In construction, curtain walling is a non-structural aluminium and glass façade hung from the face of a building: it carries its own weight and wind loads back to the floor slabs, keeps the weather out and lets daylight in, but supports no floors or roof. Curtain walling sits at the heart of most commercial façade packages. Getting the specification right at RIBA Stages 2 and 3 saves programme time and cost later. This guide covers the key decisions — system type, thermal performance, fire and blast performance, testing and how to structure the NBS specification — drawing on the curtain walling packages we deliver across London and the UK, such as Prescot Street in Whitechapel and Pentonville Road in Islington.
Types of commercial curtain walling
- Stick system — aluminium mullions and transoms assembled on site; suits low- to mid-rise buildings; lower unit cost but more site labour
- Unitised system — factory-assembled panels craned in floor by floor; faster on site with better quality control; often preferred for high-rise and time-critical projects (see what is a unitised façade)
- Structurally glazed (SG) — glass bonded with structural silicone for a flush external face; specify the sealant to BS EN 15434, and note that structural sealant glazing kits are now assessed under the European Assessment Document that replaced ETAG 002 (see what is structural glazing)
- Capped — glass clamped by an external pressure plate and cover cap, giving a visible grid; the most common and economical way to hold the glass
- Pressure-equalised and drained — drainage and ventilation paths manage water at the joints; important on exposed elevations and in high wind-driven rain zones
| Stick system | Unitised system | |
|---|---|---|
| How it is built | Aluminium mullions and transoms assembled on site, then glazed and sealed | Factory-assembled, glazed panels craned into place floor by floor |
| Best suited to | Low- to mid-rise buildings, irregular elevations and sites where crane time is limited | High-rise and time-critical projects |
| Cost | Lower unit cost, but more site labour | Costs more to engineer and make |
| On site | More work at height and on site | Faster on site, closing the building in sooner, with better quality control |
- Insulating glass unit
- Aluminium mullion
- Glazing gaskets
- Pressure plate, screwed back to the mullion to clamp the glass
- Snap-on cover cap: the visible grid line on a capped façade
- Structural silicone bond between the glass and the carrier frame
- Spacer tape: sets the silicone joint size while it cures
- Silicone weatherseal over a backer rod in the joint between panes
- Aluminium carrier frame, fixed back to the mullion
As a curtain walling company and contractor in London, we work mainly on Schüco FWS 50 — a 50 mm mullion and transom system with a highly insulated FWS 50.SI version and structurally glazed variants — and on Jansen VISS where a steel façade is specified. Our Schüco vs Cortizo guide for specifiers covers system selection in more detail.
Advantages of curtain walling
- Daylight and views: large areas of glass with slim aluminium sightlines
- Light weight: the wall carries only its own weight and the wind load, so it is far lighter than masonry cladding
- Weather performance: drained and pressure-equalised joints keep water out, even on exposed elevations
- Thermal and acoustic performance: thermally broken frames and insulating glass units meet Part L, with acoustic glass where needed
- Flexibility: one system can combine vision glass, insulated spandrel panels, opening vents, doors and louvres
What affects the cost of curtain walling
- System type: unitised and structurally glazed systems cost more to design and make than stick-built capped systems
- Glass: triple glazing, solar control coatings, laminated, acoustic, fire-rated or blast-resistant glass, and very large panes, all add cost
- Performance: fire-rated and blast-enhanced curtain walling need heavier framing and specialist glass
- Geometry and repetition: a regular grid costs less per square metre than curves, corners and one-off panels
- Access and logistics: scaffold, cranage, delivery restrictions and working hours on city sites
- Design and testing: calculations, mock-ups and approvals are a larger share of the cost on smaller packages
Price curtain walling on the actual elevations and specification rather than on a rate per square metre, because these factors can change the cost of the same area considerably.
Curtain walling U-values and Part L
Approved Document L Volume 2 (2021 edition) sets a limiting U-value of 1.6 W/m²K for curtain walling in new non-domestic buildings in England. That is a backstop rather than a target: the energy and carbon calculation for the whole building usually drives a better figure, and net-zero or BREEAM targets can push the specification further, where triple glazing and warm-edge spacers come into play. The achievable whole-system U-value (Ucw) depends on the frame, the glass and the ratio of vision to spandrel panels, so ask for a calculation on your actual elevation.
Key performance figures to specify: the whole-system U-value (Ucw), the glass g-value (total solar energy transmittance), visible light transmittance, and the risk of surface condensation, assessed using the temperature factor (fRsi). Under BS EN 13830, the curtain walling product standard, ask for evidence of air permeability (classified to BS EN 12152 from tests to BS EN 12153), watertightness (BS EN 12154 and BS EN 12155) and resistance to wind load (BS EN 13116 and BS EN 12179).
Fire-rated curtain walling
Post-Grenfell, the fire performance of external wall systems is heavily scrutinised. In England, Regulation 7(2) bans combustible materials in the external walls of relevant buildings — broadly, buildings with a storey 18 m or more above ground that contain flats or other residential or institutional accommodation — and the Building Safety Act 2022 adds a stricter regime for higher-risk buildings. Fire-resisting curtain walling is classified as E (integrity), EW (integrity with limited radiation) or EI (integrity and insulation) for a period such as 30 or 60 minutes; the period each element needs is set by the fire strategy.
Specify fire-rated curtain walling with test evidence to BS EN 1364-3 or BS EN 1364-4 and classification to BS EN 13501-2, and check that the tested configuration matches what is being built. Junctions between the curtain wall and the floor slabs need cavity barriers and fire-stopping designed in from the start — they are frequently the weakest link and must be coordinated with the structural and fire engineers. Our fire-rated glazing guide covers classifications and test standards in more depth.
Blast-enhanced curtain walling
Where a security risk assessment calls for it, curtain walling can be specified with blast-enhanced performance, using laminated glass, reinforced framing and fixings designed for the blast load. Glazing blast performance is commonly tested to ISO 16933 (arena test) or ISO 16934 (shock tube). The required level should come from the security consultant's assessment rather than being picked from a catalogue.
Writing the NBS curtain walling specification
- Use NBS section H11 (Curtain walling) as your base, and the system manufacturer's own NBS clauses where they publish them
- Define performance by output specification (U-value, air, water, wind load) rather than a proprietary product — this allows a competitive tender
- Require test evidence, system documentation and fabricator details at tender stage
- Include a mock-up and testing clause — sequence testing on a sample panel to the CWCT standard resolves interface issues before production and reduces RFIs
- Agree warranty periods at tender: the system, the glass units and installation workmanship are usually covered separately, so state the period you need for each
Frequently asked questions
What is curtain walling?
Curtain walling is a non-structural aluminium and glass wall hung from the face of a building. It carries its own weight and the wind load back to the floor slabs and keeps the weather out, but supports no floors or roof.
What are the main types of curtain walling?
Stick systems are built on site from mullions and transoms; unitised systems are made from factory-built panels. Within both, capped systems clamp the glass with an external pressure plate and cover cap, and structurally glazed systems bond it with structural silicone for a flush face.
What are the advantages of curtain walling?
Curtain walling gives large areas of glass for daylight and views, weighs far less than masonry, keeps the weather out with drained joints, and meets Part L with thermally broken frames and insulating glass. One system can also take spandrel panels, vents, doors and louvres.
What affects the cost of curtain walling?
The main cost drivers are the system type (stick, unitised or structurally glazed), the glass specification and pane size, any fire or blast performance, the geometry and amount of repetition, site access and cranage, and the design and testing required. Price it on the actual elevations rather than a rate per square metre.
What is the maximum U-value for curtain walling under Part L?
In England, Approved Document L Volume 2 (2021 edition) sets a limiting U-value of 1.6 W/m²K for curtain walling in new non-domestic buildings. Most projects need a better figure to meet the energy and carbon targets for the whole building.
What is the difference between stick and unitised curtain walling?
Stick systems are built on site from individual mullions, transoms and glass, which keeps unit cost down but needs more site labour. Unitised systems arrive as factory-assembled panels that are craned in floor by floor, which speeds up the programme and improves quality control.
Which standard covers curtain walling?
BS EN 13830 is the product standard for curtain walling. It refers to separate test standards for air permeability, watertightness and wind resistance. Fire resistance is tested to BS EN 1364-3 or BS EN 1364-4 and classified to BS EN 13501-2.