Fibre Reinforced Facade Encyclopedia: Design and detailing
The cavity is the void between the back of a rainscreen panel and the face of the wall behind it. It has three jobs and it has to do all of them at once: give water that passes the joints somewhere to go and a path out, allow air to move so that the back of the panel and the face of the wall can dry, and provide the space the substructure occupies. It is also, if nothing is done about it, a continuous vertical flue up the outside of the building.
A 50 mm cavity with a 40 mm rail in it is not a 50 mm cavity. The dimension that matters is the clear space left past the brackets and rails, and on many facades that is a third of the nominal figure.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Nominal depth | 25 to 50 mm typical | Measured from the weather barrier to the back of the panel |
| Clear drainage path | 25 mm minimum recommended | Measured past the brackets and rails, not to the wall face |
| Ventilation openings | Open area at base and head of each compartment | |
| Insect mesh aperture | 3 to 4 mm typical | Excludes insects; reduces effective open area, so increase the gross opening |
| Vermin exclusion | Perforated closers or mesh at base and head | Stainless or polymer; mild steel corrodes and fails |
| Flashing points | Base, each floor line, over every opening | Stop ends and an outward fall at each |
| Fire | Cavity barriers at floor lines and around openings | Open-state barriers where drainage and ventilation must continue |
Depth, and the dimension that actually matters
Cavity depth is almost always quoted as a nominal figure from the face of the weather barrier to the back of the panel. That number is useful for coordination and close to useless for performance, because the substructure lives inside it. A bracket standing 150 mm off the wall carrying a 40 mm vertical rail, with a 20 mm horizontal rail on top, leaves whatever remains between the rail face and the panel as the clear cavity. That remaining dimension is what drains, what ventilates, and what you should be specifying.
Twenty five millimetres of clear space behind the panel is the usual working minimum. Below that, surface tension bridges the gap, water tracks across rather than falling, and the drying air movement collapses. Deep cavities are common where the brackets also carry external insulation, and the deeper the cavity the stronger the stack effect through it, which makes fire compartmentation more important, not less.
Ventilation and drainage
Drainage and ventilation are different functions using the same void. Drainage needs a continuous downward path and an exit: a base flashing with an outward fall and stop ends, a flashing at every floor line where a cavity barrier interrupts the cavity, and a flashing over every window and door head. Every one is a point where the drainage path terminates and has to discharge to the face.
Ventilation needs openings at the bottom and the top of each compartment so that air can move through by stack effect and wind pressure. The open area required is set by the code or guidance applying to the project rather than by physics alone, and it is normally expressed as an open area per metre run of wall. New Zealand prescribes a drained and ventilated cavity through E2/AS1; the CWCT guidance in the United Kingdom and the NCC in Australia treat it as a design matter within a fire and weathering strategy.
Insects and vermin
Any opening that admits air admits everything smaller than it. The cavity behind a facade is warm, dry, dark and undisturbed, which makes it attractive to insects, spiders, wasps and, at ground level, to rodents and small birds. Nesting in a cavity blocks drainage, fouls the weather barrier and, at the base, can fill the ventilation openings entirely.
- Base and head openings get a perforated closer or a mesh, typically at a 3 to 4 mm aperture to exclude insects while still passing water
- Mesh reduces the effective open area, so the gross opening has to be increased to keep the net area
- Mesh material has to last the design life in a damp, unvisited location: stainless steel or a UV stable polymer, never mild steel or plain aluminium in a coastal zone
- At ground level, a heavier vermin mesh or a slotted closer is used, and the cavity base is kept clear of soil and mulch
- Open joints in the panel face are also an entry path, which is one reason baffled open joints are preferred at low level
The cavity as a fire path
A continuous cavity behind cladding is a chimney. Air enters at the base, is heated by a fire, accelerates upward and carries flame with it, and the cavity shields the flame from the wind and from any external suppression. The cladding fires of the 2010s spread far faster up cavities than they could have across an open face, and every code response since has focused on this.
The controls are compartmentation and material. Cavity barriers interrupt the cavity at floor lines and at compartment boundaries, and close it off around every opening in the wall. Open-state barriers use an intumescent that leaves the cavity open in normal service and seals it under heat, which keeps drainage and ventilation working. Every layer in the cavity is also subject to reaction to fire requirements: panel, insulation, weather barrier and the barriers themselves. On Type A and Type B buildings in Australia the Deemed-to-Satisfy route requires non-combustible external wall construction, and anything else is assessed as a whole system, which is what AS 5113 tests.
Standards and references
- CWCT Standard for Systemised Building Envelopes and CWCT technical notes on rainscreen cavity design (United Kingdom)
- NZBC Clause E2 and Acceptable Solution E2/AS1, drained and ventilated cavity requirements (New Zealand)
- NCC 2022 Volume One, external wall and cavity barrier provisions, and Verification Method C1V3 (check the edition adopted in your jurisdiction)
- AS 1530.1 combustibility; AS 5113 full-scale external wall fire test
- AS 4284 Testing of building facades; AS/NZS 1170.2 wind actions
- AS 4312 atmospheric corrosivity zones, for selecting mesh, closers and fixings
Related terms
Questions specifiers ask
How deep should a rainscreen cavity be?
At least 25 mm of clear space behind the panel, measured past the brackets and rails rather than from the wall face. Nominal depths of 25 to 50 mm are common, and much deeper cavities occur where brackets carry external insulation.
Does the cavity have to be ventilated as well as drained?
Yes, on a rainscreen. Drainage removes liquid water; ventilation dries what remains on the back of the panel and the face of the weather barrier. They use the same void but need different provisions.
How are insects kept out of the cavity?
With a perforated closer or mesh at the base and head openings, typically a 3 to 4 mm aperture, in stainless steel or a UV stable polymer. The gross opening is increased to allow for the area the mesh blocks.
Why is the cavity a fire risk?
It is a continuous vertical flue, sheltered from wind, that accelerates flame upward faster than fire can spread across an open facade. Cavity barriers at floor lines and around openings interrupt it.
Detailing the cavity behind a moulded panel? ShapeShift supplies panel and substructure details with the drainage, venting and barrier interfaces resolved. Talk to our facade engineers
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.