The short answer
A curtain wall hangs in front of the floor slabs and runs past them in one continuous skin. A window wall sits between the slabs, floor to ceiling, so each storey is glazed separately and the slab edges show or are covered. Window walls are usually cheaper and simpler; curtain walls give a seamless facade and better control at slab edges.
On this page
From the street, a curtain wall and a window wall can look alike: big areas of aluminium-framed glass. The difference is where they sit relative to the floors of the building, and that changes how they are supported, sealed, fire stopped and built.
The basic difference
Curtain wall
A curtain wall sits in front of the structure. It is fixed to the slab edges with brackets and runs continuously past them, so one wall can cover several storeys without a break. The slab edge is hidden behind a spandrel panel or opaque glass.
Window wall
A window wall sits between the floors. Each storey has its own framed glazing, running from the top of one slab to the underside of the next, with a head and a sill at every floor. The slab edge is either exposed or covered with a separate cladding strip.
Side by side
| Curtain wall | Window wall | |
|---|---|---|
| Position | In front of the slab edges | Between slabs, floor to ceiling |
| Support | Brackets at slab edges, spans storey to storey | Sits on each slab, fixed at head and sill |
| Look | Continuous glazed skin | Storey bands, slab edges visible or clad |
| Typical buildings | Offices, towers, landmark facades | Residential blocks, hotels, mid-rise |
| Installation | Often from outside (stick) or crane (unitised) | Usually from inside each floor |
| Slab edge | Fire stopping behind the wall | Slab itself separates floors |
| Repairs | Depends on system | Usually one storey at a time from inside |
Cost and programme
Window walls generally cost less than curtain walls of similar glazed area. They use lighter frames because each one spans only one storey, need no separate brackets, and can often be fitted from inside the building without scaffolding.
Curtain walling costs vary widely by system. Published cost data from March 2025 put a typical proprietary stick curtain wall at about £450 to £525 per m² and a unitised one at about £1,400 to £1,600 per m², supplied and fitted, excluding VAT. The cost calculator shows what that means for a given elevation.
Performance
Air and water
Curtain walls have fewer joints across the slab lines and can be designed and tested as one continuous system, which helps air and water tightness. Window walls have a head and sill at each floor, and each of those joints with the slab needs careful sealing. Poorly detailed sill joints are a common source of leaks in window walls.
Thermal
A window wall leaves the slab edge as a potential thermal bridge unless it is insulated and clad. A curtain wall puts an insulated spandrel in front of the slab, which tends to give a better result. Either way, the wall must meet the limiting U-value set in Approved Document L.
Fire
With a curtain wall, the gap between the back of the wall and the slab edge must be fire stopped to stop fire spreading between floors, as Approved Document B expects. In a window wall, the slab itself forms the separation, which makes the fire detail simpler, although the junctions still need care.
Movement
Curtain walls must accommodate movement between storeys at joints within the wall. Window walls take up slab deflection at the head of each storey, usually with a sliding head detail.
Which one to choose
Choose a curtain wall when:
- the design calls for an uninterrupted glazed facade,
- the building is tall and a unitised wall would speed up the programme,
- high and consistent air and water performance matters most,
- the slab edges should be hidden.
Choose a window wall when:
- budget is tight and the design accepts horizontal bands at each floor,
- installation from inside each floor is easier than outside access,
- the building is residential, with balconies or floor-by-floor fit-out.
Some buildings use both: curtain walling for the entrance, podium or feature corner, and window walls on the typical floors above.
Questions to ask
- How are the slab edges dealt with, thermally and for fire?
- What air and water classes has the system achieved in testing?
- How is movement between floors accommodated?
- How will a broken pane be replaced on an upper floor?
A curtain walling specialist can advise which approach suits your building. Use the form below and we will look for one that covers the project.