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Cavity barriers are one of the most commonly missed details in SFS wall construction — and one of the most important for fire safety. They prevent fire and smoke from travelling through the cavities within a wall, ceiling, or floor, bypassing the fire-resistant walls themselves. This guide explains where cavity barriers go, why they're required, and what the regulations actually say.

What is a cavity barrier?

A cavity barrier is a fire-resistant material installed within a concealed cavity — inside a wall, above a ceiling, or beneath a floor — to prevent the spread of fire and smoke through that cavity. It's not the same as the fire rating of the wall itself.

Think of a wall as a sandwich: boards on the outside, studs and insulation on the inside, and a cavity between the boards. The cavity is empty (or filled with insulation), and it acts like a chimney in a fire — hot gases and smoke can travel rapidly through it, spreading fire from one compartment to another without the wall's fire resistance being breached.

Cavity barriers seal off the cavity at strategic points, breaking it into fire compartments so that fire cannot travel freely through it.

Why cavity barriers matter

Cavity barriers are critical for several reasons:

• Prevent hidden fire spread — fire can travel through cavities unseen, reaching compartments that appear isolated from the fire source
• Protect escape routes — smoke travelling through cavities can reach corridors and stairwells, making escape difficult or impossible
• Maintain compartmentation — the fire strategy for a building relies on compartments being sealed. Cavity barriers maintain those seals
• Legal requirement — Building Regulations require cavity barriers at specific locations. Omitting them is a breach of regulations and a serious fire safety failure
• Insurers require them — most insurers require evidence of cavity barrier installation in commercial and multi-occupancy buildings

The Grenfell Tower fire brought cavity barrier failures into sharp public focus. Fire spread through the cavity between cladding and structure was a major factor in the disaster. Since then, enforcement has become stricter and specifiers are far more careful about cavity barrier detailing.

Where cavity barriers are required

For SFS walls, cavity barriers are typically required at:

1. At the junction with a compartment wall or floor

Where a fire-rated wall or floor meets another element (another wall, a floor slab, a roof), the cavity must be sealed at that junction. This maintains the compartment boundary.

2. At the top and bottom of every wall (unless sealed by another element)

The cavity at the head and base of an SFS wall must be sealed to prevent vertical fire spread through the wall. If the head meets a concrete soffit (which is fire-resistant), the seal is often achieved by the fit of the head track against the soffit. If there's a gap, a cavity barrier must fill it.

3. Around every opening (doors, windows, service penetrations)

Every opening in a fire-rated wall creates a potential path for fire through the cavity. Cavity barriers must surround each opening to prevent fire from bypassing the opening's fire-rated frame.

4. At intervals along long walls

Building Regulations require cavity barriers at intervals along long concealed cavities — typically every 20m for compartment walls, or at specified distances based on the building's fire strategy.

5. Between different fire compartments

Where a wall forms a boundary between fire compartments, the cavity must be sealed at that boundary.

6. At junctions with other elements

Where a wall meets a floor, roof, other wall, or service penetration, the junction must be sealed if the cavity is continuous on both sides.

What the regulations say

In England and Wales, cavity barriers are covered by:

• Building Regulations 2010, Approved Document B (Fire Safety) — Volume 1 (dwellings) and Volume 2 (buildings other than dwellings)
• BS 9991 — Fire safety in the design, management, and use of residential buildings
• BS 9999 — Fire safety in the design, management, and use of buildings

Approved Document B requires cavity barriers at the following locations:

• At the edges of cavities, including at the top and bottom of every wall (unless the cavity is sealed by a fire-resistant element)
• At the junction of a cavity with a fire-separating element
• Around openings in a fire-separating element
• At intervals not exceeding 20m in length (for some cavity types)

The specific requirements vary based on the building's use, height, and fire strategy. For any project where cavity barriers are required, consult the Approved Document or a fire engineer.

Types of cavity barrier

There are several types of cavity barrier product for SFS walls:

Intumescent cavity barriers

These are strips or sections of intumescent material (usually a graphite-based compound in a carrier) that expand when heated to seal the cavity. They're the most common type for SFS walls because they're thin, fit inside the cavity without reducing its width, and expand reliably under fire conditions.

Intumescent barriers are typically fixed to the studs, tracks, or adjacent structure and are available in various widths and ratings.

Mineral wool cavity barriers

Mineral wool (sometimes with a fire-resistant facing) is used as a cavity barrier, particularly at junctions with slabs or structural elements. It's compressed into the cavity to form a tight seal. Less common for narrow SFS cavities but used in some applications.

Fire-stop sealant / mastic

Intumescent or fire-resistant sealant applied to joints and gaps. Used for smaller gaps (up to about 25mm) rather than large cavities. Often combined with mineral wool for larger openings.

Composite / proprietary systems

Many manufacturers offer proprietary cavity barrier systems — pre-formed sections, wraps, or profiles designed for specific applications. These are usually the most reliable option because the manufacturer provides tested performance data.

Installation best practice

Correct position

Cavity barriers should be installed at the correct location — at the junction, edge, or interval defined by the fire strategy. Marking the position on the drawing before installation helps ensure nothing is missed.

Full compression

Many cavity barriers rely on compression to seal the cavity. If the barrier is not compressed against both faces of the cavity, it may not seal reliably under fire conditions. Follow the manufacturer's installation instructions for the required compression.

Continuous and gap-free

Cavity barriers must form a continuous seal. Gaps at the ends, joints between barrier sections, or imperfect fits create weak points that allow fire to bypass the barrier.

Correct fixing

Use the manufacturer's recommended fixings. Some barriers are friction-fit, some are stapled, some are screwed. Using the wrong fixing may compromise the seal.

Coordinate with services

Service penetrations (pipes, cables, ducts) through a cavity barrier must themselves be fire-stopped. A cavity barrier with an unsealed pipe through it is not a cavity barrier.

Documentation

Photograph every cavity barrier installation before it's covered up. This documentation is required for Building Control sign-off and for insurers, and it's essential evidence if there's ever a fire investigation.

Common mistakes

Missing barriers entirely

The most common mistake. Cavity barriers get missed because they're hidden, not visible from the surface, and not on the main drawing. Make them explicit on the installation drawing.

Wrong barrier for the cavity

Using a barrier rated for a different cavity width, or using a barrier that expands at the wrong temperature, may not achieve the required performance.

Incomplete seals

Barriers that don't span the full cavity, or that have gaps at junctions or ends, are ineffective. Fire finds gaps.

Services left unstoppered

A cavity barrier with an unsealed pipe through it is not a cavity barrier. Coordinate with the services contractor.

Covered up without documentation

Once covered by plasterboard, cavity barriers cannot be inspected. Without photographic evidence, Building Control may not accept the installation.

Wrong rating

Using a 30-minute cavity barrier in a 60-minute compartment, or vice versa. The rating must match the compartment requirement.

Summary

Cavity barriers prevent fire and smoke from travelling through concealed cavities in SFS walls. They're required by Building Regulations at specific locations — wall junctions, compartment boundaries, openings, and intervals along long cavities. Omitting them is a serious fire safety failure.

For any fire-rated SFS wall, include cavity barriers in the specification, mark them on the installation drawing, install them to the manufacturer's instructions, and document them before closing up.