Fibre Reinforced Facade Encyclopedia: Design and detailing
A curtain wall is a non-load-bearing external wall that hangs from the building structure and spans floor to floor as a complete wall in its own right. It carries its own weight and the wind load acting on its face back to the slab edge or column at every level, and it carries nothing else. In current usage the term means a framed system, normally aluminium mullions and transoms, infilled with glass, insulated spandrel units or opaque panels and sealed and tested as one assembly.
A curtain wall spans between floors and is the wall. A rainscreen panel spans between substructure rails that are themselves fixed to a wall behind, and is a skin over a wall. The two are detailed, tested and specified differently, and the words are not interchangeable.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Structural function | Self weight and wind load only | No floor, roof or imposed load |
| Typical span | 3.0 to 4.5 m floor to floor | One or two storeys per unit |
| Framing | Aluminium extrusion, 50 to 70 mm face width | Thermally broken in insulated systems |
| Anchorage | Two brackets per unit at the slab edge | One fixed, one sliding, for movement |
| Deflection limit | Span/240, or 20 mm maximum | Common Australian project requirement; confirm the brief |
| Inter-storey movement | Typically 10 to 20 mm per level | Live load deflection plus drift; from the structural engineer |
| Weight in service | 40 to 80 kg/m² | Glazed unitised aluminium systems |
| Performance testing | AS 4284 structural, air and water | Project-specific test on a representative sample |
| Design wind pressure | Site-specific, to AS/NZS 1170.2 | Serviceability and ultimate cases differ |
What makes a wall a curtain wall
Three tests separate a curtain wall from every other kind of external wall. It is non-load-bearing, so it takes no floor or roof load. It spans continuously from floor to floor, passing in front of the slab edge rather than sitting on it. And it is a complete wall, carrying the weatherline, the insulation, the vapour control and often the finished internal face in one factory-built assembly. Remove any one of those conditions and the system is something else.
The load path is short and explicit. Pressure on the glass or infill goes into the transoms, then into the mullions, then into two brackets per unit at the slab edge. Those brackets are the whole structural conversation: one is usually fixed and one slotted, so that the frame can absorb floor deflection, creep, shrinkage and inter-storey drift without the infill being squeezed. The design pressures themselves come from wind load.
Curtain wall or rainscreen cladding
Specifiers confuse the two constantly, and the confusion is expensive because it decides who is responsible for keeping water out. A curtain wall is the weatherline. It is sealed, drained internally within its own extrusions, and tested as a single assembly to AS 4284 for air infiltration, water penetration and structural adequacy. A rainscreen cladding panel is not the weatherline. Water is expected to pass its joints, and a separate weather barrier on the wall behind does the sealing. See rainscreen for that principle.
The structural difference follows from it. A curtain wall mullion spans the full floor to floor height and is engineered for that span. A rainscreen panel spans only between substructure rails or brackets at roughly 600 to 1,200 mm centres, and it is the substructure, not the panel, that reaches back to the slab or structural wall. A moulded panel weighing 5 to 50 kg per square metre is a cladding element, not a curtain wall, however large the panel is.
Stick, unitised and panellised construction
- Stick system: mullions, transoms, glass and infill are delivered loose and assembled in place. Low tooling cost, slow installation, many site-made seals.
- Unitised system: storey-high units are glazed and sealed in the factory, craned in and stitched together at split mullions. Fast installation, far better quality control, early design freeze required.
- Structural silicone glazed: the infill is bonded to the frame with structural silicone instead of being captured by a pressure plate, giving a flush external face.
- Spandrel zone: the opaque band at slab level, insulated behind, and often where a moulded panel, a metal tray or back-painted glass is used.
Unitised construction dominates Australian high rise because it moves labour off the scaffold and into a factory. It also sets the tolerance regime: unit to unit joints are typically 15 to 25 mm, and every unit is set out from a survey of the structure as built rather than from the drawings. See tolerances.
Where moulded panels meet a curtain wall
Glass fibre reinforced concrete and fibre reinforced polymer elements appear on curtain walled buildings in three positions: as an infill unit within the curtain wall grid, as the face of a spandrel unit, and as an applied feature such as a fin, blade, mullion cover or crown element carried on its own brackets off the frame or the slab. In the first two positions the panel becomes part of a tested assembly, so its weight, thermal movement and fixing method have to be agreed with the facade contractor at design stage rather than after award.
Weight is usually the reason a moulded composite is chosen here. A frame designed for glass will not accept a heavy stone or precast spandrel without redesign, but will often accept a 5 to 32 kg per square metre fibre reinforced polymer element, or a 30 to 50 kg per square metre glass fibre reinforced concrete element, with a bracket change alone. Fire performance of the spandrel zone and the perimeter fire stopping at the slab edge are separate questions; see ncc-external-walls.
Testing and compliance
AS 4284 is the project-specific facade test used in Australia. A representative sample, usually two or three bays wide and two storeys high, is built on a rig and taken through serviceability air infiltration, static and cyclic water penetration, serviceability wind pressure, ultimate structural pressure and imposed inter-storey movement. It tests the design and the workmanship rather than certifying a product, and it is the right place to discover that a detail does not work.
Under the National Construction Code (check the edition adopted in your jurisdiction), the external wall provisions apply to the curtain wall as a whole, including insulation, spandrel backing, gaskets and any applied panel, not to the aluminium frame alone. The equivalent references are the CWCT Standard in the United Kingdom, EN 13830 in Europe, and ASTM E283, E331 and E330 in North America.
Standards and references
- AS 4284 Testing of building facades
- AS/NZS 1170.2 Structural design actions: wind actions
- AS 2047 Windows and external glazed doors in buildings; AS 1288 Glass in buildings
- AS 1170.4 Structural design actions: earthquake actions in Australia
- NCC 2022 Volume One, external wall provisions and Specification 7 (check the edition adopted in your jurisdiction)
- EN 13830 Curtain walling: product standard
- CWCT Standard for systemised building envelopes (United Kingdom)
- ASTM E283 air infiltration, ASTM E331 water penetration, ASTM E330 structural performance (United States)
Where this matters on a ShapeShift project
One Sydney Harbour, Barangaroo: A high rise residential tower where moulded facade elements sit alongside the glazed envelope and have to satisfy the same external wall provisions.
Related terms
Questions specifiers ask
Is a curtain wall load-bearing?
No. A curtain wall carries only its own weight and the wind load on its face, and transfers both to the floor slabs or columns. It supports no floor, roof or imposed load, which is what the word curtain means in this context.
What is the difference between a curtain wall and rainscreen cladding?
A curtain wall is the weatherline and spans floor to floor as a complete sealed wall tested to AS 4284. Rainscreen cladding is an outer skin over a separate weather barrier, spanning only between substructure rails, and water is expected to pass its joints.
Can GRC or FRP panels be used in a curtain wall?
Yes, as infill units, spandrel faces or applied features carried on the frame or the slab. They are chosen for weight: a moulded composite at 5 to 50 kg per square metre usually needs only a bracket change, where stone or precast would need the frame redesigned.
What is the difference between stick and unitised curtain wall?
Stick systems are assembled from loose members on site and rely on site-made seals. Unitised systems arrive as complete storey-high units glazed in the factory, install far faster and hold better tolerances, but require the design to be frozen earlier.
Does a curtain wall have to be non-combustible?
The external wall provisions of the National Construction Code apply to the whole assembly, including insulation, spandrel backing and any applied panel, not just the aluminium framing. Check the edition adopted in your jurisdiction and read the provisions against the complete build-up.
Specifying a moulded panel into a curtain wall grid or a spandrel zone? We can review weight, movement and bracket interface before the facade package is awarded. Talk to our engineering team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.