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1. Curtain-walling systems

There are four main types:

• Stick This is the simplest type of curtain walling and, as its name suggests, it is composed of single elements that are assembled on site with the glass and gasket installed separately. This is a highly flexible system and can produce good results, although it is subject to the quality of the on-site workmanship.
• Modular/unitised This is composed of units made in the factory and is usually at least a storey in height and 1.5 m wide. The horizontal joint is at sill height and the unit spans to the sill line above. Quality can be better than stick because most of the wall is made in the factory. However, the replacement of units and seals can be complex.
• Bespoke Made to designers’ exacting requirements, bespoke systems are found on the most high-profile and high-budget projects. The world’s best manufacturers can create technically advanced and aesthetically beautiful solutions, and can use wall technology to create compound roofs, although at a very high price. Composite systems using timber as part of the frame are also becoming available.
• Twin wall Many manufacturers offer twin wall systems, which have a cavity of about 1 m between the outer and inner walls. These are used as a moderator for thermal and acoustic issues and also creates a maintenance platform. Twin wall has not been used much in this country to date because of its expense, but it may become more popular as demand for low-carbon designs grows.

2. Glass

The glass is just as important as the frame and should be carefully specified. Check delivery times, as glass often takes longer to arrive than curtain walling. Consider a risk assessment to ensure that the characteristics of the glass match requirements in terms of exposure, use, maintenance, cleaning and durability. Ensure that the specification includes Centre for Window and Cladding Technology standards in procurement and testing.

3. Standards

Standards such as those issued by the CWCT are key to ensuring that systems will perform adequately. These include BS EN 13051:2001 for watertightness testing, BS 6399 for assessing wind loading, BS 8200 for live loading, BS 476 for fire (despite the existence of European standards), and BS 6229, 6262 and 6180 for glazing.

4. Installation

Curtain-walling systems are only as good as the people who install them. And as installers don’t always have contractual ties with manufacturers, it is important to check their credentials during the selection process. If a specifier selects an unproven installer, chances are the expensive curtain-walling package will end up looking like a lower priced option. The specifier should be selective and inquisitive and be aware of the reputation of each installer on the shortlist.

5. Weather resistance

The weather resistance of curtain walling can usually be taken as a given – if it has been installed correctly. Typically, curtain walling features a combination of a metal frame and glass and metal panels. Metal sections are jointed together and fixed to brackets so the wall stands off the structure. The glass is separated from the metal using gaskets. As water will force its way through the joints, the systems are designed to allow it to pass through the primary seal and fall into drainage channels. The water then flows down the mullions to the base of the wall or through vents in the transoms at convenient intervals.

A secondary seal on the inside, which is subject to lower pressures, will prevent water entering the building interior if installed correctly. Water management at the junction of the transom and mullions is crucial. Most systems require careful installation at these points and many are susceptible to future problems if the installation is imperfect

6. Thermal performance

The thermal resistance of curtain walling is continuing to improve, two decades after thermal breaks first starting appearing in curtain-walling sections. This was originally achieved by pouring resin into hollow sections and then machining the walls away when the resin had set. Today, most thermal resistance is created by inserting plastic units into metal under pressure.

The materials and techniques used now are very sophisticated, although because there are few installed examples, durability has yet to be proved.

Thermal performance is still improving with the arrival of composite materials and highly insulated semi-vacuum panels. With this technology, U-values in the low single figures are now achievable for the whole wall, especially if high-tech glazing with a U-value of less than one is specified.

Thermal performance can be further improved by specifying glass with a high selectivity and integrating shading to shield the glass from the direct effects of solar radiation. Use of timber in some systems can also boost thermal performance and it has obvious sustainable benefits.

7. Samples

Before considering a system at tender, look at examples that have been in use for several years. If possible, talk to the consultants and installers and establish how well the particular system has performed. Ensure you have a clear understanding of it and the materials being used and ask for a sample that shows exactly how the components work.

Establish a sample regime. This can range from a simple sample for small projects to a larger on-site sample, which on large schemes could be a separately tested rig. At the very least, a quality control sample of some sort must be established and directly linked to the management of the works on site.