The short answer
Cladding fitted with a ventilated cavity behind it often needs cavity fire barriers, because the air gap that keeps the wall dry can also let fire and smoke spread unseen behind the cladding. Cavity barriers are fire-resisting components installed within the cavity — typically around openings such as windows and doors, at junctions, and at intervals across the wall — to close off the gap and stop fire travelling through it. Whether they are required, and exactly where, is governed by Approved Document B and depends on the building's height, use and the wall construction. Many use intumescent barriers that allow air to ventilate the cavity in normal conditions but expand to seal it in a fire. Because this is fire-critical detailing, confirm the requirement with building control or a competent professional.
The ventilated cavity that protects cladding from damp can become a fire pathway. This page explains what cavity barriers do, where they go, and how they reconcile fire safety with the need for airflow.
Cavity fire barriers and cladding
- Why neededcavity can spread fire unseen
- What they doclose the gap, resist fire
- Typical locationsopenings, junctions, intervals
- Ventilation typeintumescent barriers expand in fire
- Set byApproved Document B
Why the cavity is a fire concern
Most external cladding is built as a rainscreen with a ventilated and drained cavity behind the boards — the air gap that lets moisture dry and keeps the wall sound. The same continuous gap, however, can act as a concealed pathway: in a fire, flame and hot smoke can travel up and across the cavity behind the cladding, spreading fire to other parts of the building before it is detected. This is the problem cavity barriers are designed to solve. They are fire-resisting components placed within the cavity to interrupt that pathway, compartmenting the gap so fire cannot run freely through it. Their correct presence is an important part of how a clad wall performs in a fire, alongside the reaction-to-fire class of the materials.
Where cavity barriers go
Cavity barriers are placed where fire is most likely to enter or spread within the cavity. Subject to the requirements of Approved Document B and the specific design, this commonly includes:
- Around openings — perimeters of windows, doors and other penetrations, where fire can get into the cavity.
- At junctions — where the external wall meets compartment floors and walls, to maintain compartmentation.
- At the top and edges of the cavity, and around its boundaries.
- At intervals across large areas of wall, to limit how far fire can travel.
The exact locations, spacing and the fire resistance required depend on the building. Getting these positions right is as important as choosing a barrier product, because gaps or poor installation undermine the protection.
| Location | Purpose | Notes |
|---|---|---|
| Around openings | stop fire entering cavity | windows, doors, vents |
| At compartment lines | maintain compartmentation | floors and walls |
| Cavity edges / top | close the gap boundary | perimeter sealing |
| Across large walls | limit fire travel | per design intervals |
Indicative guidance; exact requirements are set by Approved Document B for the specific building.
Balancing fire safety with ventilation
There is a genuine tension between a barrier that seals the cavity for fire safety and the airflow that ventilates it for moisture control. The common solution is the open-state intumescent cavity barrier: in normal conditions it allows air to pass so the cavity can ventilate and drain, but when exposed to the heat of a fire it expands rapidly to close the gap and resist fire spread. This lets a wall keep its drained, ventilated rainscreen performance while still providing fire compartmentation. Choosing and positioning these barriers correctly is a coordination task between the cladding design and the fire strategy, because a barrier that blocks needed ventilation can cause damp, while a gap left for airflow can compromise fire performance.
What this means for your project
If your re-cladding includes a ventilated cavity — as most boarded systems do — assume that cavity barriers may be required and have the requirement confirmed as part of the design, not as an afterthought. The need scales with the building: it is taken especially seriously on flats, blocks and taller buildings, where compartmentation between dwellings is central to the fire strategy, but the principle of not creating a concealed fire pathway applies more widely. Use a competent designer and installer, keep records of the barrier products and their positions, and route fire-critical decisions through building control. As with all cladding fire matters, this is general guidance; the specific requirements for your wall should be verified by a qualified professional.
Frequently asked questions
What is a cavity fire barrier?
It is a fire-resisting component installed within the ventilated cavity behind cladding to stop fire and smoke spreading unseen through the gap. Many are open-state intumescent barriers that allow ventilation in normal conditions but expand to seal the cavity when exposed to fire.
Do all ventilated cladding cavities need fire barriers?
Not automatically, but they are commonly required where a ventilated cavity could spread fire, particularly around openings, at compartment junctions and on flats, blocks and taller buildings. Whether and where they are needed is set by Approved Document B for the specific building, so confirm with building control.
How do cavity barriers work with cavity ventilation?
Open-state intumescent barriers are designed to let air pass so the cavity can still ventilate and drain in normal use, then expand rapidly in a fire to close the gap and resist fire spread. This allows a wall to keep its drained rainscreen performance while providing fire compartmentation.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.