ETFE, as it it shortened to, is fast becoming the material of the moment. Its low weight and ability to transmit light have struck a chord with designers, so much so that it is beginning to replace glass as the cladding of choice for atria and rooflights. Architects that have already specified ETFE in their projects include Alsop & Störmer, Michael Hopkins and Partners and Wilkinson Eyre Architects. Its first construction application was at Arnheim Zoo in The Netherlands. Since then, applications include the bubble domes of various CenterParcs holiday camps and the atrium roof of the Chelsea & Westminster Hospital.
Despite the upsurge in ETFE’s use, little information is available on the material and its properties. This makes it difficult for designers to get the most from it and to know when it would be more appropriate than glass. It was this lack of information that prompted Stephen Tanno, manager of Buro Happold’s facade engineering group, and two of his colleagues – Syretta Robinson-Gayle and Andrew Cripps – to study the material in a DETR-funded research project. The task has taken three years, and the result, a report entitled ETFE Foil Cushions as an Alternative to Glass for Roofs and Atria, will be submitted to the DETR later this month.
ETFE is a very stable plastic that is able to resist chemical and ultraviolet attack. It is extruded as a thin foil, and, although the manufacturing process limits its width to a maximum of 1.5 m, broader sheets can be produced by heat-welding foils together.
When used for cladding, sheets of ETFE are usually assembled into air-filled cushions. These are produced from two or three sheets of foil laid on top of each other and joined at the edges by heat-sealing to form the cladding equivalent of an inflatable pillow. “Using three sheets of foil improves the cushion’s thermal properties,” explains Tanno. “The third sheet splits the void between the cushion’s two outer layers, limiting the air movement within the void and improving its insulating capabilities.”
The ETFE pillow is then clamped into a frame and inflated using a small fan. Inflating the cushion pre-stresses its outer skin, enabling it to withstand the weight of snow and pressure of wind. A hose keeps the cushion linked to the fan throughout the lifetime of the cladding, allowing the air to be intermittently topped up.
It is mainly the lightness of ETFE cushions that has attracted designers. A roof constructed with double glazing weighs 30 kg/m2; one made with single glazing 15 kg/m2. This compares with about 0.1 kg/m2 for a roof made from ETFE. As a result, ETFE roofs need far less structural support, which makes it particularly useful in atria that need lots of light.
Much more light is transmitted through an ETFE skin than through glass. Almost all light falling on an ETFE roof passes into the building. In fact, “so much passes through that it is possible to get a suntan”, says Tanno. This has obvious advantages for cladding applications such as the Eden Project greenhouses, where light is essential for plant growth, and also to reduce reliance on artificial light in other buildings.
Hand-in-hand with the flood of daylight comes solar radiation. “This often needs to be limited to stop the building overheating,” says Tanno. Fritting can be printed on to the cushion’s top surface to cut solar transmittance, and cushions made of white or tinted ETFE can be used.
Although light transmission is good, the foil’s curved surfaces result in poor transparency. This is not a problem in overhead applications, but if the view is important, glass will be more appropriate. This is why “foil cushions are unlikely to replace glazing for mainstream windows”, says Tanno.
As well as allowing almost all light to pass through, because they weigh so little the cushions also transmit nearly all sound. For applications such as public swimming pools, this has obvious advantages, as the sound can pass straight out through the roof. But for a library near a busy main road, or a school built on an airport flight path, ETFE would be an inappropriate choice of cladding.
With environmental issues high on the industry’s agenda, Tanno and his colleagues have carried out an environmental comparison of ETFE and glass. The results show that, although the two materials have similar figures for embodied energy per tonne, in practice only a thin film of ETFE is used, giving an embodied energy figure more than 10 times lower per square metre of facade than glass.
To keep their shape, the ETFE cushions need to be maintained at a pressure of about 500 N/m2. This consumes about 50 W of fan power per 1000 m2 of foil cushion, but Tanno says this is a small amount of energy compared with the heat energy saved. In single sheets, ETFE foil is not a good insulator, but when formed into cushions, its heat transmission at 2.6 W/m2ºC is significantly better than that of standard double-glazing at 3.8 W/m2ºC.
The energy lost through air gaps is also reduced using ETFE. Foil cushions and glass are both inherently airtight; it is the joints that leak, says Tanno. Foil roofs are clamped at the edge to help seal the roof, whereas glass roofs have to be loose at the edges to accommodate thermal and structural movement, and often leak air. The report concludes that the large panel sizes covered by the foil cushions result in “ETFE suffering less leakage than glass as there is a much larger area-to-joint ratio”.
Both ETFE and glass can be recycled. The softening temperature of the plastic is so low that this is not costly, and the recycled material can be added to a hopper with virgin ETFE. Float glass can also be recycled, but is sensitive to impurities that can result in catastrophic failure if it is recycled as float glass.
Recycling will take place only at the end of the material’s life. For double-glazed units, that life is likely to be 20-30 years, limited by the life of the edge seals. For ETFE, a relatively new material, manufacturers are giving guarantees of 15 years. The early installations are now approaching 20 years and the report expects ETFE to be “good for the order of 30 years without significant degradation”.
Cost comparisons between ETFE and glass are difficult. “You need to look at the whole job, since the supporting structure for the roof and the mechanical services are a significant part of the total figure,” says Tanno.
Cost-in-use comparisons are more straightforward. ETFE is closely related to the non-stick material Teflon, so dust and dirt are washed away in the rain. The extra cost of running the fan is therefore offset against savings in cleaning.
At present, two firms in the UK can supply, manufacture and install the cushions: Vector Special Projects and Skyspan. However, as the use of ETFE increases, Tanno expects the number of suppliers to grow. “ETFE will not replace glazing, but in the right application it offers the opportunity to reduce cost and achieve a more appropriate product.”