The short answer
Curtain walling is a lightweight outer wall, usually aluminium framed and mostly glass, that hangs off the edges of a building's floors. It carries only its own weight and the wind, and passes those loads back to the structure, so it can glaze several storeys without a break.
On this page
The name comes from the way it works. Like a curtain on a rail, the wall hangs from the top and is held at intervals, rather than standing on the ground and holding anything up. The Centre for Window and Cladding Technology (CWCT), the UK body that writes the main standard for these systems, describes curtain walling as a vertical building enclosure that carries no load beyond its own weight and the forces of the weather.
How a curtain wall is held up
A curtain wall is fixed to the edge of each floor slab, or to the frame of the building, with brackets. Each bracket does two jobs: it carries the weight of the wall, and it stops the wind pushing or sucking the wall away from the building.
Between the floors, the wall spans on its own. Vertical members called mullions run from bracket to bracket, and horizontal members called transoms run between the mullions. The glass and any solid panels sit in the grid those members form.
Because the wall is not built into the structure, it has to move independently of it. Floors deflect under load, tall buildings sway, and aluminium expands in the sun. Joints at each floor and between panels take up that movement without breaking the seals.
The main parts
| Part | What it does |
|---|---|
| Brackets | Fix the wall to the slab edge and allow it to be adjusted true |
| Mullions | Vertical members that span floor to floor |
| Transoms | Horizontal members between mullions |
| Glass units | Usually double glazed, sometimes triple, with coatings for heat and solar control |
| Spandrel panels | Insulated solid or opaque glass panels in front of slab edges and ceiling voids |
| Gaskets and seals | Keep water and air out and let the glass move |
| Pressure plates and caps | Clamp the glass in capped systems and finish the outside face |
Types of curtain walling
There are three broad ways to build one.
Stick systems
The frame is assembled on site, piece by piece, then glazed. This is the most common approach on low and mid-rise buildings, entrances and refurbishments. See stick curtain walling.
Unitised systems
Storey-high panels are made and glazed in a factory and lifted onto the building, where they interlock. They cost more per square metre but go up quickly and need less work from outside, which suits tall buildings. See unitised curtain walling.
Structural and point-fixed glazing
The frame is hidden or replaced by bolts, brackets or glass fins, so the outside reads as a sheet of glass. It is used for entrances, atriums and feature walls. See structural glazing.
Curtain walling compared with windows and cladding
Windows sit in holes in a wall that is built some other way, usually masonry or a framed and clad wall. Curtain walling is the wall itself, running past the floor slabs in one continuous skin.
Rainscreen cladding is another kind of facade, where an outer layer of panels shelters an insulated, waterproofed backing wall behind it. Many buildings mix the two: curtain walling for the glazed areas and rainscreen for the solid ones. The CWCT standard covers both, along with sloped glazing.
Where curtain walling is used
- Offices and commercial buildings, where daylight and views are selling points.
- Schools, colleges and universities, often for entrances, halls and stair cores.
- Hospitals and health centres.
- Residential towers and hotels.
- Showrooms and retail.
- Larger private houses, usually for a glazed gable or a wall of glass to a garden.
The rules that apply
Curtain walling counts as building work, so the Building Regulations apply whether it is new or a replacement.
- Energy. Approved Document L sets limits on heat loss. For buildings other than dwellings in England, the 2021 edition sets a limiting U-value of 1.6 W/m²K for new or replacement curtain walling. Read more in curtain walling U-values.
- Fire. Approved Document B deals with fire spread over and through the wall, and some glazed walls must be fire rated. On residential and some other buildings of 18 m or more, regulation 7(2) restricts the materials in the external wall; glass, frames, gaskets and seals are exempt, but insulation in spandrel panels is not.
- Safety. Glass that people could fall against or walk into must meet the safety requirements of Approved Document K.
Performance is proved by testing. The CWCT Standard for systemised building envelopes and the product standard BS EN 13830 set out tests for air, water and wind resistance. See CWCT testing.
What it costs
Published cost data from early 2025 puts a proprietary aluminium stick system at about £450 to £525 per m², supplied and fitted on a flat elevation, and a unitised system at about £1,400 to £1,600 per m², both excluding VAT, access and design. Better glass, a flush outer face or ceramic frit add to that. The curtain walling cost calculator works through an elevation of your own size.
How long it lasts
The aluminium frame can last more than 60 years. The parts that wear are the sealed glass units, at around 25 years, and the gaskets and sealants, which last between 5 and 30 years depending on type. A well-maintained curtain walling facade usually needs a major refurbishment of glass and seals every 25 to 35 years rather than full replacement.
If you have a project in mind, the form below takes two minutes and we will look for a specialist that covers it.
Where our information comes from
- Centre for Window and Cladding Technology (CWCT)
- British Glass, the CWCT Standard for systemised building envelopes
- Approved Document L, conservation of fuel and power (GOV.UK)
- The Building Regulations 2010, regulation 7 (legislation.gov.uk)
- RIBA Journal, Costed envelope and cladding materials, March 2025