Curtain Wall Façade

What a curtain wall actually is

A curtain wall is a glazed envelope hung in front of the building frame. It carries its own weight and the wind that hits it back to the slab edges, and it carries nothing else. No floor load passes through it. It is a skin on a skeleton.

That one sentence decides everything else on this page, because a skin that hangs in front of a frame has to move when the frame moves, drain the water that gets past its outer face, and be fixed to a concrete edge that was poured to a much coarser tolerance than the wall itself.

Three things get called curtain wall. Only one of them is.

• Curtain wall runs continuously past the slab edges. The floor stops short and the wall passes in front of it. The slab edge is never seen from outside.
• Window wall sits between slabs, floor to soffit, one bay at a time. It is cheaper and simpler, but the slab edge is exposed and has to be clad or finished separately, and each bay is independent.
• Storefront is a framed glazed screen that stands on the floor slab rather than hanging from it. It is a ground-floor system. It is not designed to absorb storey drift and it should not be used above the second floor.

• Curtain wall Hung from the slab edges and carried past them. It bridges every floor, so it has to take one floor moving against the next.

• Window wall Set between slabs, floor to soffit, one bay at a time. Cheaper and simpler — but the slab edge is exposed and has to be finished separately.

• Storefront A framed screen standing on the floor slab. A ground-floor system, not designed to absorb storey drift.

Sections, outside on the left.

The difference that matters is not appearance. It is what happens when the building moves. A curtain wall bridges the slab, so it has to accommodate one floor moving relative to the next. A window wall does not bridge anything, so it does not. That is the whole distinction, and it is why a window wall detailed as a curtain wall leaks and a curtain wall detailed as a window wall cracks.

Mullion depth

50 – 250 mm

Largest unitised panel

1,500 × 4,200 mm

Glass build-up

28 – 40 mm

Air and water class

AE 1200 · RE 1200

Five systems, two decisions

Ask for “a curtain wall” and you have specified about half of what the fabricator needs. The rest divides into two questions that are answered separately.

Question one — how is the wall assembled? Stick, unitised, or semi-unitised. This is a decision about where the labour happens, what the site can deliver, and how much tolerance the wall can absorb. It is a logistics decision, and it belongs at scheme stage.

Question two — how is the glass held? Captured in a pressure plate, bonded with structural silicone, or bolted through with point fixings. This is a decision about the sight line, the external appearance and how the wall is maintained. It is a design decision, and it can be settled later.

The two are independent. A unitised wall can be structurally glazed. A stick wall can be point-fixed. Captured glazing is available on all three assemblies.

The three assemblies

• Stick Mullions and transoms assembled on the building, then glazed bay by bay. Every joint is made — and adjusted — on site.

• Unitised Storey-height units glazed in the factory, hooked on and interlocked. The stack joint between them takes the movement.

• Semi-unitised Mullions installed as sticks, then pre-glazed panels hung onto them. The panel-to-mullion joint is both the seal and the tolerance joint.

Elevations over two storeys; slab edges dashed.

01 · Stick curtain wall

Mullions and transoms are delivered as lengths, assembled in place on the building, and then glazed. The frame goes up first and the glass follows.

What it is good at. Stick work forgives. Every joint is made on site, which means every joint can be adjusted on site, which means the wall can be built onto a frame that is not quite where the drawings said it would be. On an irregular elevation, on a refurbishment, on any building where the geometry changes from bay to bay, this is not a minor advantage — it is the difference between a wall that can be built and one that cannot.

It is also the cheaper system to fabricate. The factory cuts and machines; it does not assemble and it does not glaze.

Where it hurts. All the labour is on the building, at height, in whatever weather there is. The elevation needs access for its full height for the whole duration, which means scaffold or mast climbers, which means cost and programme and a congested site. And the wall is not weather-tight until a bay is completely glazed, so the follow-on trades wait.

Choose it when the elevation is not repetitive, the building is not tall, the site can take scaffold, or the geometry demands site adjustment.

The Ansar Cultural Center is a stick-built curtain wall — 2,000 m² across a cultural building in eastern Tehran, with a single family of profiles carried across every face.

02 · Unitised curtain wall

The wall is built in the factory as complete storey-height units — frame, glass, gaskets, insulation, spandrel panel, all of it — and delivered finished. On site the unit is lifted, hooked onto its brackets, and interlocked with the units already installed. There is no second operation.

What it is good at. The work moves off the scaffold and into a controlled environment. Glazing happens on a bench at ground level under a roof, which is better for quality, better for safety, and immune to weather. Installation is fast and it needs almost no external access — units are commonly craned or hoisted floor by floor, so the elevation is never occupied by scaffold. Each unit is weather-tight the moment it is hung, so the building closes progressively from the bottom and the internal trades follow you up.

Where it hurts. The unit is finished before it leaves the factory, so every dimension is fixed before it leaves the factory. There is nothing to adjust. All the tolerance has to be taken up in the bracket and in the stack joint between units, and that means the slab edges must be surveyed and must be within the range the bracket can absorb — before production, not after. A unitised wall released to production against a drawing rather than a survey is the most expensive mistake available in this trade.

It is also front-loaded. Design and approval take longer, the first unit takes months, and the money is spent before anything appears on the building. On a project that stops and starts, that is a real risk.

Choose it when the elevation repeats, the programme is tight, the building is tall, or the site cannot take scaffold.

03 · Semi-unitised curtain wall

Semi-unitised sits between the two, and it is the least standardised of the three — the word means slightly different things to different fabricators. The common version installs the mullions as sticks, then hangs pre-glazed panels onto them. The vertical structure is made on site; the glass arrives already framed.

What it is good at. It keeps the site adjustment that stick work gives you in the vertical direction, while taking the glazing off the building. On a mid-rise with a crane available but an elevation that does not repeat enough to justify unitising, it is often the honest answer.

Where it hurts. Two interfaces instead of one. The panel-to-mullion joint has to be both a weather seal and a tolerance joint, and it is doing a job that the stack joint does in a unitised wall and the transom does in a stick wall. It needs to be detailed properly or it becomes the leak.

Choose it when the elevation is mid-rise and semi-repetitive, the crane exists, and full unitising cannot be justified by the repeat count.

The two glazing methods — and the default

Captured (pressure-plate) glazing is the default and it is what most of the world builds. The glass sits on setting blocks in the frame, and an aluminium pressure plate clamps it mechanically against the gasket from outside. A cap covers the plate. It is mechanical, it is inspectable, it is repairable, and the glass can be replaced from outside without touching anything structural. The visible result is a grid of aluminium caps, typically 50 – 65 mm wide, over every joint.

• Captured A pressure plate clamps the glass against gaskets and a cap covers it. Mechanical, inspectable, and the glass is replaced from outside.

• Structurally glazed Glass bonded to the frame with structural silicone. The bond width — the bite — is calculated from wind pressure, pane size and the sealant’s design stress.

• Point-fixed Drilled glass bolted to fittings on a structure behind. The fitting has to rotate as the glass deflects, or it levers the hole.

Plan sections through a vertical joint, outside at the top. Simplified — real profiles vary.

04 · Structurally glazed (structural silicone glazing)

The pressure plate is removed on two or four sides and the glass is held to the frame by a structural silicone bond instead. From outside there is no aluminium on the glazed line — only glass, and a silicone joint between panes. The frame is still there; it is behind the glass.

What it is. A bonded joint carrying wind load in shear and tension. The bond width — the bite — is not a detail chosen for looks. It is calculated from the design wind pressure, the short dimension of the pane and the allowable design stress of the silicone. Make the pane bigger or the wind higher and the bite grows.

What it changes. Three things, and all of them are programme items rather than drawing items.

• Fabrication moves indoors and slows down. A structural bond has to be made in controlled temperature and humidity, on surfaces prepared and primed to the sealant manufacturer’s procedure, and then left to cure. It is a factory operation with a recorded quality trail. It is not a site operation.
• Glass replacement is no longer a glazing job. A captured pane comes out by removing a pressure plate. A bonded pane comes out with its frame, or it comes out by cutting a structural joint at height and remaking it on the building — which is exactly the operation that could not be done in the factory conditions the bond required. Decide at design stage how a broken pane will be replaced, and detail for it.
• It is assessed under its own approval route. Structural sealant glazing is not covered by the curtain wall product standard alone; bonded kits are assessed separately. State which route your project is following, because it determines what documentation the bond has to carry.

Choose it when the elevation is meant to read as glass rather than as a grid, and the maintenance route has been thought through.

05 · Point-fixed glazing

No frame at the glass at all. The pane is drilled and bolted at its corners to fittings — spiders, clamps, or countersunk bolts — which are carried back to a structure behind. The joint between panes is silicone. The structure is a separate object that you see through the glass rather than a frame that surrounds it.

What it demands of the glass. Drilling means the glass must be toughened, because annealed glass is not strong enough around a hole to carry the load. Toughened glass carries a small but real risk of spontaneous fracture from nickel sulphide inclusions, and the mitigation is heat soaking. On a vertical wall over an entrance, a lobby, or any occupied space, that is not an optional refinement — it is the reason heat soak testing exists.

What it demands of the structure. The fitting is a hinge. A point fixing has to rotate as the glass deflects, or it will lever the hole and split the pane. Which means the fitting, the glass thickness and the support stiffness are one calculation, not three.

Where the boundary is. A spider fitting on a rigid frame is a detail. A spider fitting on a cable is a system. If the structure behind the glass is a mullion, a fin or a truss, it belongs on this page. If it is a tensioned cable or a cable net, it belongs to Cable-supported Curtain Wall or Cable Net Structures, where the glass fixing has to absorb a much larger deflection and the anchorage into the primary structure becomes the governing problem.

The Moghaddas Ardebili Office is the cable version — a cable-net wall to the street elevation with the frame set back behind the glass line.

Stick or unitised: the decision that sets the programme

This is the single highest-consequence choice on the page, and it is usually made too late or made by default. Five things decide it.

1 · How many times does the unit repeat? Unitised economics come from repetition. The tooling, the first-article approval, the jigs and the test unit are largely fixed costs, and they are spread across the number of identical units you build. A tower with eight hundred of the same unit spreads them to nothing. A building with forty units, all different, spreads them across forty and the number becomes visible. Count your typical units before you decide anything else.

1. 1 · Paid once per unit type Tooling, first-article approval, jigs and the test unit are largely fixed.
2. 2 · Spread across the repeat Eight hundred of the same unit spreads them to nothing.
3. 3 · Few repeats, and it shows Forty units, all different, and the front-end cost is visible in the price.

Schematic — the shape of the curve, not a price.

2 · How tall is it, and what does access cost? Scaffolding a four-storey building is a line item. Scaffolding a twenty-storey building for eighteen months is a project. The taller the building, the more of the unitised premium is paid for by access you no longer need. This is why the crossover is not a fixed number of storeys — it is the point at which access cost exceeds factory cost, and that depends on your site.

3 · What can the site physically do? A crane with the reach to place units at the top floor, or a monorail on the roof, or neither. A street that can take deliveries of storey-height units on a just-in-time schedule, or a site with nowhere to lay anything down. Unitised needs delivery discipline more than it needs space, because units cannot be stacked indefinitely and cannot be stored outside forever.

4 · How accurate are the slab edges, and do you know? This is the question nobody asks in time. A unitised bracket has a finite adjustment range in three directions. If the slab edges are outside it, the bracket does not work, and the options are all bad — shim packs, cast-in remedial steel, or re-making units. The survey belongs before production release. It costs a day and it has saved more projects than any other single act of diligence in this trade.

5 · When does the building need to be weather-tight? Unitised closes progressively — each unit is sealed as it is hung, so the building weather-tightens from the bottom and the internal trades follow. Stick work closes a bay at a time and only once that bay is fully glazed. On a programme where the finishing trades are pressing, that difference is worth more than the price difference between the two systems.

• Stick Scaffold or mast climber on the whole elevation for the whole installation. A bay is weather-tight only once it is fully glazed.

• Unitised Units lifted by crane or roof monorail, floor by floor. Each is weather-tight as it is hung, so the building closes from the bottom and the trades follow it up.

Elevations, part-way through installation. Schematic.

How the wall is held up: the bracket

Every curtain wall on earth is hung from a bracket at the slab edge, and the bracket is where most of the difficult engineering lives.

It does four jobs at once.

• It carries dead load. The weight of the wall goes into the slab at one point per unit or per mullion.
• It carries wind load in and out. Positive pressure and, more onerously at corners, suction.
• It allows movement. Only one bracket per mullion takes the weight. The others must let the mullion slide vertically as it expands, contracts and as the building settles. A mullion fixed at both ends will buckle or will tear its own fixings out.
• It absorbs tolerance in three directions. In and out, up and down, along the elevation. That adjustability is the only thing standing between a slab poured to construction tolerance and a wall built to fabrication tolerance.

• Weight and wind The wall’s weight goes into the slab at one point per mullion. Wind pushes in and — more onerously, at corners — pulls out.

• One fixed, the others slide Only one bracket per mullion takes the weight. The rest hold it against wind and let it slide as it expands, contracts and settles.

• Adjustable in three directions In and out, up and down, along the elevation — the range that absorbs a slab poured to construction tolerance under a wall made to fabrication tolerance.

Sections, outside on the left. The dotted circle marks a channel running along the wall. Simplified — real brackets combine these in one assembly.

Two consequences worth designing for.

The bracket is a thermal bridge. The profile can have the best thermal break available and the bracket will still be a continuous metal path from the outside face to the slab. Isolate it, or accept that the wall’s real performance is worse than the frame’s figure suggests.

The bracket decides whether the wall can be installed at all. Its adjustment range is a published number. The slab edge survey is a measured number. If nobody compares them before production, they will be compared on site, in the worst possible week.

Mullion depth: why the same system ranges from 50 mm to 250 mm

The specification says 50 – 250 mm, set by span and wind load. Here is what sets it.

A mullion is a beam spanning between brackets, and in a curtain wall the brackets are almost always at the slab edges — so the span is the storey height. Wind load on the glass goes into the mullion, the mullion bends, and the mullion has to be deep enough to keep that bending within limits.

It is almost always deflection, not strength, that sets the depth. A shallower mullion would be strong enough. What it would not be is stiff enough — and a mullion that deflects too far breaks the glazing seal, pulls the gasket off the glass and puts the pane into a bending it was not designed for. The glass fails long before the aluminium does.

• Deflection governs The mullion is a beam spanning floor to floor. Long before it is overstressed, a flexible one pulls the gasket off the glass and bends the pane.

• Span Deflection rises steeply with span. A 5 m atrium mullion needs considerably more than five-thirds the depth of a 3 m storey mullion.

• Where the wind is Pressure rises with height and exposure, and sharply in the corner zones and at the top of the elevation. Those zones set the design, not the middle.

Deflection exaggerated. In the span drawing each bar’s length is the span and its width the depth, for a box section of the same width and wall thickness held to the same deflection limit.

So the depth is driven by:

• Storey height. Deflection rises steeply with span. Going from a 3 m storey to a 5 m atrium mullion does not increase the depth by two thirds; it increases it by considerably more.
• Wind pressure, which rises with building height, with exposure, and sharply at corners and at the top of the elevation. The corner zone of a tall building can carry several times the pressure of the middle of the same façade.
• Whether there is a second line of support. A mid-height restraint, a fin, or a floor-level transom that spans horizontally all reduce the effective span, and depth follows.

Which is why 50 mm serves a two-storey shopfront and 250 mm serves a tall atrium wall, in the same system, with the same gaskets and the same glazing method. It is the same wall. Only the beam changed.

Where the depth needed to control deflection has grown to the point where it dominates the elevation, that is the point at which Cable-supported Curtain Wall starts to make sense — tension has no depth.

The largest panel we make, and why that number

The specification gives 1,500 × 4,200 mm for a unitised unit. Four separate limits land near there and the smallest one wins.

• Glass weight and handling. A pane of that size in a laminated insulating build-up is heavy. It has to be lifted onto a bench, glazed, moved, loaded, unloaded, and hung — handled five or six times by machine and by hand.
• Transport. Width on the road is regulated everywhere; height on the lorry is limited by bridges. A unit is transported in a frame, which adds to both.
• Crane capacity at reach. A crane’s lifting capacity falls as the jib extends. The governing case is not the heaviest unit — it is the heaviest unit at the furthest corner of the building.
• Storey height. 4,200 mm is a generous floor-to-floor. The unit is sized so that one unit equals one storey, because that is what makes the stack joint work.

• Storey height One unit per storey is what makes the stack joint work — and 4,200 mm is a generous floor-to-floor.

• Weight and handling A laminated insulating unit this size is heavy, and it is handled five or six times between the bench and the building.

• Transport Width on the road is regulated and height is limited by bridges — and the unit travels in a frame that adds to both.

• Crane at reach Capacity falls as the jib extends. The governing lift is the heaviest unit at the furthest corner, not the heaviest unit.

The unit in the first drawing is to scale.

If your project needs more, say so at concept. It is not always impossible — but it stops being a catalogue decision and becomes a project decision about glass supply, transport route and lifting plant, and those have lead times.

Reading the specification

These are the lines that appear on every curtain wall specification and get copied without being understood.

Product standard — EN 13830

EN 13830 is the harmonised product standard for curtain walling. It defines what a curtain wall kit must declare and how each characteristic is tested and classified: air permeability, watertightness, resistance to wind load, impact, thermal transmittance, airborne sound.

What it is not is an installation standard, and it is not a performance level. It does not tell you that your wall is good. It tells you which tests were run and what classes came back. Two walls can both be “to EN 13830” and be several classes apart on every line that matters. The classes are the specification. The standard is only the language.

Air permeability — Class AE 1200 to EN 12152

EN 12152 classifies how much air leaks through a fixed curtain wall at a given test pressure. The classes run A1 to A4, and AE is the class above them — used when the wall is tested beyond the standard range, with the number giving the test pressure in pascals.

AE 1200 means the wall was held to the permeability limit up to 1,200 Pa. For scale: 1,200 Pa is roughly the dynamic pressure of a wind of about 44 m/s — around 158 km/h. It is a high bar, and it is a bar you should only ask for when the building’s height and exposure justify it.

Watertightness — Class RE 1200 to EN 12154

Same structure. EN 12154 classifies the pressure up to which a wall stays watertight under a specified water spray. R4 to R7 are the standard classes; RE is the class above, with the number again in pascals.

The important thing about this line is what it implies about the detailing. A wall does not reach RE 1200 by being well sealed on the outside. It reaches it by being drained — which is the next section, and it is the part of curtain wall design that is most often misunderstood.

1. 1 · The standard classes stop at 600 Pa A1 – A4 for air and R4 – R7 for water, in steps of 150 Pa.
2. 2 · E is the class above The wall is tested beyond the standard range, and the number is the test pressure.
3. 3 · 1,200 Pa is a severe storm Roughly the dynamic pressure of a 44 m/s wind. Ask for it where height and exposure justify it.

Test pressures on one scale.

Frame transmittance Uf, and overall Ucw

Two different numbers doing two different jobs.

Uf is the frame alone — 1.4 to 2.6 W/m²K for a thermally broken aluminium profile, with the range set by the depth of the section and the width and design of the break.

Ucw is the whole wall: frame, glass, and the spacer bar at the edge of the glass unit where the two meet. 1.2 to 1.8 double-glazed, 0.9 to 1.3 triple.

• Frame alone, Uf 1.4 – 2.6
• Whole wall, double glazed, Ucw 1.2 – 1.8
• Whole wall, triple glazed, Ucw 0.9 – 1.3

0.00.51.01.52.02.53.0

Lower is better

Design note 02 on this page says frame transmittance dominates the wall U-value at typical framing ratios, and that is the practical point. The frame is a small share of the elevation area and a disproportionate share of its heat loss. Specifying excellent glass into an average frame moves the number far less than people expect. So does ignoring the edge of the glass unit: the spacer bar is a short thermal bridge repeated around the perimeter of every pane, and the difference between an aluminium spacer and a warm-edge one is visible in Ucw.

Acoustic performance — Rw 36 to 48 dB

Twelve decibels is an enormous range, and it is almost entirely the glass. The top of it needs specific glass: laminated with an acoustic interlayer, asymmetric pane thicknesses so the two leaves do not resonate together, and a cavity chosen for sound rather than for heat.

Two things will hold you at the bottom of the range regardless of what glass you buy. Opening vents — every operable unit is a joint that has to seal against a gasket rather than being permanently sealed. And the perimeter: the junction between the wall and the slab, and the wall and the return, will set the achieved figure on site no matter what the laboratory said about the wall itself.

Glass build-up — 28 to 40 mm

28 mm is a conventional double unit: two panes and a cavity. 40 mm gives room for a laminated outer leaf, a wider cavity, or a triple build-up. Within that range you are trading thermal performance, acoustic performance, safety, weight and cost against one another, and the weight feeds straight back into the mullion depth and the bracket.

The wall is not sealed. It is drained.

This is the single most misunderstood thing about curtain wall, and it explains half the leaks in the industry.

A curtain wall is not a sealed box. It is a drained and pressure-equalised assembly, and it is designed on the assumption that some water will get past the outer face — because under wind-driven rain, it will.

There are two gasket lines in every properly detailed curtain wall.

• The outer gasket sheds the bulk of the water. It is a rain shield. It is not the seal.
• The inner gasket is the air seal and the last line. The cavity between the two is drained, and the pressure in it is allowed to equalise with the outside through small openings.

Equalising the pressure is what stops the water. Water is driven through a gap by a pressure difference across it; remove the difference and water has to climb in against gravity alone, which it will not do. Any water that does get in lands in the drained cavity, runs down to the nearest transom, and is discharged back to the outside through weep slots.

• Sealed at the face One line of seal with the wind behind the rain. Any gap — and over the years there will be one — has water driven straight through it.

• Drained and pressure-equalised The outer gasket sheds the bulk. The cavity is vented to outside pressure, so wind has nothing to push with, and water that gets in runs out through the weeps. The inner seal is the one that has to be perfect.

Vertical sections through a transom, outside on the left. Simplified — real profiles vary.

Three consequences that should change how you detail.

• Never seal the weeps. They look like defects. They are the drainage.
• The drainage has to be continuous to the outside. Every transom, every stack joint, every corner and every interface with another system has to pass water outward. A drainage path that terminates inside the wall is a reservoir.
• The inner line is the one that has to be perfect. Attention usually goes to the visible outer joint. The seal that stops the leak is the one you cannot see.

Three movements, one joint

Design note 03 on this page says stack joints are designed for the sum, not the largest. Here are the three, and why the sum is the only number that matters.

Inter-storey drift. Wind sway and seismic movement displace each floor relative to the one below. The wall bridges that gap, so the wall absorbs the displacement. In a seismic region this is the governing movement, and it is horizontal.

Thermal movement. Aluminium expands about 0.023 mm per metre per degree. A 4.2 m unit through a 60-degree annual swing between a winter night and a sunlit summer elevation moves close to 6 mm on its own. A dark finish on a south elevation reaches the top of that range; a light finish on a north elevation does not.

Slab deflection and long-term creep. A concrete slab deflects under its own load, then keeps deflecting for years under creep and shrinkage. The floor above closes down on the wall slowly and permanently, and the wall has to keep working while it happens.

• Inter-storey drift Wind sway and seismic movement shift each floor against the one below — horizontal, and in a seismic region the governing case.

• Thermal movement About 0.023 mm per metre per degree: a 4.2 m unit through a 60-degree swing moves close to 6 mm on its own.

• Slab deflection and creep The slab deflects under load and keeps deflecting for years. The floor above closes down on the wall, permanently.

Movements exaggerated.

These do not arrive one at a time. They add, and the joint has to take the sum, in both directions, for the life of the building.

This is also where stick and unitised differ most. Stick work spreads the movement across many small joints — every transom, every anchor, every splice contributes a fraction. Unitised concentrates it into one engineered joint: the stack joint between units, an interlocking male-female profile designed to slide while staying sealed and drained. Distributed tolerance versus concentrated tolerance. It is the real reason a unitised wall demands an accurate slab edge and a stick wall tolerates a rough one.

• Stick spreads it Every transom, anchor and splice takes a fraction of the movement — and absorbs a rough slab edge the same way.

• Unitised concentrates it One engineered joint between units, interlocking, that slides while staying sealed and drained. That is why a unitised wall needs an accurate slab edge.

Simplified — real stack joints carry more gasket lines than drawn.

The thermal break is not a detail

Design note 02 says the break is not a detail, it is the performance. Three things follow from that.

What it is. Aluminium conducts heat several hundred times better than the insulating material used to interrupt it. A thermally broken profile is two aluminium sections — outer and inner — mechanically locked to insulating strips so that there is no continuous metal path from outside to inside. The strips also have to carry structural load between the two halves, which is why the break is a structural component and not a filler.

• The break Outer and inner aluminium locked to insulating strips, so no metal runs through. The strips carry load between the halves — structural, not filler.

• Where it is undone At the spacer bar round every pane, at the bracket from mullion to slab, and at every anchor and interface. The wall on the building is the frame plus all of these.

Plan sections, outside at the top. Simplified.

Where it is undone. At the bracket, which is solid metal from the mullion to the slab. At the edge of the glass unit, where the spacer bar bridges the cavity. At every penetration, every anchor and every interface with another system. The frame figure on a datasheet is a clean laboratory number; the wall on the building is the frame plus all of these.

What it is worth arguing about. The depth and design of the break, the spacer bar, and the isolation at the bracket. Those three move the number. Most of the rest is decoration.

Testing: the mock-up, and when to programme it

Design note 04 says air, water and structural performance are verified on a project mock-up before production release on anything of significant scale. Here is what that actually involves.

A project mock-up is a section of the real wall — usually two bays wide and two storeys high, including at least one typical joint, one corner or interface, and one opening vent if there are any — built as it will be built and tested in a chamber. The sequence matters more than any single result:

1. Air permeability
2. Static water penetration
3. Dynamic water penetration, where specified
4. Structural performance at design wind load, positive and negative
5. Air and water repeated
6. Structural at the amplified load
7. Site hose test on the finished wall

• The mock-up A section of the real wall — typically two bays by two storeys — with a typical joint, a corner or interface, and an opening vent if there are any.

• The sequence Air, water and structural load at design wind — then air and water again. Passing after being loaded is the result that matters.

Step five is the reason to do any of this. A new wall that passes air and water proves the wall was built well. A wall that passes them again after being loaded proves the wall will still be weather-tight after its first storm. A test programme that skips the repeat is testing the wrong thing.

Programme it early. A mock-up must be designed, built, tested, and have its failures corrected before production is released. That is months, not weeks. If it is not in the programme with a slot of its own, it will be squeezed and then dropped — and the first full-scale test of the wall will be the building, in the rain, with the client inside.

What to decide, and when

1. 01

   Concept

   Whether it is a curtain wall at all, or a window wall, punched openings or a rainscreen. Where the glass line sits relative to the slab edge. Storey height.

2. 02

   Scheme

   Stick, unitised or semi-unitised — this sets the programme, the access strategy and the site logistics, and it is expensive to reverse. The grid. Whether there are opening vents.

3. 03

   Tender

   The performance classes: air, water, wind resistance, thermal, acoustic. The mock-up scope and its slot in the programme. Which approval route applies if the wall is bonded.

4. 04

   Production

   Glazing method — captured, structurally glazed or point-fixed. Sight line and profile. Glass specification and spacer. Finish and colour.

What drives the cost

Curtain wall is priced per square metre of elevation, which hides where the money goes. In order of leverage:

1 · How many unit types there are

The dominant factor, and the one most easily controlled by the architect at no cost.

A thousand square metres built from twelve unit types is a different price from a thousand square metres built from ninety. Each type carries its own drawing, its own approval, its own cutting list, its own jig setting and its own margin for error. Rationalising an elevation down to a small family of repeated units is the cheapest saving available on a façade, and it is available only before the elevation is fixed.

• A small family, repeated One drawing, one approval and one jig setting per type, spread across every unit of that type.

• Every unit a type Each carries its own drawing, approval, cutting list and jig setting. The same square metres, a different price.

Elevations. Panels of one type share a shade.

This is the same argument made on Free-form Structures about panel sets. It runs through every system in the catalogue.

2 · Stick or unitised, against height and access

Covered above. Stated as a cost: stick moves money from the factory to the site and into access; unitised moves it the other way and adds a fixed front-end. Which is cheaper depends on how many times the unit repeats and what a full-height scaffold costs on your site.

3 · The glass

Frequently the largest single material line. Double or triple, laminated or monolithic, the coating, the acoustic interlayer, the spacer. Each step up is justified by something — thermal, acoustic, safety, solar — and each is worth paying for when that something is actually required and worth nothing when it is not. Specify from the performance you need, not from the datasheet.

4 · Everything that is not a flat rectangle

Corners, returns, parapets, soffits, the junction with the roof, the junction with the ground, the junction with every other system on the building. These are a small share of the area and a large share of both the cost and the risk, because each one is engineered individually and none of them repeats.

The Yas Shopping Mall is exactly this problem at full scale — a skylight, a curtain wall and a punched metal screen meeting on one building. The value there was not in any of the three systems. It was in the junctions between them.

5 · Opening vents

Every operable unit is a hole in a sealed wall with a mechanism in it. It costs more per square metre than fixed glazing, it is the most likely place for the wall to leak, and it will limit the acoustic and air permeability classes the wall can reach. Sometimes you need them. Count them, and know what each one costs you.

6 · The performance classes you ask for

AE 1200 and RE 1200 are high classes, and reaching them costs money at every joint, every gasket line and every interface. On a tall exposed tower they are appropriate. On a three-storey building on a sheltered site they are money spent on a number nobody will ever test. Specify to the exposure, not to the best class available.

7 · The mock-up

A real cost with a real programme slot, and cheaper than every alternative. The only thing more expensive than testing a mock-up is not testing one.

8 · Slab edge accuracy

Not a façade cost at all — a concrete cost that arrives as a façade cost. Edges within the bracket’s range cost nothing extra. Edges outside it are paid for in shims, remedial steel, remade units and delay.

Profiles

CORTEX produces its own aluminium profiles. For a curtain wall that matters in two specific situations: when the sight line, the cap geometry or the section depth needed by the design does not exist in a standard catalogue system, and when a project needs a profile matched to another system on the same building so that the sight lines run through. Neither is a reason to make a bespoke profile on every project — a standard system is cheaper and faster when it fits. It is a reason to ask the question rather than designing around a catalogue.