Building components · Layers & Barriers
Ventilated Drained Cavity
Explains why a cavity behind cladding must do two things at once, and what happens when it only does one.
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 drained cavity is
Behind a cladding or covering that is not sealed, a space is provided that does two distinct things. It drains: water passing the outer face runs down it and out at the bottom. And it ventilates: air moves through it so anything that gets wet can dry.
The two functions are separate and both are needed. A cavity that drains but does not ventilate leaves the backing damp; one that ventilates but does not drain lets water accumulate at the bottom.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a wall cavity between masonry leaves. That cavity drains and is commonly not deliberately ventilated; this one is narrower, behind a supported cladding, and ventilation is part of its purpose.
- It is not a service void. A service void is inboard and is meant to be occupied by services.
- It is not a drainage layer. A drainage layer is a material with void space; this is an open air space.
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.
- drained and ventilated cavity
- ventilated void behind cladding
- back-ventilated cavity
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Being open at the bottom so water can leave, and open at both bottom and top where ventilation is required.
- Being continuous vertically, since obstructions dam water and interrupt airflow.
- Not being obstructed by rails, insulation or debris.
- Being reconciled with fire compartmentation, which requires the same continuous space to be interrupted at defined positions.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Drying capacity is what distinguishes a build-up that survives occasional wetting from one that accumulates moisture over years.
A continuous ventilated cavity is a path through a facade, so compartmentation of the cavity and its ventilation function are in direct tension and must be reconciled by design.
Air moving in the cavity is outside air, so the insulation must be inboard of it and must be protected from that air movement.
Horizontal rails, insulation and construction debris all obstruct the cavity, and it is concealed as soon as the cladding goes on.
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
- External Wall
- Facade
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 behind cladding is a construction gap. It is a designed drainage and ventilation space with openings at specific positions.
- That ventilating a cavity makes it colder. The insulation sits inboard of the cavity, so it is outside the insulated envelope by design.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this cavity drained, ventilated, or both, and where are the openings?
- Is the cavity continuous vertically, and what obstructs it?
- How are fire compartmentation and cavity ventilation reconciled here?
- Is the insulation protected from air movement within the cavity?
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, ventilation opening area, or compartmentation arrangement is stated here.
- Cavity ventilation and fire compartmentation in external walls are regulated matters requiring qualified design.
Envelope Layers & Barriers
Continuous positioned layers within a build-up — insulation, separation, protection, drainage and root barriers — named by the job they do rather than the product used.
Browse all layers & barriers entries →