For owners of office and mixed-use buildings
Curtain Walling Explained: How It Works, Which System, and What It Costs
Sviatoslav JerdetchiiCEO & Founder of Craftex. Updated , 7 min read
The short answer
Curtain walling is a non-load-bearing facade of aluminium framing and glazed or solid panels, hung from the building's structure. It carries only its own weight, wind and weather.
- Stick systems are built on site. Unitised panels arrive finished from the factory.
- The frame can outlast the glass units and seals by decades.
- Replacing a curtain wall brings Part L into play and, on tall residential buildings, regulation 7.
Curtain walling is a facade of aluminium framing filled with glass or solid panels, hung from the building's structure. It keeps the weather out and carries only its own weight and the wind.
This guide is for owners and asset managers of office and mixed-use buildings, and for anyone specifying a new facade. It covers the systems, how long each part lasts, the rules on replacement and what it costs.
Our curtain walling contractors survey, cost and replace facades in occupied buildings. Is yours leaking or fogging? Get a free survey and cost plan.
What curtain walling is
A curtain wall is the outer skin of a building, hung in front of the floors like a curtain. It is not part of the structure. The fire safety guidance describes curtain walls as walls that transmit only self weight and wind loads, and no floor load.
The skin is a grid. Vertical aluminium sections called mullions run from floor to floor. Horizontal sections called transoms divide each bay. Between them sit infill panels: glass units, or solid spandrel panels that hide the floor edge. Brackets fix the grid to the slabs, and gaskets and sealants make it weathertight.
Offices, hotels, schools, hospitals and mixed-use blocks all use it. A wall more than 25 years old is already past the expected life of its glass units, so many are now due a decision.
Stick, unitised and other curtain wall systems
Systems differ in where the work happens. Stick systems are built piece by piece on site. Unitised systems arrive as finished panels.
| System | How it is built | Where it suits | Site or factory |
|---|---|---|---|
| Stick | Mullions and transoms fixed to the slab edge, then glass and panels fitted | Lower buildings, complex shapes, repairs | Mostly on site |
| Unitised | Frames glazed and sealed in a factory, then lifted onto the slab as whole panels | Tall buildings, repeated bays, tight programmes | Mostly in the factory |
| Semi-unitised | Factory-made frames or panels combined with glazing fitted on site | Schemes where a pure system does not fit | A mix of both |
| Structurally glazed | Glass fixed with structural silicone or point fixings, so little frame shows | Flush elevations that look almost frameless | Depends on the frame system |
GreenSpec says stick systems are site assembled and carry a higher risk of workmanship non-conformities. Unitised systems are factory manufactured. The difference matters in an occupied building, because a unitised bay spends less time open to the weather.
BS EN 13830 is the product standard for curtain walling kits. Any new system should come with test evidence against it.
If a facade is already leaking, our curtain walling contractors can survey it and cost the options for you.
How long curtain walling lasts
Curtain walling rarely fails all at once. Its parts wear out at different speeds. GreenSpec's whole life guide gives an expected life for each.
| Part | Expected life | What shortens it |
|---|---|---|
| Aluminium frame | More than 60 years | The finish: 15 to 30 years or more, by type |
| Insulating glass units | 25 years | Edge seal failure lets moisture in |
| EPDM gaskets | 15 to 30 years or more | Hardening and shrinking |
| Neutral-cure silicone sealant | 5 to 20 years | Weather and movement at joints |
So the frame often outlasts the glass. A wall that leaks or mists at 25 years is usually failing at the seals and units, not the aluminium.
Look for these signs.
- Misted glass units, where moisture has entered the sealed cavity
- Water marks or leaks at transoms and around the floor edge
- Cracked, hard or shrunken gaskets and sealant joints
- Corrosion or a flaking finish on the aluminium and its brackets
Survey before you decide. A survey records which parts have failed and how many. You then pay to fix what is broken, not the whole wall.
Curtain walling and the Building Regulations
Three regulatory sources touch a curtain wall, plus the product standard. Which apply depends on the work and the building.
| Rule | What it says | When it applies |
|---|---|---|
| Part L, Approved Document L Volume 2 | Maximum U-value of 1.6 W/m²K | New or replacement curtain walling, buildings other than dwellings |
| Regulation 7(2) | External wall materials A2-s1,d0 or A1 | Storey 18 m or more above ground, with flats, an institution or residential rooms |
| Part B, Approved Document B Volume 2 | Fire-stopping where each compartment floor meets the external wall | Buildings with compartment floors |
| BS EN 13830 | Product standard for curtain walling kits | The system you buy |
Part L: the 1.6 U-value
Approved Document L sets 1.6 W/m²K for curtain walling in Table 4.1, as an area-weighted average. It applies to new curtain walling and to replacement elements in existing buildings. The 2026 edition keeps 1.6 and takes effect on 24 March 2027 for most building work.
Whether reglazing alone triggers the standard depends on the scope. Agree it with building control before you price.
Regulation 7: combustibility on tall residential buildings
Regulation 7(2) requires materials in the external wall of a relevant building to be class A2-s1,d0 or A1. A relevant building has a storey at least 18 m above ground and contains flats, an institution or rooms for residential purposes. A tall office building with none of these is outside it.
Window frames and glass are exempt, as are seals, gaskets, fixings, sealants and backer rods. The exemptions do not name spandrel panels or insulation, so check each against the regulation.
Part B: fire-stopping at the floor edge
Approved Document B says the fire resistance of a compartment floor must be kept where it meets the external wall. Fire-stopping does that. On a curtain wall, this is the gap between the slab edge and the glazing line, hidden behind the spandrel. A replacement is the moment to open it up, inspect it and repair it.
Craftex has completed a range of both small and large-scale projects on our commercial property with exceptional precision and quality. Dmitriy consistently delivers courteous, prompt service and has shown great flexibility in accommodating last-minute changes to plans and timelines. I highly recommend Craftex for their professionalism and outstanding workmanship.
Refurbish, reglaze or replace
There are four ways to deal with a failing curtain wall. Each fixes a different failure.
- Reseal and regasket. Replace failed gaskets and sealant joints. It stops leaks at the joints and is the cheapest route.
- Reglaze. Replace misted glass units in the existing frame. It restores the view and the thermal performance, if the frame can take the new units.
- Refurbish the frame. Repair corroded fixings and refinish the aluminium. It suits a sound frame with tired parts.
- Replace the system. Strip the old wall and fit a new one. It resets the lifespan and meets current Part L, but costs most and takes longest.
Knight Frank's Spring 2026 landlord guide gives London benchmarks. It uses Central London as the base and excludes VAT and professional fees. We converted its per ft² figures to per m².
| Work | £ per ft² | £ per m² |
|---|---|---|
| Repairs to window seals and gaskets, including access (per ft² of facade) | £1 to £5 | £11 to £54 |
| Replace glazed windows with triple glazing, including access (per ft² of glazing) | £10 to £25 | £108 to £269 |
| Replacement of facades and windows | £20 to £100 | £215 to £1,076 |
Outside London the guide cuts its rates. In the South East, for example, it takes 10% off. These are benchmarks, not a quote. Access, glass specification and the state of the frame decide the real price.
If the building is let, its energy rating matters too. Our guide to MEES rules for commercial property explains the standard landlords must meet. Timing the facade with a Cat A refurbishment of the floors means the new glazing is in before the next tenant moves.
Replacing a curtain wall in an occupied building
A curtain wall can be replaced while tenants stay in. The work follows a sequence that keeps every floor weathertight overnight.
- Survey and open up. Survey the wall, then open a sample bay to see the floor edge, the brackets and the fire-stopping.
- Design the sequence. Split the facade into bays or floors, so only a small part is open at any time.
- Weather each bay. Fit temporary weathering to each open bay before the old wall comes out.
- Work from outside. Use mast climbers, cradles or scaffold where you can, to keep work out of tenants' space.
- Hand back floor by floor. Finish, check and clean each bay before the next one opens.
CDM 2015 and notification
CDM 2015 applies to the work. A project is notifiable in two cases under regulation 6. One is work scheduled to last longer than 30 working days with more than 20 workers at once. The other is work that exceeds 500 person days. A large facade can pass either test.
Our guide to CDM 2015 client duties explains what the client must do.
Working around live tenants
At Portsoken House in Aldgate we delivered 1,550 m² of offices in seven months around occupied tenants. The Portsoken House reception was remodelled over nights and weekends, with tenants undisturbed.
Neither was a curtain wall job. They show how we phase work around the people in the building. Our guide to office refurbishment while occupied covers the method.
Use this list before you commit. Send us the address and photos, and get a free survey and cost plan. Already have a quote? Send it to us. We'll beat it.
- A survey that records which glass units, seals, gaskets and brackets have failed, with photos
- A sample bay opened to check the floor-edge fire-stopping and fixings
- Building control's view on whether the 1.6 W/m²K standard applies to your scope
- Regulation 7 checked, if a storey is 18 m or more above ground with flats or an institution
- Test evidence against BS EN 13830 for any new system
- Reseal, reglaze, refurbish and replace all priced, so you can compare
- Temporary weathering and the access method agreed for each phase
- Tenant notice, working hours and CDM notification agreed
