Curtain wall details: a technical guide to components and assembly

Here at Wrightstyle, we supply product advice as well as products – advice that's backed up by years of industry expertise.

There comes a time in many projects when technical reference materials become a must. This can be especially demanding when the product that needs installing consists of multiple components, all of which need to be considered.

This is true of curtain walls – those non-load-bearing wall systems that are made from aluminium or steel with glass infills.

Essential curtain wall components include mullions, transoms, pressure plates, gaskets and capping. These form parts of the framing system, glazing retention system (the components that securely hold glass panes in place) and weather-sealing system.

In this guide, we take a deep dive into these components and how they work together to create a complete curtain wall system.

Curtain wall components explained

Mullions

Mullions form the spine of the curtain wall. They run vertically between floors or along the facade.

They transfer wind loads, transom loads and glass loads (horizontal components) back to the building via anchors and brackets.

They're often made from extruded aluminium. At Wrightstyle, our mullions are made from either mild steel or stainless steel for extra durability.

Transoms

Transoms are the horizontal members that work with the mullions to form the curtain wall grid. They split the facade into panels and support the infills.

Transoms transfer building loads into the mullions and often include drainage paths and glazing support details.

Pressure plates

Pressure plates retain the glass against the curtain wall frame. They're normally screwed to the mullion or transom and clamp the glass or other infill into the frame.

Pressure plates are typically covered by the capping, meaning they can't be seen from the exterior.

Gaskets

Gaskets are seals that serve several functions within a curtain wall system. They're often made from ethylene propylene diene monomer (EPDM) rubber.

Gaskets seal the glass to the aluminium framing. This prevents the penetration of water and air. Without this protection, the curtain wall system is vulnerable to everything from structural failure to mould to thermal inefficiency.

They also allow for small movements between the glass and frame and prevent the glass and aluminium or steel from coming into direct contact.

Gaskets also play a role in the aesthetics of the curtain wall. They create clean lines around the gridded exterior and are often coloured and finished to match the overall curtain wall system.

Capping

The capping is the cover that's attached to the pressure plate. It's usually made from extruded aluminium.

It does double duty, concealing structural elements and providing external protection. It conceals the pressure plate and fixings, helping to complete the overall aesthetic of the facade.

At the same time, it protects glazing-retention system components against wind, rain and other climatic conditions.

At Wrightstyle, our SR series curtain wall systems can be provided with no external capping or an 18mm capping option. Our fire-rated curtain walls, meanwhile, come in 15mm and 18mm capping options.

How the components fit together

So, how do mullions, transoms, pressure plates, gaskets and capping fit together?

Perhaps the best way to conceive of the assembly is to picture three separate but connected systems.

These three systems are the structural framework, the glazing retention system and the weather management system.

The structural framework is made up of the mullions and transoms and creates the aluminium or steel grid. This is connected to the building with anchors.
The glazing retention system consists of pressure plates, fixings and gaskets. This positions the glass in the frame and holds it in place.
Finally, the weather management system acts as an outer skin. It stops water, wind and extreme temperatures from affecting the overall curtain wall system.

Depending on the curtain wall system, the weather management system could include drainage channels, weep holes, pressure-equalisation cavities, internal seals, glazing pockets or thermal breaks.

These mean any water that enters the system can be controlled, drained and sent back to the exterior.

The gaskets, pressure plate, framing cavities and drainage paths all work together to manage the ingress and egress of air and water.

A thermally broken curtain wall is one with a thermal break – a low-conductive material that's fitted between the interior and exterior. This helps keep indoor temperatures stable and reduce heating and cooling bills.

What details do you need to specify for a curtain wall system?

When specifying a curtain wall system, you need to specify a number of details. The essentials are:

Glass
Gaskets
Transoms and mullions
Pressure plate and cover cap
Thermal breaks
Insulation
Backpans and spandrels
Drainage cavities
Brackets and anchors
Slabs and edges
Fire stopping
Perimeter seals

You should also consider the type of glass used for the infills. In particular, you need to specify whether you need any low-e or solar-control coatings or safety glazing (toughened, laminated or toughened and laminated glass).

Fire performance is another issue that requires careful attention. The whole assembly must have the appropriate fire rating, not just the glass.

This means specifying cavity barriers, fire-rated spandrels, interfaces and fire stopping around the brackets.

How curtain wall systems are assembled

There are two main ways that curtain wall systems are assembled: stick curtain wall systems and unitised curtain wall systems.

In the case of stick curtain walling, parts are provided and assembled on site. These include mullions, transoms, glazing and panels.

Unitised curtain wall systems, however, are large, factory-assembled panels and units. They're delivered to the site and connected to the building structure.

The choice between a stick and unitised curtain wall system will depend on a range of factors, from the nature of the project to your budget.

Wrightstyle is a UK supplier of high-performance steel glazing systems, including blast-resistant, ballistic-resistant and fire-resistant glazing systems. If you have any questions, please don't hesitate to contact our experts for comprehensive product advice.