Fibre Reinforced Facade Encyclopedia: Testing and standards
A cavity barrier is a fire stopping element installed inside the ventilated cavity of a rainscreen or ventilated facade to interrupt the vertical and horizontal path that the cavity would otherwise give to flame, hot gas and smoke. It closes the cavity at floor levels, at compartment boundaries, around openings and at the perimeter of the facade, so that a fire starting at one level cannot travel behind the cladding to the next. It is a component of the external wall, not a lining, and it is assessed as part of the wall system rather than on its own.
A 30 millimetre cavity behind a facade is a chimney. It has an unlimited supply of air at the bottom, no obstruction over the full height of the building, and radiant surfaces on both sides. Fire in a cavity travels upward several times faster than fire on an open face.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Function | Interrupts fire, hot gas and smoke spread within the facade cavity | Not a weather or acoustic element |
| Horizontal barriers | At each floor line, and at the head of the cavity | Aligned with the compartment floor |
| Vertical barriers | At compartment walls and at the facade perimeter | Including at building corners where required |
| Around openings | At the reveals, head and sill of windows and doors | Also at service penetrations through the wall |
| Closed-state type | Mineral wool with a foil facing, or galvanised steel | Blocks the cavity permanently; blocks drainage and airflow |
| Open-state type | Intumescent strip on a perforated or slotted carrier | Leaves a ventilation gap that closes under heat |
| Typical intumescent activation | Around 150 to 200 °C, product specific | |
| Cavity depth accommodated | Typically 20 to 150 mm, product and size specific | The barrier must match the built cavity, not the nominal one |
| Evidence | Tested as installed within the wall system, e.g. AS 5113 or BS 8414 | A barrier tested in isolation does not qualify the system |
Why the cavity is the problem
A rainscreen works because the cavity behind the panel is drained and ventilated. That same cavity is an ideal fire path. Air enters at the base and is drawn upward as it heats, the two facing surfaces radiate to each other so heat is not lost, and the geometry concentrates flame into a narrow vertical channel. Fire spread rates measured in facade cavities are several times those on an exposed face of the same material, and the flame front is hidden from view and from a hose stream.
That is why every full-scale facade test standard, AS 5113, BS 8414 and NFPA 285 alike, builds the cavity and its barriers into the specimen rather than testing a bare panel.
Where barriers go
Placement follows the fire strategy for the building, not a standard detail. The usual set is:
- Horizontally at every floor line, aligned with the compartment floor so the cavity is divided storey by storey
- Vertically at every compartment wall that meets the external wall, so the cavity does not bridge two compartments
- Around every opening: head, sill and both reveals of windows, doors and louvres
- At the top of the cavity, at the junction with the roof or parapet
- At the base of the cavity, and at any change in facade plane or material where the cavity would otherwise continue
- At the junction with a fire rated shaft, a plant room or a stair enclosure
The barrier must be continuous and must meet something on both sides. A horizontal barrier that stops 20 millimetres short of the back of the panel, or that is cut around a bracket and not made good, leaves the cavity continuous. Most defects found on inspection are workmanship failures of that kind, not product failures.
Open-state and closed-state
A closed-state barrier fills the cavity permanently, typically with a foil faced mineral wool sleeve compressed between the sheathing and the back of the cladding, or with a folded galvanised steel section. It is the simplest device and the surest one in a fire. It also stops drainage and ventilation dead at every line where it occurs, which on a drained and ventilated facade means either an interrupted drainage path or a flashed break that vents to the outside above and re-enters below.
An open-state barrier leaves a deliberate gap, usually 20 to 30 millimetres, held open by a perforated or slotted carrier, with an intumescent strip mounted on it. In normal service the cavity drains and ventilates through the gap. In a fire the intumescent expands, typically to many times its original thickness, and closes the cavity. That expansion needs heat and it needs time, and the amount of expansion is finite, which is why the product must be selected for the actual cavity depth and why an open-state barrier in a cavity deeper than its tested range is not a barrier at all.
The tension between ventilation and compartmentation
The two requirements pull in opposite directions and the conflict is real rather than notional. A cavity has to be open enough to drain water and dry the back of the panel, and closed enough that fire cannot pass. Open-state barriers are the compromise the industry has settled on. They work, but they trade a degree of fire certainty for drainage and they depend on an intumescent behaving as tested.
The practical resolution is to design the drainage path around the barriers rather than around their absence. Flashings at each horizontal barrier take water out of the cavity and a vent above it lets air back in. Where a designer is tempted to reduce the number of barriers to protect ventilation, the question for the fire engineer is what the full-scale test configuration contained, because that result was achieved with a specific barrier arrangement.
Barriers, cladding material and the compliance route
Cavity barriers are required whether the cladding is combustible or not. A non-combustible facade on the Deemed-to-Satisfy route still has a cavity, and the fire strategy will usually still call for it to be interrupted at compartment boundaries. ShapeShell-RC is Not Deemed Combustible under AS/NZS 1530.1, tested by Ignis Labs, and that says nothing about whether its cavity needs barriers; the building does.
Where the cladding is a polymer composite the barriers become part of the compliance argument itself. Fibre reinforced polymer is combustible and cannot pass AS 1530.1, so the route is a Performance Solution under NCC 2022 Verification Method C1V3 (check the edition adopted in your jurisdiction), whose conditions include cavity barriers. ShapeShell-RT passed the full-scale BS 8414 and AS 5113 test at BRE Global in a specific configuration, and the barrier arrangement in that configuration is part of what was tested. Departing from it is a design change, not a detail.
Standards and references
- NCC 2022 Volume One C1V3 (verification method for spread of fire via external walls) and C2D10 (check the edition adopted in your jurisdiction)
- AS 5113 Classification of external walls of buildings based on reaction-to-fire performance (barriers form part of the tested system)
- BS 8414-1 and BS 8414-2 with BR 135 classification criteria
- Approved Document B Volume 2, Section 8 (cavity barriers), England
- AS 4072.1 Service penetrations and control joints in fire-resisting separating elements
- CWCT Standard for Systemised Building Envelopes (drainage and ventilation of rainscreen cavities)
Related terms
Questions specifiers ask
What is the difference between an open-state and a closed-state cavity barrier?
A closed-state barrier fills the cavity permanently, usually with foil faced mineral wool or steel, and stops drainage and ventilation at that line. An open-state barrier leaves a ventilation gap held open by a perforated carrier with an intumescent strip that expands under heat to close the cavity in a fire.
Are cavity barriers needed if the cladding is non-combustible?
Usually yes. The requirement follows the building’s fire strategy and compartmentation, not the combustibility of the panel. A non-combustible facade still has a cavity that can carry hot gas and smoke between storeys and compartments.
Where do cavity barriers go?
At every floor line, at compartment walls, around the head, sill and reveals of openings, at the top and base of the cavity, and at junctions with fire rated shafts and stairs. Placement is set by the fire strategy for the specific building.
Can cavity barriers be substituted on site?
Not without assessment. If a full-scale facade test such as AS 5113 or BS 8414 supports the compliance argument, the barrier type, depth and layout were part of the tested system, and a substitution changes it. The fire engineer decides whether the change is within the field of application.
Do cavity barriers block drainage?
Closed-state barriers do, which is why a flashing and a vent are detailed at each one. Open-state barriers are designed to keep the cavity drained and ventilated in normal service and to close only under fire conditions.
Detailing barriers into a moulded rainscreen? ShapeShift publishes the tested configuration for each system so the cavity arrangement can be checked against it. See ShapeShell fire test data
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.