What is a cavity fire barrier?
Unsealed cavities can allow air to be drawn in and smoke to vent out, enabling the fire spread to accelerate through the building. This is known as the ‘chimney effect’ and enables flames within a cavity to be able to extend between 5 and 10 times higher than a flame that is not within a cavity. This is regardless of whether or not the surfaces of the cavity are combustible.
A cavity barrier are blocks of fire-resistant materials that seal up these cavities in a wall to prevent the spread of fire and smoke. This is achieved by the fire-resistant material used to make the barrier expanding in the event of a fire and as a result sealing off the space. This helps with the compartmentation of a building during a fire. The performance of a cavity barrier must be at par throughout the building’s lifespan, taking into account any building movement through things such as shrinkage and subsidence.
That being said, if a cavity closer has been made using fire-resistant material, they could be considered a cavity barrier despite not being designed for that purpose.
Older existing buildings may not have been built to this standard or may have been built before it was a legal requirement. As a result, their fire stopping provision should be regularly checked as part of their fire risk assessment.
Things such as building movements, the collapse or deformation of structures and surfaces, and the failure of the fixing should be considered when installing a cavity barrier. Opening and penetrations in cavity barriers should always be kept to a minimum in order to restrict the flow of smoke.
When cavity barriers are installed, it’s important to record all of their locations and the relevant maintenance information. This should all be put in the building’s fire safety manual. Building regulations require records of cavity barriers, including drawings of the positions and specifications in all compartment walls and doors. These can then be used to inform any future cavity barrier inspection and maintenance.