What's the difference between cavity barriers and fire stops?

As both cavity barriers and fire stops help contain fire and smoke within defined compartments, they are often assumed to be the same. In reality, they perform different roles, are installed in different locations, and are designed to achieve different levels of fire resistance. Understanding the distinction is crucial for compliant fire-safe design, especially within façade and cladding systems.

Cavity barriers

Cavity barriers are installed within the cavities behind cladding or within structural gaps to slow the spread of fire and smoke by closing these voids so that fire cannot travel from one compartment to another.

Open-state (ventilated) cavity barriers and closed-state barriers are the two main types used in rainscreen cladding. Rainscreen systems typically rely on open-state barriers, which allow ventilation and drainage during normal conditions but rapidly expand to seal the cavity during a fire. Cavity barriers also come in two orientations: horizontal cavity barriers prevent vertical “chimney” spread of flames, while vertical barriers limit lateral fire movement, working together to compartmentalise the façade for effective fire protection.

Cavity barriers are usually made from stone wool or intumescent materials, which expand or resist fire when exposed to high temperatures. 

Fire stops

Fire stops perform a completely different role in passive fire protection. They are designed to maintain the fire resistance of compartment walls, floors and ceilings by sealing any gaps or penetrations created for building services such as pipes, ducts, cables and structural fixings. Without fire stopping, even the smallest opening can allow fire, heat and smoke to pass from one compartment to another.

Materials include fire-retardant sealants, wraps and collars that are tested to restore the fire performance of the original wall or floor.

By reinstating the fire resistance of compartment walls and floors, fire stops prevent fire and smoke from spreading vertically or horizontally through gaps formed where pipes, cables or ducts pass through walls and floors.

Approved Document B

Approved Document B sets out the core legal requirements for cavity barriers and fire-stopping in buildings. ADB mandates cavity barriers wherever a cavity crosses a compartment wall or floor, and around openings such as windows and doors, to restrict hidden fire spread. It also requires fire-stopping wherever services or façade connections penetrate a fire-rated wall or floor, ensuring that the original fire resistance of the compartmentation is maintained. ADB further emphasises that these measures must be installed continuously, correctly sealed, and in accordance with tested performance.

ADB states cavity barriers should be made of materials achieving at least 30 minutes integrity, although many façade cavity barriers are tested to 30/60/90 minutes depending on specification. Fire stops, on the other hand, do not have a fixed universal fire-resistance duration. Instead, they must provide at least the same fire resistance (integrity and insulation) as the element they are protecting.

In summary

Although both systems contribute to a building’s passive fire protection strategy, cavity barriers and fire stops operate in very different contexts.

Cavity barriers address fire movement within concealed voids, while fire stops prevent fire and smoke from passing through gaps in fire-rated walls and floors. Cavity barriers protect the façade build-up; fire stops maintain compartment lines. Together, they stop fire from spreading through hidden cavities or breaching walls and floors.
When cavity barriers and fire stops are used correctly and in the right locations, they work together to maintain safe compartmentation and protect the building and its occupants.