Building components · Wall Leaves & Backing
Cavity Barrier
Explains why an intentionally continuous cavity is deliberately interrupted, and what the interruption is for.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a cavity barrier is
A cavity barrier is placed inside a concealed space to stop it running continuously. The same continuity that makes a cavity useful for drainage makes it a path through a building, and barriers subdivide it at defined positions.
Positions are typically at compartment lines, around openings, and at the perimeters of a cavity, so that the concealed space is bounded rather than open through the construction.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a cavity closer. A closer deals with the cavity at an opening reveal, primarily for thermal and weather reasons; a barrier subdivides the cavity within the wall.
- It is not insulation, even where insulating materials are used to form one. Its purpose is interrupting a concealed path, not reducing heat loss.
- It is not a cavity tray. A tray directs water out of a cavity; a barrier interrupts the cavity itself.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- cavity barriers
- cavity closure at compartment lines
- fire barrier in cavity
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Being continuous and tight against both sides of the cavity, since a barrier with a gap around it has not interrupted anything.
- Not defeating the cavity's drainage function where the two requirements meet.
- Being present at every position the design identifies, including around openings and at edges.
- Remaining undisturbed by later work in the cavity, such as services or retrofit insulation.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The reason cavity barriers exist is that concealed continuous spaces are a route through a building, and regulation addresses this directly.
A barrier crossing a drained cavity has to be arranged so that it does not dam water, which is a design tension resolved case by case.
Barriers are installed as the cavity is built and are concealed immediately, so whether they are continuous is very hard to verify later.
What barriers were installed, and where, is worth having recorded because the answer is not observable afterwards.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Junctions
Interfaces this element takes part in
Junctions where the condition between this element and another governs how the construction behaves. These name the junction and the subjects it raises; they are not details, arrangements or approved ways of building.
Cavity Barrier Across a Cavity
Where a barrier interrupts a concealed cavity, and where two requirements of that cavity are in direct tension.
Raises: Fire continuity · Moisture · Drainage · Buildability
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Building Envelope
- Building Interior
- Ceiling
- External Wall
- Facade
- Internal Partition
- Separating Construction
Clarifications
Commonly misunderstood
- That cavity barriers and cavity closers are the same element. They address different problems at different positions.
- That any material stuffed into a cavity acts as a barrier. Continuity, tightness and the material's behaviour all matter, and this is a regulated area.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Where does the design require cavity barriers in this construction?
- How is continuity achieved where the barrier meets both faces of the cavity?
- How is cavity drainage maintained where a barrier crosses it?
- What record will exist of the barriers installed?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes what a cavity barrier is for. It states no position, material, dimension, fire rating or regulatory requirement.
- Cavity barrier provision is a regulated matter determined by the building's fire strategy and by qualified designers.
Wall Leaves & Backing
The distinct vertical layers that make up a wall — outer and inner leaves, the cavity between them, and the backing that supports what is fixed to the face.
Browse all wall leaves & backing entries →