Curtain Wall Systems – Components, Details and Technical Working Principles

Curtain wall systems are among the most advanced façade solutions developed for modern buildings. They wrap the structure with a lightweight but highly durable outer skin, providing not only a clean architectural expression but also a high–performance environmental buffer between inside and outside.

The aim of a curtain wall is not simply to “close” the façade. It is to manage wind, rain, sun, noise and temperature in a controlled way while admitting natural light, reducing energy consumption and allowing architects to design flexible, transparent and modular building envelopes.

1. General Overview of Curtain Wall Systems

A curtain wall is a non–load–bearing façade system that is structurally independent from the primary building frame. It is designed to carry only its self–weight and environmental actions such as wind pressure and suction, while transferring these loads safely back to the main structure.

The system is a composite of glass, aluminium profiles, gaskets, anchors and various connection components that work together to regulate heat, air, moisture, sound and light between inside and outside. When designed correctly, a curtain wall provides high transparency and architectural freedom, without compromising on energy performance, comfort or long–term durability.

2. Structural Frame: Mullions and Transoms

The structural backbone of the curtain wall

The most critical component of any curtain wall is its aluminium supporting frame. This frame is built from vertical mullions and horizontal transoms, forming a structural grid that defines module sizes, derailing behaviour and overall façade stiffness.

2.1 Mullions (Vertical Main Members)

Mullions act as the vertical spines of the façade. They collect wind loads from the glass or panel surfaces and transfer these loads through anchor brackets into the concrete or steel structure.

Main functions of mullions

• Wind load transfer: Carry positive and negative pressures from panels to the anchors.
• Thermal movement accommodation: Work with fixed and sliding anchors to allow aluminium expansion.
• Support for glass and infill panels: Provide edge support and limit deflections.
• Architectural grid definition: Typical axes (1.20 / 1.35 / 1.50 / 1.80 m) define the visual rhythm.
• Housing of drainage and pressure equalisation chambers: Internal multi–chamber geometry hosts gaskets and weep paths.

2.2 Transoms (Horizontal Members)

Transoms establish the horizontal lines on the façade, usually aligned with floor levels and glass joints. Vision and spandrel glass rest on transoms, and water is collected and drained through these members.

Main functions of transoms

• Support the dead load of the glass and transfer it to mullions.
• Carry wind loads in the horizontal direction as secondary beams.
• Integrate internal drainage channels and weep holes.
• Define horizontal banding and joint alignment.

2.3 How Do Mullions and Transoms Work Together?

“Mullions carry the load, transoms shape the module.”

The curtain wall frame only performs as intended when mullions and transoms are engineered as one system: the mullion provides vertical stiffness and load path, while the transom controls glass support, water drainage and the horizontal visual grid. A weak link in either member will reduce the performance of the entire façade.

3. Stick, Unitized and Ladder Curtain Wall Frames

The way the structural frame is fabricated and installed has a direct impact on quality, cost, programme and risk profile of the project. Stick, Unitized and Ladder systems share the same mullion–transom logic, but use different strategies for prefabrication and site assembly.

A) Stick System

In a stick curtain wall, mullions and transoms are delivered to site as loose bars and assembled piece–by–piece on the building. It is the most common and often the most economical approach for low– to mid–rise projects.

A.1 How it works

• Vertical mullions are fixed to the slab edges or beams.
• Horizontal transoms are connected between mullions on site.
• Glazing, pressure plates, cover caps and gaskets are installed afterwards.

A.2 Advantages

• High flexibility for last–minute dimensional changes.
• Lower initial system cost and no need for heavy factory investment.
• Suited to projects with difficult access where large units cannot be craned.

A.3 Disadvantages

• Quality depends heavily on site conditions and workmanship.
• All joints and gaskets are assembled on site → higher risk of leakage.
• Longer installation time compared to unitized façades.

B) Unitized System

A unitized curtain wall is factory–built into large pre–glazed panels that are simply hung on the building frame. It is the preferred solution for high–rise, fast–track and premium façade projects.

B.1 How it works

• Mullions and transoms are assembled into complete frames in the factory.
• Glass and spandrel panels are installed in controlled conditions.
• Gaskets, structural silicone and pressure plates are applied in the workshop.
• On site, each unit is craned into position and locked to the anchors.

B.2 Advantages

• Highest level of quality control and repeatability.
• Very fast site installation, minimal exposure to weather.
• Excellent performance for high–rise buildings.

B.3 Disadvantages

• Higher system and logistics cost.
• Requires factory capacity and experienced fabrication teams.
• Limited flexibility after units have been produced.

C) Ladder System

The ladder system is a hybrid between stick and unitized. Mullions and transoms are pre–assembled into “ladder” frames, but glazing still takes place on site.

C.1 How it works

• Mullions and transoms are pre–fixed together into ladder units.
• These ladders are installed on the structure similar to stick mullions.
• Glass and infill panels are then mounted on site.

C.2 Advantages

• Faster than stick, more economical than full unitized.
• Smaller units → easier transport and handling.

C.3 Disadvantages

• Key weather–tightness details are still site–dependent.
• Glazing quality depends on installation crews.

D) Summary Comparison

Criterion	Stick	Unitized	Ladder
Installation speed	Medium	Very high	High
Workmanship quality	Variable (on site)	Very high (factory)	Medium–high
System cost	Lowest	Highest	Medium
Suitability for high–rise	Limited	Excellent	Medium

4. Glass and Infill Panels (Vision, Spandrel, Metal Panels)

Curtain walls are not just aluminium skeletons. The actual façade performance in terms of daylight, solar control, thermal insulation, acoustics and fire behaviour is largely governed by the glass and infill panels.

4.1 Vision Glass

Vision glass forms the transparent areas of the façade. It provides visual connection, daylight and a modern appearance. At the same time, it has to manage solar gains, heat losses, sound and safety.

• Light transmission (VLT): Typically 30–60% to balance daylight and glare.
• Solar factor (g–value / SHGC): Controlled by low–E and selective coatings.
• Thermal insulation (U–value): Achieved with double or triple IGUs.
• Safety: Toughened and/or laminated glass is mandatory.
• Acoustics: Asymmetrical build–ups and laminated interlayers improve sound reduction.

4.2 Spandrel Panels

Spandrel zones are opaque areas covering floor slabs and structural elements, while maintaining a continuous external appearance between vision zones. They must provide thermal insulation, fire protection and a visually consistent façade.

Typical spandrel materials

• Back–painted or fritted glass.
• Aluminium composite panels.
• Aluminium or steel sheet panels.
• High–performance mineral panels.

Behind the spandrel, insulation (rockwool, PIR, XPS/EPS) and vapour control layers must be designed carefully to avoid condensation and “ghosting” stains on the outer glass.

4.3 Alternative Infill Panels

• Aluminium composite for lightweight continuous bands.
• Steel panels for high–impact or industrial applications.
• Natural stone or sintered surfaces for prestige façades.

5. Pressure Plates, Cover Caps and Anchors

At first glance a curtain wall looks like glass and aluminium frames. In reality, its performance is defined by “hidden” mechanical components: pressure plates, cover caps and anchors. These elements secure the glass, transfer loads and keep the façade stable over decades.

5.1 Pressure Plates

Pressure plates are aluminium or sometimes steel profiles bolted to the mullions and transoms. They clamp the glass against setting blocks and gaskets, distributing wind loads into the frame.

Main functions

• Transfer wind loads safely from glass edges into the frame.
• Prevent glass from sliding or popping out under suction.
• Provide continuous support to gaskets for air and water tightness.

5.2 Clip–On Cover Caps

Cover caps are aesthetic profiles that snap onto the pressure plates. They hide fixings and help create the final visual joint pattern of the façade.

• Define vertical and horizontal sightlines (e.g. 50–80 mm).
• Protect critical sealant edges from UV and pollution.
• Available in different shapes, colours and finishes.

5.3 Anchors and Connection Hardware

Anchors connect the curtain wall frame to the primary structure. Together with fixing bolts and shims, they form the last link in the structural load path.

• Fixed anchors: Resist vertical loads and in–plane forces.
• Sliding anchors: Allow for thermal expansion and building movements.

5.4 Typical installation errors

Error	Potential consequence
Insufficient tightening torque on pressure plates	Glass movement, gasket failure, air and water leakage
Poorly engaged cover caps	Risk of cap detachment under wind suction
Incorrect anchor selection or position	Excessive mullion deflection or even partial collapse
Misplaced or missing gaskets	Failure of drainage and pressure equalisation

6. Gaskets, Structural Silicones and Drainage

No façade is 100% watertight. The goal of a modern curtain wall is not to block all water, but to manage it. Gaskets, sealants and drainage details are therefore among the most critical – yet least visible – components of the system.

6.1 EPDM / Silicone Gasket Systems

EPDM gaskets are the industry standard for curtain walls because of their high resistance to UV, ozone and temperature changes. A large part of the air and water tightness is achieved through correctly designed and installed gaskets.

• Control air leakage between inside and outside.
• Form water barriers between glass and aluminium.
• Absorb thermal and structural movements elastically.
• Separate pressure chambers and guide drainage paths.

6.2 Structural Silicone Glazing (SSG)

Structural silicones are used in silicone–glazed curtain walls, where glass is bonded to aluminium frames without visible mechanical retainers. In these systems, the silicone is a load–bearing element that transfers wind loads from the glass to the frame.

• Enables flush, cap–less glass façades.
• Requires strict cleaning, priming and curing procedures.
• Must be tested for tensile strength, elongation, UV and long–term stability.

6.3 Drainage and Pressure–Equalised Rainscreen

Modern curtain wall systems use internal drainage channels and pressure equalisation cavities to control water ingress.

• Small amounts of water enter at glass joints or gasket edges.
• Water is directed into internal channels and flows downward by gravity.
• Weep holes at transoms discharge the water to the exterior.
• Pressure equalisation reduces the driving force pushing water inwards.

6.4 Common drainage mistakes

Error	Result
Weep holes blocked or painted over	Water accumulates inside profiles, leading to leakage and corrosion
Gaskets installed in the wrong orientation	Pressure chamber fails, water is driven inward
No slope on transoms	Standing water, staining and early deterioration of components

7. Thermal Insulation and Energy Performance

The thermal performance of a curtain wall strongly influences the overall energy consumption and comfort of the building. Poorly designed façades lead to heat loss in winter, overheating in summer, condensation, increased HVAC loads and user complaints.

7.1 Thermal Breaks

Aluminium is lightweight and strong but also an excellent conductor of heat. To prevent thermal bridges, modern curtain wall profiles use polyamide thermal breaks that separate inner and outer aluminium sections.

• Reduce conductive heat flow across the frame.
• Shift the condensation plane towards the exterior.
• Can significantly improve overall U–values.

7.2 Spandrel Insulation

Spandrel zones cover floor slabs and are often weak spots in the thermal envelope. Rockwool, PIR/PUR or XPS/EPS boards are commonly used behind the spandrel, together with vapour barriers and fire stops.

• Correct thickness is selected based on climate and code requirements.
• Vapour barriers must be continuous to prevent condensation.
• Insulation must not be in direct contact with the back of the glass.

8. Optional Components: Sunshades, Vents and Smart Glass

Modern curtain walls are often enhanced with additional components that improve comfort, reduce energy use and add architectural depth. The most common are external sunshades, operable ventilation vents and dynamic glazing technologies.

8.1 Sunshades / Louvers

External sunshades are passive solar control elements, typically made of aluminium. They reduce solar heat gains, mitigate glare and provide a strong three–dimensional expression on the façade.

• Can reduce cooling energy demand by 15–35% on exposed façades.
• Improve visual comfort and screen computer glare.
• Available as fixed or adjustable, horizontal or vertical configurations.

8.2 Natural Ventilation Vents

Operable vents integrated into curtain walls allow the building to “breathe” in a controlled way. They are often designed with minimal visible difference from fixed glass.

• Reduce reliance on mechanical cooling and improve indoor air quality.
• Can be motorised for smoke exhaust in fire scenarios.
• Acoustic liners can maintain sound insulation while allowing airflow.

8.3 Smart Glass / Dynamic Glazing

Smart glass technologies allow the optical properties of glass to change with voltage, temperature or light intensity. This enables automatic solar control and comfort optimisation.

• Electrochromic glass – tint level controlled by low–voltage current.
• Thermochromic and photochromic glass – respond to temperature or UV.
• SPD glass – uses suspended particles aligned by an electric field.

9. Why Curtain Wall Detailing Is Critical

Curtain walls may look simple from a distance, but their true value lies in the invisible details. A façade is more than a combination of mullions, transoms, glass and caps. Its real performance is measured by how it behaves under wind, rain, temperature and building movements over many years.

Structural safety, air and water tightness, energy performance, acoustic comfort, fire resistance and long–term stability are all controlled by the quality of the details. A small mistake – an incorrectly installed gasket, a blocked drainage path, or an under–designed anchor – can lead to serious issues across thousands of square metres of façade.

In short: Good details = safe, durable, high–performance curtain walls.

10. Related Technical Details, Calculators and FAQ

10.1 Related technical detail pages

• Curtain Wall Technical Detail
• Aluminium Single–Sash Window 60 mm Detail
• Aluminium Double–Sash Window 50 mm Detail
• Technical Façade Detail

10.2 Curtain wall & façade calculators

• Aluminum Composite Panel Cutting Optimization
• Aluminum Sheet Cutting Optimization
• Capped Curtain Wall Take–Off Calculator
• Ceramic Façade Quantity & Cost Calculator
• Double–Leaf Door Take–Off Calculator
• Double–Leaf Outward Opening Door Calculator
• Façade Calculator Hub
• Feet–Inch ⇄ Metric Converter
• Fixed + Operable Sash Window Calculator
• Fixed Window Quantity Calculator
• Silicone Curtain Wall Take–Off Calculator
• Single–Leaf Door Calculator
• Single–Leaf Door with Threshold Calculator
• Single–Leaf Outward Opening Door Calculator
• Single–Sash Window Calculator
• Top–Hung Window Calculator
• Wind Load Calculator

11. Download the Full PDF Guide