Cavity barriers are fire-resisting barriers installed within concealed cavities and voids to restrict the hidden spread of fire and smoke. They are an important part of passive fire protection and building compartmentation.
Because cavity barriers are often hidden in walls, roof spaces, ceiling voids, facades and floor zones, defects may not be obvious during day-to-day building management. Missing, damaged or incorrectly installed barriers can allow fire to travel unseen through the building fabric.
This guide explains what cavity barriers are, where they are normally found, how they differ from fire-stopping and cavity closers, and when they may need to be checked as part of fire safety management.
Quick Answer
A cavity barrier is a fire-resisting barrier used to close or subdivide a concealed cavity, such as a wall cavity, roof void, ceiling void or facade cavity. Its purpose is to restrict fire and smoke spread through hidden spaces. Cavity barriers should be correctly specified, installed, maintained and checked where defects are suspected.
What Is a Cavity Barrier?
A cavity barrier is a construction element that closes or subdivides a concealed cavity to restrict the movement of fire and smoke. It may be made from mineral fibre, intumescent material, fire-resisting board or another tested system, depending on the location and construction type.
Cavity barriers are used where hidden spaces could otherwise allow fire to spread around compartment walls, floors or protected routes. They are different from visible fire doors or walls because they are usually concealed inside the building fabric.
Why Are Cavity Barriers Important?
Concealed cavities can allow fire and smoke to travel quickly through a building without being seen. This can undermine compartmentation, threaten escape routes and allow fire to spread beyond the area where it started.
Cavity barriers help to:
- Restrict hidden fire and smoke spread
- Support fire compartmentation
- Protect escape routes and protected staircases
- Limit spread through roof spaces, wall cavities and facade voids
- Support the assumptions made in the fire strategy or fire risk assessment
Where Are Cavity Barriers Normally Found?
The exact locations depend on the building design and applicable guidance, but cavity barriers are commonly found in:
- External wall cavities and facade systems
- Roof spaces and roof voids
- Ceiling voids and floor voids
- Junctions between compartment walls, floors and external walls
- Edges of cavities around openings
- Concealed spaces above protected corridors
- Timber frame construction and modular construction interfaces
Approved Document B gives guidance on cavity barriers in England, but the correct solution always depends on the building, construction type, tested system and relevant design information.
Cavity Barriers in External Walls and Roof Voids
Cavity barriers are particularly important in external walls, facade systems and roof voids because these concealed spaces can allow fire and smoke to spread beyond the room or compartment where the fire started. The correct barrier type, position and fixing method depends on the building design, construction type and relevant fire safety guidance.
In blocks of flats and other residential buildings, cavity barrier issues may also be considered alongside external wall risk, compartmentation and evacuation strategy. Where there are concerns about external wall systems, further review may be needed through a fire risk assessment, compartmentation review or FRAEW.
Cavity Barrier, Fire Stop and Cavity Closer: What Is the Difference?
| Item | Purpose |
|---|---|
| Cavity barrier | Subdivides or closes concealed cavities to restrict hidden fire and smoke spread. |
| Fire-stopping | Seals gaps, joints and service penetrations through fire-resisting construction. |
| Cavity closer | Closes a cavity for weather, thermal or construction purposes. It is only a cavity barrier if it has suitable fire performance for that use. |
Common Cavity Barrier Defects
- Missing barriers where cavities should be subdivided
- Barriers displaced during refurbishment or maintenance
- Incorrect products used for the cavity type
- Poor fixing or incomplete installation
- Gaps around junctions, openings or services
- Barriers damaged by later contractors
- No evidence that the barrier matches the required tested system
How Cavity Barrier Defects Are Usually Identified
Cavity barrier defects may be identified during refurbishment works, intrusive inspections, external wall reviews, compartmentation surveys or fire risk assessment follow-up actions. Because many cavity barriers are hidden, a standard visual fire risk assessment may not confirm every barrier without further investigation.
Where defects are suspected, the responsible person may need competent advice to decide whether a targeted inspection, sample survey or more detailed fire compartmentation survey is appropriate.
When Should Cavity Barriers Be Checked?
Cavity barriers may need to be checked where the building history, fire risk assessment or survey findings suggest potential defects. This is particularly relevant after refurbishment, external wall works, roof works, service installations or intrusive works affecting concealed spaces.
Checks may also be relevant in blocks of flats, care homes, hotels, schools, healthcare buildings, mixed-use buildings and other premises where compartmentation is important to the evacuation strategy.
Are Cavity Barriers Covered by a Fire Risk Assessment?
A fire risk assessment may consider obvious or known cavity barrier concerns, especially where they affect escape routes, compartmentation, external walls or residential building fire safety. However, hidden cavity barriers are not always visible during a standard non-intrusive assessment.
If there is doubt, the assessor may recommend further investigation, a compartmentation survey, intrusive inspection or specialist review.
How Fire Risk Assessment Network Can Help
Fire Risk Assessment Network can help responsible persons arrange fire risk assessments and related fire safety support. Where cavity barrier or compartmentation concerns are identified, we can help you understand the appropriate next step, including fire-stopping review, compartmentation survey, fire door inspection or external wall fire safety support.
Concerned About Cavity Barriers or Hidden Fire Spread?
If your building has had refurbishment, external wall works, service installations or compartmentation concerns, we can help arrange suitable fire safety support.
Frequently Asked Questions
What is a cavity barrier?
A cavity barrier is a fire-resisting barrier installed in a concealed cavity or void to restrict the spread of fire and smoke.
Where are cavity barriers required?
Cavity barriers are commonly used in wall cavities, roof voids, ceiling voids, floor voids and at junctions with compartment walls and floors. The exact requirement depends on the building design and relevant guidance.
Is a cavity closer the same as a cavity barrier?
No. A cavity closer is not automatically a cavity barrier. It must have suitable fire performance for the specific use if it is relied on as a cavity barrier.
Can a fire risk assessment check cavity barriers?
A fire risk assessment may identify obvious or known concerns, but hidden cavity barriers may need specialist inspection or intrusive investigation if defects are suspected.
What happens if cavity barriers are missing?
Missing or defective cavity barriers can allow fire and smoke to spread through concealed spaces, undermining compartmentation and increasing risk to escape routes and occupants.
Who can inspect cavity barriers?
Cavity barriers should be inspected by a competent person with suitable knowledge of passive fire protection, compartmentation and the relevant building construction.
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