Skip to content
English
  • There are no suggestions because the search field is empty.

What is a cavity barrier?

Cavity barriers are elements of passive fire protection systems designed to prevent fire spread.

Topics covered:

  • How cavity barriers prevent fire spread.
  • The main types of cavity barriers.
  • Performance criteria of cavity barriers.
  • Where cavity barriers should be installed.
  • Importance of compatible cavity barriers.

Cavity barriers are installed within the cavities behind cladding or within structural gaps to restrict the spread of fire and smoke from one compartment to another. Without cavity barriers, airflow is facilitated within these gaps, accelerating the spread of fire. Getting cavity barriers right is one of the most effective ways to prevent façade fire spread and ensure compliance.

There are two main types of cavity barriers: open state barriers (or ventilated) and closed state barriers. Rainscreen cladding systems typically use open state barriers as they are designed to remain open in normal conditions to allow airflow and drainage, however seal shut under fire conditions, expanding to close the void. They work using materials that expand or react when exposed to heat, such as intumescent strips, effectively sealing the cavity during a fire.

How do rainscreen cladding systems prevent fire spread?


Closed state barriers, on the other hand, are designed to remain closed at all times, making them ideal for use in non-ventilated systems or areas where airflow isn't required.

Approved Document B and Cavity Barriers

Building regulations relating to cavity barriers can be found in Approved Document B.

The performance criteria for cavity barriers are outlined in ADB Vol 2 Section 9.10, which states that cavity barriers should provide at least 30 minutes fire resistance (integrity only)or be constructed from materials of limited combustibility (Euroclass A2-s3,d2 or better). By providing 30 minutes of fire resistance, cavity barriers delay the spread of fire and allow enough time for inhabitants to escape and firefighters to arrive.

Approved Document B also outlines where cavity barriers are required. Key positions include:
  • At every floor slab level – to prevent vertical fire spread up the building
  • Around all window and door openings – to prevent fire entering or exiting
  • At party wall junctions – to maintain horizontal compartmentation
  • At rooflines and parapets – to prevent fire wrapping around the structure
  • At the junctions between different materials or façade zones

As a design rule of thumb, cavity barriers should be installed in any location where a fire could leap between compartments or exit the building envelope.

An important final point on cavity barriers is that they must be compatible with the overall cladding system. More specifically, the entire cladding setup (with the same type of cavity barrier) should have passed a BS 8414 fire test.