Maximum transparency at the World Trade Organisation

The previously open inner courtyard of the Centre William Rappard now features a transparent domed roof made of ETFE foil cushions. It offers a high degree of freedom of form coupled with exceptionally good heat insulation properties. The NOWOFLON ET 6235Z foils are made of the high-performance 3M Dyneon ETFE material.

The Centre William Rappard was built in the 1920s and was the first building in Geneva designed specifically for an international organisation. The World Trade Organisation (WTO), which is responsible for the regulation of international trade and business relations, has been based here since 1995. In addition to around 750 employees, almost 1,000 delegates and ambassadors from the WTO's 159 member states now use the complex for meetings, conferences and training seminars. In 2008, the WTO, in conjunction with the Swiss authorities, decided to renovate and extend the existing building. An international competition to design a new extension attracted 115 entries and was won by Stuttgart-based architects Wittfoht.

Energy efficiency and good insulation properties

As with all building projects for UN organisations, strict regulations applied for the WTO project with regard to energy efficiency. The tender stipulated compliance with the Swiss MINERGIE-P standard defining low-energy construction methods. The permissible values correspond with those for a 3-litre house.

The original building's open inner courtyard planted with trees now features a transparent roof made of pneumatically-supported, triple-layered foil cushions affording extremely good heat insulation properties. The foil cushions were calculated and constructed by Swiss company Texlon, a specialist in hangar construction as well as foil and membrane constructions. The company, which operates throughout Europe and Central Asia, has countless foil roofs to its name.

The roof construction with triple layered foil cushions

The roof is formed of a total of 104 triple-layered foil cushions of which 22 feature an opening mechanism. The cushions lend the domed roof a distinctive curved form. Due to the special steel support structure, Texlon calculated a total of 26 different three-dimensional cuts and forms for the foil cushions of around 3 metres times 3.5 metres in size. The individual foils were manufactured in the company's ETFE welding plant, which is unique in Switzerland. For the WTO roof, Texlon sought to further develop the profiles commonly used.

This further development enabled the fitters to make fine adjustments while clipping the foils in place as well as allowing for easier re-tensioning. These foil cushions weigh about 95 percent less than a comparable glass construction, and their extremely fine support structure enables unbroken views of the skies over Geneva. The foils allow both visible light and the ultraviolet UV-A light essential to plants to penetrate virtually unfiltered.

The unique features of ETFE foils

The foils were extruded by Siegsdorf-based specialists Nowofol Kunststoffprodukte GmbH & Co. KG. The NOWOFLON ET 6235Z foils come in a thickness of 12-300 micrometres. The company has the production capabilities to produce foils that are both transparent and of almost any RAL colour. 3M Dyneon ETFE is Nowofol's material of choice.

This high-performance material from the fluoropolymers family affords virtually universal chemical resistance and meets the B1 fire class criteria (according to DIN 4102). The tensile strength can reach up to 50 N/mm² and the elongation at break more than 300 per cent. For extrusion of the foils, Nowofol requires neither stabilisers nor softeners, which can evaporate over time and compromise the roof's positive qualities.

Due to the foils' extremely smooth surface on which fungi and bacteria are unable to take hold, rain is sufficient to keep the roof clean. Texlon has tailored the stability of the foil cushions according to the wind and snowfall expected for Geneva of 100 kg/m². NOWOFLON ET 6235Z foils made from 3M Dyneon ETFE are the only foils to have been used in architecture for more than 30 years - this means unsurpassed experience in all types of climate.

Source: Dyneon GmbH