How the two differ in kind
A rain-screen wall works by accepting that the cladding leaks and managing the water behind it. The cavity between the cladding and the weather-resistive barrier is what does the managing, and it has two jobs that are commonly collapsed into one instruction: leave a gap.
DEPTH is what makes the cavity a DRAINAGE path. Water that passes the cladding — through a joint, around a fastener, driven by wind — has to run down a continuous vertical space and out at the base without touching anything it should not. A cavity too shallow to stay open, or bridged anywhere along its height, is not draining: water crosses the bridge to the barrier, and the assembly behind it gets wetter than the design assumed.
VENTILATION AREA is what makes it a DRYING path. Openings at the BASE and at the HEAD, sized as a net free area per unit length of wall, let air move through the cavity — driven by the stack effect and by wind pressure — and carry away the moisture that drainage alone leaves behind: water on the back of the cladding, vapour from the assembly, and whatever wetting the barrier has taken. A cavity with depth but no openings drains and then stays damp, which is a slower version of the problem it was built to solve.
The two are therefore complementary rather than alternative, and a specification that states one without the other is incomplete.
Both depend on CONTINUITY, and that is where the failures happen. HORIZONTAL furring interrupts the vertical path unless it is detailed to let water and air past — notched, discontinuous, or crossed by a second layer of vertical battens — and a wall furred horizontally without that detail has a series of dams up its height. Openings are lost to insect mesh of the wrong specification, since mesh reduces free area substantially. And the base is frequently closed by the ground, by later landscaping, or by a trim fitted tight to the cladding.
The factors that actually differ
Show
| Cavity depth | Ventilation opening area | |
|---|---|---|
| What it provides | A drainage path — a continuous vertical space for water to run down and out. | A drying path — airflow through the cavity to carry moisture away. |
| Where it applies | The full height of the cavity, continuously. | At the base and at the head, as a net free area per unit length. |
| If only this one is provided | The cavity drains and stays damp, because nothing dries it. | The openings connect to a cavity that is not continuous, so nothing flows through. |
| What interrupts it | Horizontal furring, fixings, compressed insulation, mortar droppings, and any trim crossing the cavity. | Insect mesh of the wrong specification, buried base details, and closed head trims. |
| Net free area | Not applicable. | The figure that counts — mesh, louvres and perforations all reduce it well below the physical opening. |
| Horizontal furring | The classic defect. It dams the cavity unless notched, discontinuous, or crossed by vertical battens. | The same interruption blocks airflow as well as drainage. |
| At the base | Must be open to discharge, above finished ground, and it must stay above it after landscaping. | The intake for the airflow, so the same opening serves both — which is why burying it fails twice. |
| At the head | Not directly relevant. | The outlet. Without it the cavity is a chimney with a cap and no flow develops. |
| Effect on the barrier behind | Keeps water off it, so it handles incidental wetting rather than running water. | Lets it dry, which is what extends its life and the assembly's. |
| Both required | Yes. A specification stating one is incomplete. | Yes. |
Which one, and when
Choose cavity depth when…
- Setting out the furring, where the depth and its continuity are decided.
- Where the cladding is heavy or the fixings are deep, and the cavity risks being bridged.
- Where insulation sits in or against the cavity and could be compressed into it.
- Any wall furred horizontally, where the drainage path has to be created deliberately.
Choose ventilation opening area when…
- Detailing the base and head, where the openings are formed and where they are most often lost.
- Specifying the insect mesh, since its free area is what counts rather than the opening's size.
- Any assembly relying on drying — which is every rain screen with a moisture-sensitive backing.
- Where the cladding is impermeable, so the cavity is the only drying route.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does horizontal furring cause a problem?
- Because it lies across the path water and air need to take. A rain-screen cavity drains downward and ventilates upward, both vertically, and a horizontal furring member spanning the wall is a continuous obstruction to both — water arriving at it runs sideways and collects on top of it, and airflow stops. The wall then has a series of small dams up its height, each holding water against the barrier. The detail is common because some cladding types fix horizontally and therefore need horizontal support. The resolutions are established: notch the furring at intervals so water and air pass through, use short discontinuous lengths with gaps between them, or cross-batten — vertical battens first, horizontal over them — which keeps a continuous vertical space behind everything. What does not work is continuous horizontal furring with nothing done about it.
- What is net free area and why is it so much less than the opening?
- The actual open area through which air can pass, after everything obstructing it is deducted — and for a ventilation opening at the base or head of a rain screen, the obstruction is usually insect mesh. A mesh fine enough to exclude insects blocks a substantial proportion of the opening it covers, so a slot of a given physical size delivers considerably less free area than its dimensions suggest. Perforated trims, louvres and profiled closers do the same. The consequence is that the opening has to be sized on the manufacturer's stated net free area for the product being used rather than on the gap's dimensions — and that a detail designed without mesh and then meshed on site to keep insects out has quietly lost most of its ventilation.
- Why does the cavity need openings at both base and head?
- Because airflow needs somewhere to enter and somewhere to leave, and a cavity open at only one end does not flow. Ventilation in a rain-screen cavity is driven by the stack effect — air warmed by the cladding rises — and by wind pressure differences across the wall, and both require a path. An opening at the base with a closed head is a chimney with a cap: air can enter and has nowhere to go, so it does not move. An open head with a closed base is the same in reverse and also removes the drainage outlet. The base opening does double duty — it is both the drainage discharge and the air intake — which is why a buried or blocked base defeats the cavity twice over and is the single most consequential detail on the wall.
- How deep does the cavity need to be?
- Deep enough to stay open and to drain, and the figure comes from the cladding system and the local guidance rather than being universal — but the reasoning is consistent. A very shallow cavity is easily bridged: by a fastener head, by a run of sealant, by a slightly proud barrier lap, by insulation expanding, by mortar or adhesive squeezed out, or simply by construction tolerance in a wall that is not perfectly flat. Once bridged, water crosses to the barrier at that point. A deeper cavity tolerates those imperfections and drains and ventilates more readily. Guidance in wet and exposed climates tends to require more depth than in dry ones, and cladding systems state their own requirement — which is a minimum to design to rather than a target to achieve on the best part of the wall.
- What happens at the base as landscaping changes?
- The opening gets buried, and the cavity stops working — which is the failure that appears years after the building was correctly built. The base of a rain screen is detailed to discharge above finished ground with a clearance, and that clearance is a requirement rather than a preference: it keeps the opening clear of splashing, of debris, and of the ground itself. Then a path is laid, a bed is raised, gravel is topped up, or decking is built against the wall, and the opening disappears. Once it does, water in the cavity cannot leave and air cannot enter, so the wall drains into the ground at the base and stays wet. It is worth recording on the building's documentation and worth checking on any older rain-screen wall showing damp at low level.
- Does a drainage mat count as a cavity?
- As a drainage path, largely yes; as a ventilation path, much less. A drainage mat or entangled-mesh product is a thin layer that guarantees a drainage gap behind a cladding or render system, and it is genuinely effective at that — it is what makes a drained system out of an assembly that would otherwise be bonded. What it provides far less of is airflow, because the space is thin and tortuous rather than open, so drying is slower than in a true ventilated cavity. Which is adequate depends on the assembly: where the cladding is permeable and the backing tolerant, drainage alone may be enough; where the backing is moisture-sensitive or the cladding impermeable, a ventilated cavity with real depth and real openings is what the assembly needs.
- What else has to be continuous?
- The cavity closers and fire barriers, which is a separate requirement that interacts with this one. In many jurisdictions a ventilated cavity behind cladding has to be interrupted by cavity barriers at specified intervals and around openings, to prevent fire and smoke spreading unseen up the cavity — and those barriers have to be detailed so that they close the cavity to fire while still permitting drainage and ventilation, which is what ventilated cavity barriers are made for. Using a solid closer instead dams the cavity permanently. The same applies at every opening: the reveal detail has to close the cavity against fire and weather while letting the water that arrives there drain out, usually through a flashing and weeps at the head and sill.
- How do I tell whether an existing cavity is working?
- By looking at the base and the head, which is where both requirements are visible without opening anything up. At the base: is the opening there, is it clear, is it above the finished ground with a clearance, and can you see daylight or feel airflow through it? At the head: is there an opening, or has the cladding been closed tight to the soffit? Then look for the signs of a cavity that is not draining: damp or staining at low level on the wall inside, deterioration of the cladding's bottom edge, and vegetation or moss growing at the base line. A borescope through an opening shows whether the cavity is bridged. None of it is conclusive, and all of it is more informative than assuming the gap behind the cladding is doing what it was drawn to do.