Glossary · Two-word term
Fire Barriers
In short
components designed to restrict the spread of fire and smoke in cavities
Fire barriers are critical components within building cavities designed to restrict the spread of fire and smoke. This reference guide explains their technical function, regulatory context in the UK, and their role in cladding remediation and safety.
Last updated
On this page
Fire Barriers
Definition and Function of Fire Barriers
In the context of the UK built environment, fire barriers are components designed to restrict the spread of fire and smoke within concealed spaces, or cavities, of a building’s external wall system. While the term is often used broadly, it specifically refers to elements installed to subdivide a cavity, ensuring that the void created between the structural wall and the external cladding does not act as a chimney, which could otherwise facilitate the rapid vertical or horizontal spread of fire.
These components are distinct from "fire stopping," which typically refers to sealing penetrations where services—such as pipes or cables—pass through fire-separating elements. Fire barriers are instead integral to the cavity design itself. They are manufactured from non-combustible or limited-combustible materials, such as stone wool or intumescent-clad steel, and are positioned at strategic intervals determined by the building's design and fire safety strategy.
Integration within External Wall Systems
Modern rainscreen cladding systems rely on a ventilated cavity to manage moisture and provide thermal performance. This creates a technical challenge: the cavity must remain open to allow air circulation under normal conditions, yet it must be sealed immediately in the event of a fire. To address this, industry practice utilizes "open-state" fire barriers.
These components feature a reactive intumescent strip bonded to a rigid mineral wool core. Under the heat of a fire, the intumescent material expands rapidly to bridge the ventilation gap, effectively closing the cavity. In contrast, "closed-state" barriers are used where no ventilation is required, such as at the junction between a floor slab and the external wall, providing a permanent seal. The correct specification and installation of these components are fundamental to the performance of the external wall system as a whole.
Regulatory Context in England and Wales
The requirements for fire barriers are governed by the Building Regulations, specifically Approved Document B (Fire Safety). This guidance stipulates where cavity barriers must be fitted, such as at the edges of cavities, around openings like windows and doors, and at the junction between the external wall and a compartment wall or floor.
The regulatory framework distinguishes between "cavity barriers," which generally require a minimum fire resistance of 30 minutes integrity and 15 minutes insulation, and "fire stops," which are expected to match the fire resistance of the compartment they are sealing. In buildings over 11 metres or 18 metres in height, the requirements for the combustibility of the materials used within the external wall assembly, including those used in fire barriers, become significantly more stringent. The Building Safety Act 2022 further emphasises the necessity of maintaining a "golden thread" of information, ensuring that the specification and installation of these components are documented throughout the building's lifecycle.
Role in Testing and Remediation
Fire barriers are a primary focus during intrusive inspections, such as those conducted for an External Wall Fire Risk Assessment (EWFRA) or an EWS1 (External Wall System) survey. During these assessments, a competent professional—typically a fire engineer or building surveyor—will verify the presence, placement, and condition of the barriers.
Common issues identified during remediation projects include:
- Missing Barriers: Cavities that were designed to have barriers but where they were omitted during construction.
- Incorrect Orientation: Barriers installed upside down or with the intumescent strip facing the wrong direction.
- Improper Fixings: The use of plastic fixings or incorrect spacing, which may lead to the barrier failing or falling away under heat stress.
- Inadequate Sizing: Gaps between the barrier and the cladding panels that exceed the expansion capability of the intumescent strip.
Where deficiencies are found, remediation typically involves the removal of cladding panels to retrofit or replace the barriers to meet current safety standards and the original design intent.
Fire Barriers
Technical Specifications and Performance
The performance of fire barriers is verified through standardised testing, most notably BS 476 or the more contemporary BS EN 1366 series. These tests measure "integrity" (the ability to remain intact and block flames) and "insulation" (the ability to limit the transfer of heat).
| Attribute | Description |
|---|---|
| Integrity (E) | The duration the barrier prevents the passage of flames and hot gases. |
| Insulation (I) | The duration the unexposed side stays below a specific temperature. |
| Expansion Rate | The speed at which intumescent material reacts to heat. |
| Void Gap | The maximum width of the cavity the barrier is certified to seal. |
Building owners and managers should note that a barrier’s certification is often specific to the substrate it is fixed to (e.g., masonry, steel frame, or timber) and the width of the cavity it is intended to close. A barrier tested for a 50mm cavity cannot be assumed to perform in a 100mm cavity without specific evidence.
Considerations for Building Owners and Managers
For those responsible for high-rise residential buildings, such as Resident Management Companies (RMC) or Right to Manage (RTM) directors, understanding fire barriers is essential for compliance and insurance purposes. Unlike visible cladding panels, fire barriers are hidden, meaning their presence and efficacy cannot be confirmed by a visual walk-around.
Managing agents must ensure that any maintenance or repair work involving the external wall does not compromise the integrity of the fire barriers. For instance, installing new signage or telecommunications equipment that pierces the cladding could inadvertently damage a barrier or create a new, unsealed cavity. A robust "permit to work" system is often recommended to ensure that contractors do not disturb these critical safety components.
Related Terminology and Misunderstandings
In technical discourse, terminology is often conflated, leading to confusion during the procurement of remediation works. It is important to distinguish between the following:
- Intumescent: A substance that swells as a result of heat exposure, increasing in volume and decreasing in density. While many fire barriers are intumescent, not all intumescent products are fire barriers (e.g., intumescent paint).
- Spandrel Panels: The sections of a curtain wall located between the head of a window on one floor and the sill of the window on the floor above. These areas frequently require specific fire barrier detailing to prevent floor-to-floor fire spread.
- Compartmentation: The division of a building into fire-resistant "cells" to contain a fire. Fire barriers are the components that ensure this compartmentation is maintained across the cavity of the external wall.
A common misunderstanding is that fire barriers make a building "fireproof." In reality, they are designed to buy time for evacuation and for the fire service to intervene by slowing the spread of fire. Their effectiveness is entirely dependent on the quality of installation and the compatibility of the barrier with the surrounding wall components.
Fire Barriers