Building envelope · Facades & Cladding
Curtain Walling System
This entry explains why curtain walling behaves as one continuous system: brackets that must absorb structural movement, a glazing rebate designed to drain rather than to seal, and control layers that have to be made continuous across every unit joint and floor edge.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What curtain walling is
A framework spans from floor to floor and is hung back to the structure at each level, and the openings within that framework are filled with vision glass, opaque spandrel panels, doors and vents. Nothing beneath supports it; the wall below carries none of it, and each length of frame carries only itself and the wind that presses on it. The building's envelope, its thermal line, its air control and its appearance all arrive as a single designed package.
Stick and unitised construction are two ways of dividing the same problem. Stick systems arrive as members and infill assembled in place, so the joints between components are site joints. Unitised systems arrive as complete storey-height units, so most joints were made in a factory and only the joints between units are made on site. The difference is not quality but where the joints are and who made them, and that governs where the assembly is most likely to leak.
The nearest sibling is the ventilated rainscreen facade system, and the test is what stands behind the visible layer. A rainscreen is a screen set in front of a backing wall, and that wall carries both the load and the control layers; curtain walling has no wall behind it, because the framework and its infill are the envelope. A second visible check is the floor line: curtain walling passes in front of the slab with a spandrel zone and a joint at every level.
The least intuitive part is that the glazing rebate is not sealed. Glass sits on setting blocks in a rebate that is vented to the outside and drained, so that water and vapour reaching it are returned rather than trapped. The assembly is designed to be wet in defined places, which is why sealing a drainage slot with the intention of improving the facade is one of the most reliable ways to damage it.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- curtain wall
- stick curtain walling
- unitised curtain walling
- glazed envelope system
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Enclose a building with a self-contained envelope that spans between floors instead of resting on a wall below.
- Carry only its own weight and the wind acting on it, and deliver both back to the structure at chosen points.
- Admit water at the glazing rebate by design and return it outside, rather than relying on a sealed line at the outer face.
- Maintain continuity of air, water and thermal control across every joint between units and at every floor edge.
- Let the structure shorten, deflect and sway without transmitting that movement into rigid frames and brittle infill.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportMullions and transoms
The vertical and horizontal members forming the grid. Mullions usually span between floors and take the wind load; transoms divide the openings and support the infill. Together they are the only structural element in a facade that carries nothing else.
- AttachmentBrackets back to the structure
Connections at each floor, typically one taking weight and others restraining only. They also absorb the deviation between a factory-made frame and a structure built on site, and they must let the floor above deflect without loading the frame below.
- Finish surfaceVision glass and spandrel infill
The panels within the grid: glazed units where there is view and daylight, opaque panels where the floor zone or a service riser sits behind. Both are held by the same framing, but only one of them has a room on the other side of it.
- Drainage planeDrained and vented glazing rebate
The channel around each infill panel, deliberately open to outside air and drained downwards and outwards. Water crossing the outer gasket collects here and is returned, which only works while the slots stay clear and the path runs continuously downward.
- Jointing and sealingGaskets and unit-to-unit joints
The outer and inner gaskets around each panel, and the joints between units or members. The inner line does most of the air control work while the outer sheds the bulk of the water, and the space between them is where drainage happens.
- Control layerAir and vapour continuity through the frame
The chain of gaskets, seals and connections that keeps the air control line unbroken across a facade made of many parts. It runs through the frame rather than behind it, so its weakest point is wherever a site-made joint meets a factory-made one.
- Thermal layerThermal breaks and spandrel insulation
Isolating components within the frame profiles, and insulation behind the opaque panels. The break sits inside the load path, so it is a structural component as well as a thermal one, and its position governs where the coldest internal surface will be.
- Cavity or voidSpandrel zone at the floor edge
The concealed space between the back of an opaque panel and the edge of the floor slab. It usually contains insulation, a back pan and provisions determined by others, and it is one of the least visited spaces in a finished building.
- Edge and terminationHead, cill and abutment closures
Where the framing stops: at the roof edge, at the ground, against a different facade type or against an internal wall. Each closure has to weather, carry the control layers off the frame onto something else, and accept movement from both sides.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The frame is hung, not stood up, and the bracket arrangement is what keeps that true. One connection per unit takes weight and the others restrain only, so that as the structure shortens and each floor deflects, the frames below are not asked to carry any of it. Where a restraint locks instead of sliding, the weight of the framing above comes to bear on the frame beneath it, and the structure's shortening and deflection are forced into a member that was designed to be free of both. The consequence appears as distorted frames and broken glass rather than as a movement crack.
The drained rebate is the part most often defeated by good intentions. Water passing the outer gasket is meant to reach a channel, run down, cross to the outside and leave, and every stage of that depends on the next. Sealing over a vent slot, filling a rebate with sealant or breaking the downward path at a transom converts a zone designed to be wet into a closed one that holds water against a glazing unit's edge seal, which is where it does the most harm.
Continuity is the harder half of the design. The frame is a grid of separate pieces, so the air and water control lines are a chain of gaskets, corner blocks and joints rather than a sheet. Unitised construction moves most of those joints into a factory and leaves the split between units on site; stick construction leaves nearly all of them on site. Either way, the weakest joint in the chain, not the average one, sets what the facade does.
Thermal breaks sit inside the load path, which makes them structural components with a second job. Where they fall determines which internal surface runs coldest, and that in turn is what governs where condensation is most likely to form on a cold morning. Whether it will form at all depends on the internal conditions, the assembly and the climate, and is assessed by a qualified professional rather than read off the frame.
The spandrel zone is where the most parties meet in the least accessible place. The facade, the floor structure, the air control line and provisions determined by fire and acoustic design all converge behind an opaque panel at every level. Each is designed by someone different, installed at a different time, and covered permanently once the floor finish goes down, which is why the coordination happens on paper or not at all.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Framing families of these kinds are commonly encountered as mullions and transoms. What a member can span, and how it behaves with an isolating component inside its section, is set by the system documentation and by facade engineering.
Finish surface
Glass families of these kinds are commonly encountered as infill. Which is encountered in a given opening follows from the facade design, the exposure and requirements determined elsewhere, and never from the appearance of a neighbouring building.
Jointing and sealing
Sealing components of these families are commonly encountered at unit joints and abutments. Because the rebate itself is drained rather than sealed, where sealant belongs and where it must not go is defined by the system documentation.
Thermal layer
Insulating components of these kinds are commonly encountered behind opaque panels and within glazing units. What any of them contributes is calculated for the complete assembly by a qualified professional rather than inferred from the parts.
Control layer
Tape and sheet families of this sort are commonly encountered where the control line leaves the frame at abutments and floor edges. Whether one adheres to a coated metal profile is a compatibility question for both products' documentation.
Attachment
Bracket components of these families are commonly encountered connecting frames to the structure. Contact between dissimilar metals, and the adjustment each connection provides, are matters for the facade engineer and the structural design.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Slab edge and cast-in fixings
Brackets reach a slab edge whose position and level are known only within a range, and whose shortening continues after the facade is installed. Cast-in channels have to be placed before the pour, so a facade decision lands inside the structural programme.
Read about Concrete Frame →Deck edge and its tolerance
A steel and deck floor presents an edge that is lighter and more variable than a cast slab, and one that deflects visibly under load. The bracket arrangement has to take up both the setting-out deviation and the movement that follows occupation.
Read about Composite Metal Deck Floor →Control line leaving the frame
The frame carries the air control line across the facade, but at every abutment it has to be handed onto a wall, a slab or a roof. That handover involves two trades and two materials, and it is where an otherwise continuous line is most often left open.
Read about Air Barrier System →Internal wall meeting a mullion
Where a partition or separating wall runs up to the facade, it lands on a frame that is continuous past it. The junction has to close a gap that moves, and what happens there is governed by acoustic and fire design carried out separately.
Read about Separating Wall →Head of the facade at the roof
At the top the framing stops and the roof takes over, with a coping, an upstand and the roof's own control layers arriving from a different package. Water shed from the roof reaches this junction first, and the closure decides where it then goes.
Read about Parapet System →Elements hung from the mullions
Fins and louvres are frequently fixed directly to facade frames, which asks a member designed for wind to carry a cantilever as well. The fixing also crosses the frame's isolating component, so the connection is a thermal question at the same time.
Read about External Shading →Meeting a different facade type
Where curtain walling stops and a punched opening or a solid wall begins, two entirely different weathering philosophies meet along one line. The drained system has to discharge at that boundary rather than into the construction it abuts.
Read about Window Opening Interface →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- This facade is designed to admit water in defined places and return it, so the presence of moisture in a rebate is expected. What matters is whether the path out remains continuous and clear from the point it enters to the point it discharges.
- Thermal
- The frame crosses the insulation line everywhere by definition, and isolating components inside the profiles are what interrupt it. The result is calculated for the specific system and arrangement by a qualified professional, not taken from the glass.
- Acoustic
- A largely glazed envelope, and the gap between its frame and each floor, both influence how sound travels into and around a building. Behaviour belongs to the tested assembly and the flanking construction, and belongs with a qualified acoustic professional.
- Fire
- The space between the facade and each floor edge, and the materials within the spandrel zone, are the subject of fire design carried out separately. This entry makes no determination about any of it, and the relevant authority and the tested details govern.
- Movement
- Structural shortening, floor deflection, sway and the thermal movement of long metal members all arrive at the same brackets. Provision for each is made in the connection rather than in the frame, because the frame itself is not free to move.
- Durability
- Gaskets, seals and the edge seals of glazing units age at different rates from the metal around them, so parts of this facade have a shorter working life than others. Which parts can be renewed without dismantling the rest is a design outcome.
- Maintenance and access
- Cleaning, gasket replacement, resealing and glass replacement all happen from the outside face at height. The access provision, whether cradle, davit or platform, is designed with the building and constrains everything that can be done later.
- Documentation
- A facade of near-identical bays is difficult to describe after the fact, so unit marks, glass types, gasket references and drainage details form the only reliable record. Without it, a later repair begins by identifying what is already there.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Whether the system is stick or unitised determines where the joints are and how many of them are made on site rather than in a factory.
- Which bracket at each level takes the weight and which merely restrain, and whether the restraints can genuinely slide, governs how movement is absorbed.
- How the drained rebate discharges at each transom and at the base of each unit is worth tracing on the drawings before installation begins.
- Where the air control line leaves the frame, and who owns that handover at the roof, the ground and each abutment, needs settling early.
- What is inside the spandrel zone at every floor edge, and who installs each part of it, is a coordination question with several owners.
- Which internal surface of the frame will run coldest, and what that means for the internal conditions expected in these rooms, is worth asking now.
- How glass, gaskets and seals will be replaced in the future, and from what access equipment, constrains the design rather than follows it.
- Where curtain walling meets another facade type, which system discharges into which is a decision with consequences at that line for decades.
- What record of unit positions, glass types and drainage details will exist at handover determines what any later repair has to start from.
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- The glazing rebate is assumed to be sealed, when it is deliberately vented and drained and depends on staying that way.
- Sealant applied over a drainage slot is thought to improve the facade, when it closes the route that keeps water off the glazing unit edge.
- Curtain walling is read as a wall with a lot of glass, when it carries none of the building and spans between floors instead of standing on one.
- Stick and unitised are treated as quality grades, when the real difference is where the joints are and who made them.
- A leak is assumed to come from the visible outer line, when it usually reports a blocked or interrupted drainage path further in.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Is this system stick or unitised, and where does that put the joints that were made on site?
- Which connection at each level supports the frame, and how do the restraint connections allow the structure to move?
- How does the glazing rebate drain at each level, and where does the water finally discharge to the outside?
- Where does the air control line pass from the frame onto the surrounding construction, and who is responsible at each of those points?
- What has fire design determined for the zone between the facade and each floor edge on this building?
- How has the coldest internal surface been identified, and what internal conditions were assumed when it was assessed?
- What access is provided for cleaning, resealing and glass replacement on each elevation?
- What will be recorded at handover about unit positions, glass types, gasket references and drainage details?
What this page does not do
- No part of this facade excludes water on its own; performance belongs to the complete system, its drainage paths and its junctions with the building.
- Applying sealant to a curtain walling face without knowing which openings are drainage can close the path the design depends on.
- Fire and acoustic provisions at the floor edge are determined by qualified professionals for the specific building, and this entry states none.
- Fixing anything to a mullion after completion loads a member designed for wind and may cross the isolating component within its section.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Facade and Cladding Systems
Outer skins carried on a wall that is already complete behind them, where the support, the joint and the cavity carry the durable knowledge rather than the finish.
Browse all facades & cladding entries →