Building components · Wall Leaves & Backing
Wall Cavity
Explains the cavity as a designed element rather than an absence, and what filling or bridging it changes.
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 is
A cavity is a space built deliberately between two leaves of a wall. It exists so that water reaching the back of the outer leaf cannot continue to the inner one: there is nothing for it to travel across, and it runs down the cavity face instead.
Whether it remains a void is a design decision. Cavities are commonly partly or wholly filled with insulation, which changes what the space does — the separation function then depends on the insulation's own properties and on how it is installed rather than on the absence of material.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not simply a gap. A gap is what is left; a cavity is designed, drained at the bottom, trayed at interruptions and vented in some arrangements.
- It is not a ventilated drained cavity behind cladding. That is a narrower space behind a rainscreen with a specific ventilation and drainage function; a wall cavity is between two masonry leaves.
- It is not a service void. A service void is inboard of the construction and is intended to be occupied by services; a wall cavity should not be.
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
- wall void
- the gap between leaves
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Being kept clear of what would bridge it — mortar droppings, debris, and badly installed insulation all create paths across.
- Drainage at its base and at every interruption, so that water reaching it can leave.
- Whether it is filled, partially filled or clear, since each is a different arrangement with different requirements.
- How anything crossing it — ties, supports, services — is arranged so that it does not become a water path.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The cavity's core function is breaking the path water would otherwise take from outer leaf to inner leaf, and bridging is what defeats it.
An unfilled cavity that is open to outside air can allow air to move around insulation, which is a thermal bypass rather than a thermal benefit.
A continuous cavity is also a continuous path within a wall, which is why cavity barriers are part of the design in many buildings.
Cavities are kept clean during construction or not at all; debris at the base is very difficult to deal with afterwards.
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.
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.
- External Wall
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That a cavity is empty space with no requirements. It is drained, trayed, tied across and in many buildings compartmented.
- That filling a cavity with insulation is always an improvement. It changes how the wall handles water, and whether it is appropriate depends on exposure and the condition of the wall.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this cavity clear, partially filled or fully filled, and what does the design assume?
- How does water reaching the cavity get out, and where are the trays?
- Are cavity barriers required here, and are they in place?
- What is the exposure of this wall, and does it affect whether the cavity should be filled?
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
- No cavity width, fill type, barrier arrangement or drainage provision is stated here.
- Whether an existing cavity is suitable for insulation is an assessment made on the specific wall and its exposure, not a general question.
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 →