effect.’ Simply, these gaps allow heat, flames, gas, and smoke to travel unhindered from one building area to another.
The upward flow of air, heat and flames creates significant 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.
What is a Cavity Barrier?
Cavity barriers, also known as stop socks, 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 and create a window of time for a building to be safely evacuated and for firefighters to arrive.
What is the Difference Between a Cavity Barrier And a Firestop?
The terms “cavity barrier” and “firestop” are often used interchangeably. While both are passive fire protection methods, there are a few key differences. As mentioned, a cavity barrier is a piece of fire-resistant material placed in enclosed, typically unused spaces within buildings. They are designed to prevent the spread of fire and hot gasses.
In comparison, while firestops are also used to prevent the spread of flames and smoke, they typically come in the form of putty or intumescent clamps and are used to seal joints or gaps between pipes, conduits, or sockets and the walls through which they pass. They can also be used around fire doors, windows, and ceiling voids. This helps to maintain the fireproofing of a wall or floor by closing small yet still problematic gaps. In short, firestops seal any imperfections between building elements that should be fire-resistant.
Cavity Barrier Installation and Fire Barrier Building Regulations
There are strict regulations in place to ensure that cavity fire barriers function effectively. 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.
Fire Safety Approved Document B2 requires that, as a minimum, cavity barriers provide 15 minutes of fire resistance and hold their structural integrity for 30 minutes. Approved Document B2 also specifies that the following factors cannot undermine the performance of cavity barriers:
- 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 penetrating the cavity barriers, including ducts, wires and pipes. (To learn more about service penetrations, consult our guide to firestopping methods.)
These regulations are in place to ensure maximum protection. However, regular or annual maintenance can ensure that cavity fire barriers remain uncompromised. Tampering, shrinkage, and third-party damage can all affect the quality and performance of a cavity barrier.
How Should Cavity Barriers Be Installed?
Cavity barriers should be installed near the edges of internal cavities, such as 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 particular material or thickness may not require additional cavity barriers placed at junctions.
Before installing cavity barriers, it’s essential to consider building movements and deformations where the barriers are placed. Settlement, sway, thermal expansion, and penetrations by deformations can negatively affect the efficacy of a cavity barrier.
Cavity Barrier Inspection and Maintenance
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 implemented. That’s why it’s vital to assess a building’s compartmentation and fireproofing features thoroughly.
Unfortunately, a recent report following a tower block fire in Worcester Park, South West London, found ‘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.
Persimmon, a property developer that constructs roughly 16,000 houses a year, reviewed their process after the tragedy. The inspection uncovered a widespread problem where the cavity barriers in timber frame properties were either incorrectly installed or not installed at all.
That reinforces the need for regular inspections of cavity barriers. The tower block, which was almost entirely destroyed in the fire, was owned by Metropolitan Thames Valley Housing (MTVH). You can visit our dedicated local authorities page if you’d like to learn about CLM Fireproofing’s work with housing associations.
When cavity barriers are installed, it’s crucial to record all locations and maintenance information 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.
Cavity barriers play a critical role in preventing the spreading of fire and hazardous gasses and protecting properties and lives. However, proper evaluation, installation, and maintenance are vital to ensure cavity barriers function correctly.
At CLM Fireproofing, we install cavity barriers and acoustic seals 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.