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How to Specify: Curtain Walling

Curtain walling refers to non-load bearing or non-structural walls, first developed in the late 19th century in Britain and America, which consist of light frames of aluminium, steel, plastic or occasionally timber infilled with glass, but also encompass frames with panels of metal, stone, terracotta or other materials.

These infill panels literally hang from the structure of the building at each storey, rather than supporting the weight of the facade from the bottom.

Curtain walling is very common in large non-domestic buildings, particularly skyscrapers, where structural walls would be impossible due to the demands of the building’s own weight or the weight of its contents.

Curtain walling comes in a wide variety of configurations and finishes, and manufacturers are continually innovating to create new systems with better performance.

Curtain walling is so flexible because it does not have to perform structurally except to support its own weight and resist wind loads. Because of this, curtain walling is free from constraints on thickness like traditional masonry walls, and can perform on purely aesthetic, as well as thermal and acoustic terms.

Curtain walling can be used on non-vertical surfaces as well as vertical ones, and in some cases can be used at low pitches on faceted buildings where required.

Please note: This article will discuss stick and unitised curtain walling systems, rather than point-loaded structural glazing walls, which are also often considered a form of curtain walling.

Technical Details of Curtain Walling

Curtain walling generally falls under two broad categories; the more traditional stick systems which date back almost 100 years, and newer unitised systems which date to the 1980s.

In a stick system, individual components of the curtain walling such as extruded metal profiles, glazing and other infill panels, and insulation are installed and assembled on site by specialist contractors, so that up to 90% of the work can be done on site.

These systems tend to rely upon mechanical pressure clamps to hold glazing in place, which are capped with covers to achieve whatever aesthetic is desired. In stick systems, dividing mullions and transoms between glass or infill panels will usually be visible from the outside of the building.

In a unitised system, curtain walling panels are prefabricated to various extents in a factory environment, then transported to site for installation, so that only 10% of the work is done on site.

Semi-unitised systems also exist, where approximately 50% of the work is done on site, and the rest off site in a factory environment. In both of these types of systems, glazing tends to be held to mullions and transoms using structural sealant, which in turn means that the outer surface of the glass will appear relatively seamless, while the facade’s structural framework is visible on the inside.

Curtain walling sometimes incorporates elements based on the principle of rainscreen cladding as an outer layer. For more information on rainscreen cladding, see our article on the topic here.

Specifying Curtain Walling Solutions

When specifying curtain walling, its important to remember the different factors which will affect the design of the system.

The height of the building and the area to be covered are the most important factor, but so too, you should consider the distance between slabs and the span of each panel that will be required.

Many manufacturers will allow custom or varying panel sizes, so don’t be put off requesting these as part of your specification.

Just as important to consider is the location of your building and situation of the site during constructions.

Consider what kinds of provisions available on site for the storage of curtain walling.

Particularly when specifying unitised systems, the storage space needed can be considerable, and may need to be weatherproof as well. It may not be feasible to store all of the material on site, but nor may it be feasible to transport new material to the site every day.

You should also be aware of the kinds of guarantees and service packages offered by the manufacturer of whatever curtain walling system you choose.

The typical service life of a polyurethane sealant is roughly 7 to 10 years, while for a silicone sealant it is approximately 15 to 20 years, meaning that some maintenance or resealing will be required within the reasonable lifetime of your building.

Many facade manufacturers offer servicing packages, and remember that with some facade systems, warranties and guarantees will only remain valid if approved contractors are used for repairs, due to the proprietary and often highly specialised nature of the work.

There are three essential points that relate to the proper performance of curtain walling systems so as to gain maximum lifespans:

  • Understanding of preventive maintenance routines specified by manufacturer
  • Regular inspections, cleaning and repairs
  • Written records of both of the above

Advantages of Curtain Walling

Curtain walling is so widely used that it has become commonplace in a wide variety of applications across the world. In this sense, the benefits of specifying curtain walling over other types of cladding/structural system are very broad. They can be summarised as:

  • Possibility of large expanses of glass
  • High speed of installation
  • Low overall installation and lifespan Costs
  • Good quality control
  • Aesthetic and thermal/acoustic versatility
  • Structural flexibility in relation to main building structure (particularly in seismic areas)

Common Uses of Curtain Walling

Curtain walling systems are used in an enormous variety of applications, in both small and large buildings, and both residential and public buildings.

Curtain walling was first used in factories and warehouses, buildings where a large amount of natural light was desired, but structural constraints due to loading meant that large traditional windows were not possible.

This continues to be true to this day: buildings where large amounts of natural lighting are required are a very common application for curtain walling.

Even where light is not the primary concern however, curtain walling is often used, for example, in very tall or large buildings, where the slow speed of constructing traditional facades would make the project unsustainable, or where scaffolding cannot be used.

How Do I Choose a Curtain Walling System?

The most crucial choice you will make when selecting a curtain walling system for specification will be between a stick system or unitised system.

Stick systems offer the benefits of lower costs, both in their standard manufacturing profiles (custom profiles require new tools to be made at a very high cost and time lag), and in the fact that contractors will be familiar with their installation.

They are particularly suitable for shopfronts, residential uses, or other small areas of glazing or panelling, or for industrial applications at heights of three stories or less.

Stick systems require scaffolding however, can be prone to water leakage depending on the skill of their installer and on structural or ground subsidence of the building.

Stick systems will tend to take between four and five months to be delivered from order date, and can then be installed at a rate of around 50 m2 per week per installer.

Unitised systems often take longer, between five and six months, to be delivered on site from the time of ordering, but then can be installed at a much quicker 75 to 150 m2 per day per installer, depending on their complexity.

You should consider both of these time factors when specifying curtain walling systems and choosing a product or manufacturer.

Unitized systems offer many more features at a higher cost than stick systems, including higher installation speed, higher accuracy in their production tolerances, and can accommodate building movement.

They do not require scaffolding, which can reduce their overeall costs in comparison to stick systems.

Unitised curtain wall systems are not recommended for buildings of less than three stories, or applications above 3000m2 of facade but instead for high buildings and skyscrapers, which are almost exclusively clad in unitised systems, and have been for thirty years.

Along with higher costs, unitised systems require specialist designers, engineers, and contractors to be involved in their design and installation, often in form of third party facade companies which may offer all three services.

When selecting a supplier for your curtain walling, make sure to look their design experience and track record, examples of their work and in-house resources and facilities.

They should be willing to work integrally to the design team from an early stage in the project, and to be involved in workshops with different members of the team.

Make sure you understand whether you will be given a turnkey service, and will have a single, available point of contact at the company. In turn, you should make sure to use their advice and involve them as much as possible, due to the technical complexity of modern curtain walling.

Other Considerations When Specifying a Curtain Wall

Its also important to consider how your building will be designed to accommodate your chosen facade system.

While it would be convenient if a facade type could be chosen to suit a building, without any modifications required to the building’s design, this is rarely the case.

Curtain walling often requires integrated thinking in its design, so as to successfully combine it with an appropriate structural system, appropriate thermal insulation, etc.

The integrity and inbuilt tolerances of your building’s structure are key to the lifespan of any curtain walling you might specify.

Curtain walling can be affixed to the building structure in a variety of ways, each with its own implications.

Some of these include:

  • Between floor slabs
  • Over floor slabs
  • Onto edge beams

Preventing water entering the building will clearly be a primary consideration when specifying a curtain wall. If installed correctly, curtain walling should not leak, although careful design and consideration must be given to junctions between standardised curtain walling systems and non-standard elements such as floor slabs, roof slabs, internal or structural walls or columns, windows and doors.

While previously popular, fully face sealed systems, which rely entirely on the performance of sealants or gaskets to prevent water penetration, have fallen out of use in Britain due to their unreliability. Instead, most curtain walling systems in use today make provision for some water ingress through one or more condensation chambers connected to drainage channels and weep holes.

Increasingly, curtain walling which relies on the rainscreen principle of pressure equalisation to prevent this is being specified, particularly in the UK where rain and lack of sun make water ingress in buildings so common.

See our article on rainscreen cladding here for more details.

Regulations to Consider when Specifying Curtain Walling

While there are no specific regulations referring to curtain walling, the following regulations are relevant to its specification:

For further safety and installation standards and codes of practice, check:

  • BS EN 13830 – Specification for Curtain Walling
  • BS 6262 – Code of Practice for Glazing of Buildings

Search for ‘curtain walling’ solutions on SpecifiedBy: https://www.specifiedby.com/category/structural-frames-walls/facade-glazing/curtain-wall-systems

SpecifiedBy Editor