Tensile and pneumatic applications in constructions have always been characterised as a complementary system to the larger structure. Recently, however, the need for bigger buildings with better designs and larger spans has propelled tensile and pneumatic applications in construction to be characterised as the larger structure itself. Further, a growing environment consciousness has consequently triggered the increasing use of sustainable features and materials in today’s construction.
Ethylene tetra?uoroethylene (ETFE) is regarded as one such material. Excellent material properties, robust structural behaviour and good architectural performance coupled with significant environmental benefits in comparison with other transparent alternatives have been propelling the polymer’s gain in popularity [1–3]. Presently, ETFE has been finding increasing use in structures like airport terminals, stadiums and greenhouses however, its impact on total energy consumption of buildings as well as its versatility is slowly propelling its usage in residential and commercial buildings as well.
- Introduction to ETFE
ETFE, a modified co-polymer made from ethylene and tetrafluoroethylene, was first introduced in the 19th century by DuPont. In 1981, the invention of the drop bar welding technique paved the way for the use of ETFE on a commercial scale. Properties like high corrosion resistance, self-cleaning (due to its nonstick surface) and recyclability (ETEF is a 100 percent recyclable material), amongst others propelled the end-user demand of the material across multiple industries, including the building & construction industry.
In the building & construction industry, ETFE predominantly finds its use in the roofing and facade sectors. Here, it is either used as a single layer membrane supported by a cable net system (figure 1) or as a series of pneumatic cushions made up of two to five layers (figure 2).
The first project to employ the use of ETFE films (or foils) for roofing was the Burgers zoo project of the Netherlands in 1981 . At just 1% of the weight of glass, the use of ETFE films significantly reduced the weight of the structure, the amount of supporting architecture and therefore, the overall costs. The polymer’s fantastic resistance to chemical degradation and UV light has resulted in the original ETFE roof still being fully functional to this day, and showing no sign of visible wear and tear, even after more than 30 years. ETFE films were chosen for this project due to the polymer’s ability to transmit visible and UV light, while withstanding the various natural forces .
In the case of ETFE cushions, multiple films (or foils) are kept continually pressurised by a small inflation unit which maintains the pressure at approximately 220 Pa. The use of pressurised air gives the foil a structural stability and the roof some insulation properties.
The use of ETFE cushions was first employed for the geodesic domes of the Eden Project in Cornwall. The cushions were made with two or more ETFE films with an aluminium perimeter with a pocket of pressurised air inside. The air, which was dried prior to use to avoid condensation, provides structural stability and thermal insulation to the roof. To maintain the air pressure within the cushions, one 60–100W unit per every 1,400 square metres of roofing is required.
Following these two successes, several projects across the globe began employing the use of ETFE polymer for roofing solutions. In 2005, the Tropical Islands resort in Germany built a 20,000 m2 window of ETFE film while in 2008 the largest ETFE cushion structure in the world (National Aquatic Centre for the Beijing Olympics) was built. The National Aquatic Centre building is completely cladded with more than 100,000 m2 of ETFE pillows that are only 0.2 mm (1/125 of an inch) in total thickness. The use of ETEF over glass for the National Aquatic Centre building has resulted in more light and heat penetration resulting in a 30% decrease in energy costs.
- Global ETFE Market
Presently, the ETFE market is projected to grow from USD 352.4 million in 2018 to USD 518.4 million by 2023, at a CAGR of 8.0% between 2018 and 2023. Amongst its various applications, the films & sheets segment of the ETFE market (which is used in the building & construction industry) accounted for the largest share of the ETFE market in 2017, in terms of both, volume and value.
- Factors propelling the use of ETFE
Using ETFE as a roofing solution is nowadays, often considered in projects that would traditionally use glass. This is because, in comparison to glass ETFE transmits more light; insulates better; costs 24 to 70 percent less to install; is only 1/100 the weight of glass; has properties that makes it more flexible as a construction material and a medium for dynamic illumination; and is eco-friendly and recyclable. These properties have been driving the growth of the ETFE market across the globe. Below we take a look at some of the polymer’s more prominent advantages.
3.1 Light Transmission
ETFE foils transmit 94-97 percent light and 83-88 percent UV light, respectively . Across the visible spectrum, light of all wavelengths is evenly transmitted, unlike in the case of other materials (see figure 3). Recent studies have also shown that ETFE foils have much better light transmittance levels than PVC- polyester fabric and PTFE-glass fabric .
3.2 Insulation and solar control
ETFE film systems that use a multilayered approach offer increased thermal performance as compared to other transparent alternatives. The pressurised air that serves to stabilize ETFE cushions is adjusted to achieving varying R-values, depending on the requirement. For instance, in a single-layered application, ETFE will achieve an approximate R-value of less than 1 while in a two-layered system will reach approximately R-2.0, and a three-layer ETFE system will have an R-value of approximately 2.9. [A Case History Review of ETFE on Today’s Current Projects]
Similarly, in the case of solar control, ETFE foil systems can incorporate a number of frit patterns on one or multiple layers to alter their solar performance. This is achieved by printing the foil with various standard or custom patterns. This in turn provides varied levels of solar transmission or reflection. Depending on the angle of the sun (seasonal change), more or less solar gain can be planned (Figures 4 and 5). Further customization is possible by adding pressure control devices that raise and lower the pressurisation between the second and third foil opening and closing the frit pattern based on operational needs. [A Case History Review of ETFE on Today’s Current Projects]
3.3 Sustainability / Cost Effectiveness
From the manufacturing stage to post installation, ETFE promises a lower impact on the environment, savings in project costs and savings in energy requirements during the life cycle phase of a building.
In the production process for ETFE involves the polymerisation of the monomer TFE into ETFE, which is a water based process with no requirement for the use of solvents . It is then extruded to the required thickness, a process which requires very little energy .
In the construction phase, ETFE is welded into large sheets. This process also requires low energy consumption. Additionally, its low weight results in lower C02 emissions and requires far less structural support than other transparent building systems. In fact, the carbon footprint of ETFE is said to be 80 times lower than that of comparable transparent systems .
The ETFE system outperforms any other transparent material systems in terms of insulation, translucency and recyclability. It, therefore, provides a lot of opportunities in terms of reducing indoor lighting costs, temperature control costs as well as repair and maintenance costs. Further, the versatility of the material allows dynamic manipulation of light transmission to match specific requirements.
The use of ETFE in construction can reduce build costs by 10% on small projects and up to 60% on large-scale projects . ETFE cushion roofs typically weigh 450 g per square meter, making it the lightest available transparent roofing system. In comparison to alternative transparent systems, using ETFE cushion roofs reduces the weight of a roofing system by 100 to 250 times. Further, ETFE structures are easy to fabricate and fast to erect; eliminating the need for scaffolding and large cranes.
In an evolving landscape that is increasingly becoming environment-conscious, ETFE foils are emerging as sustainable and cost effective alternative that not only reduces the overall environmental burden but also reduces the energy burden during a building’s lifetime.
3.4 Energy benefits
Building energy consumption accounts for more than 40 percent of the total primary energy . This is a key concern for engineers and architects. ETFE is continually emerging as a material of choice to address this concern as apart from having a very low carbon footprint (as compared to other cladding materials) in the manufacturing process, ETFE offers enhanced building energy management solutions due to its versatility.
By adjusting layer number, pigmentation and printed patterns of ETFE cushions at the design stage, designers can determine envelope systems that are best suited for the project’s energy management. Further, by introducing a variable air pressure system within the cushion, the buildings heat loss and solar gain can be controlled leading to further energy savings.
3.5 Fire safety
ETFE contains fluorine in its chemical structure and a low oxygen index. This means that, intrinsically, ETFE is a self-extinguishing material. It is also a self-venting material, since in most cases the ETFE foils shrink away from the plume of hot gases and vent fire to the atmosphere. Any remnants of ETFE foils are swept up and away by escaping gases and do not fall onto occupants. The self-venting nature also prevents any form of heat build thereby negating any chance of an explosion or flashover. Due to these properties, several national and international standards have rated the material as self-extinguishing with no burning drops. The melting point of ETFE films is around 500°F (260°C).
3.6 Maintenance and cleaning
Due to the similar non-adhesive surface properties as a fluorocarbon polymer, ETFE is a ‘self-cleaning’ material. The co-efficient of friction (typically 0.23) prevents deposits of dirt, dust, and debris from sticking to the ETFE material and are washed away during rains. However, like for all mechanical equipment and their components, inspection and necessary checks must be conducted to prevent possible damages. Maintenance of ETFE systems is required approximately every 3 years.
ETFE is resistant to degradation both from UV light and atmospheric pollution. It exhibits less than a 10 percent decrease in material strength after 10,000 hours of concentrated artificial weathering, an effect that is offset by similar increase in strength due to wind-generated molecular realignment. With upto 30 years exposure at natural test sites in Germany and in Florida ETFE has suffered no perceptible adverse effects. It does not become brittle, nor does it discolour or deteriorate. Importantly, this durability is intrinsic to ETFE, not the result of applied coatings that could themselves be vulnerable to decay.
As a relatively new material, ETFE’s life expectancy is unknown, however, with no evidence of degradation to date, it is well on its way to acquiring the credentials for belonging in the stable of durable long-life materials.
- Limiting Factors of ETFE
4.1 Assemblies and supporting systems
In all cases, ETFE systems should be considered custom design-build aspects of a particular project. While designing an ETFE system, interfaces with adjoining materials, assemblies, and systems will always occur. Whether the ETFE is interfacing with a membrane roofing system, a curtainwall glazed system, metal panels, or even a brick veneer, the continuity of barriers needs to be maintained. To date, this proves to be the biggest hurdle to overcome when working with an ETFE system as, in most cases, an elaborate transition assembly is needed to be developed to allow for movement and provide a traffic walkway.
4.2 Contradictions to building conventions
The ETFE cushion’s unusual form may work in its disadvantage. This is not because of it physical traits, but rather, a result of people’s building traditions and trends. For instance, while conventional building materials are judged on their strength, i.e. becoming stronger when more material is added, ETFE does quite the opposite. Adding material results in failure, because ETFE uses its flexibility to absorb loads through its elasticity and plasticity [12, 13]. The same contradictions are found in movement; while windows concentrate movement at specific locations, e.g. reinforced junctions, ETFE spreads movement over the whole layer [12, 13]. Also, ETFE disappears with fire, while previously it was always the intention for conventional materials to withstand fire [12, 13]. Therefore, it is clear that there is a certain contradiction between ETFE and normal building conventions. While most building components try to work against nature, ETFE works with it.
4.3 Design Complexities
The design of an ETFE system requires a project by project evaluation of the driving force behind the requirement of the system. Whether the function of the ETFE system is architectural imagery, transparency, structural reasons or thermal performance has to be established well in advance. This is why ETFE structures are generally specified as design-build projects or a subcontracted portion of a design-build project (delegated design). The unique characteristics of the system calls for highly specialized and experienced designers. Additionally, the complexity of the cushion system also deters the use of this material on residential projects.
4.4 Sound Transmission
The airborne sound insulating power of ETFE films is 8 dB, which means that almost all sound is transmitted through the film. Pre-stressed ETFE foils allows 100 percent acoustic transparency of low frequency sounds (31.5–250 Hz) while allowing 30 percent acoustic transparency for mid to high frequency sounds (250–8000 Hz) . During rains, the sound on an ETFE system is comparable to that of the sound of rain hitting a metal roof. For these reasons, ETFE systems require significant intervention in the form of innovations to control indoor sound levels.
Two possible solutions have been put forward by Vector Foiltec, one of the biggest firms in ETFE-applications. The first one is used ‘for environments where the absorption of medium or high frequency sound is critical or where the acoustic environment is particularly sensitive’. In this case ‘a kind of Helmholtz resonator can be added to the cushion system. This consists of a microporous ETFE layer on the underside of the cushions that, through pneumatic manipulation of air filled tubes, can be tuned to selective frequency absorption’ . The second solution applies only to rain noise; ‘An ETFE foil cushion is fitted with a device which reduces the effects of rain generated noise by reducing the vibration of the external layer of the inflated ETFE foil cushion by dampening it with a liquid. In addition, the sound reduction index of the inflated ETFE foil cushion is increased due to the increased mass of the ETFE foil cushion due to the addition of the liquid’ .
The future for this relatively new material is undeniably optimistic. Presently, the material enjoys notable success in stadium structures, retail complexes and recreational facilities. However, the material has the potential to impact much further afield.
Propelled by real-world requirements and scientific advancements, constant innovation is taking place in the realm of multifunctional ETFE surfaces (ETFE-mfm). These surfaces allow for the integration of photovoltaics and façade lighting with control electronics and battery storage, to generate electricity in the day which can be used for illumination in the evenings leading to ‘zero-energy’ structures. Further, ETFE is a very ‘green’ material as 100 percent of it is recyclable. Waste material from the site can be recycled into new ETEF foils for use elsewhere. Couple this with the growing attention being given to energy conservation and sustainability and the future of ETFE seems only set to rise. (figure 7)
Below are few case studies showcasing real world applications
- Case Studies
6.1 The U.S Bank Stadium
- Sector – Sports stadium
- Structure – Steel
- Application Type – Roof Enclosure
The US bank stadium is an enclosed stadium that was opened to the public in 2016. To date, it is the largest ETFE installation in North America and is the only stadium in the nation with a clear ETFE roof.
The stadium consists of a fixed roofing system that comprises of 60 percent (240,000 sq. ft.) ETFE. The remaining 40 percent of the roof is made up of the traditional single ply roof over metal decking (figure 8). Although the ETFE section of roof does not cover the entire field, the angle of the roof allows sunlight over its entirety (figure 9). Rain and snow is managed due to the sloping architecture of the roof and slides off into a large gutter that encircles the building. An heating system is attached to the gutter to melt the ice to prevent and blocks and build up.
The ETFE section of the roof consists of 75 cushions with some spans measuring over 300 ft in length. Since the construction was done during the winter seasons, it was observed that the ETFE system of the roof managed to shed snow better than the single-ply membrane roof on the opposing slope.
The engineering firm involved with the US bank stadium construction was Thornton Tomasetti while the architecture firm was HKS.
6.2 The Avenues Kuwait Phase 4/4b
- Sector – Retail
- Structure – Steel
- Application Type – Roof and Facade
The Avenues project is the largest and most prestigious retail project in Kuwait and one of the most renowned shopping destinations in the region with over 850 stores and a parking capacity accommodating over 8500 vehicles. With 3 colossal phases already open to public, Phase 4/4B extends the collaboration between Gensler and Pace, to design an additional floor area of 222,000 sqm. (Figure 11)
The Phase 4/4B of the project comprises of open streets set under an ETFE roofing system. Over 42,000 m2 of ETFE was used to cover the area. The lightweight steel structure of the Dome (figure 10) supports nine unique foil cushion sizes, the largest spanning 20 m² and the smallest cushions along the edge are 3 m².
-  Schwitter, The use of ETFE foils in lightweight roof constructions, in: Spatial, Lattice and Tension Structures, ASCE, 1994, pp. 622–631.
-  Robinson-Gayle, M. Kolokotroni, A. Cripps, S. Tanno, ETFE foil cushions in roofs and atria, Constr. Build. Mater. 15 (7) (2001) 323–327.
-  Moritz, R. Barthel, Transparent architecture-building with ETFE membranes, Detail 12 (2002) 1620.
-  LeCuyer, ETFE: Technology and Design, Birkäuser Verlag AG, Berlin, 2008.
-  Ploszajski, Anna (2nd March 2016) Material of the month; ETFE, Available at: http://www.iom3.org/materials-world-magazine/feature/2016/mar/02/material-month-etfe(Accessed: 8th August 2018).
-  Tregenza, D. Loe, The Design of Lighting, Routledge, 2013.
-  ETFE Film, Available at: http://www.birdair.com/.
-  Pérez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (3) (2008) 394–398.
-  ELIZABETH WOYKE, 24/04/2007.
-  ARCHITEN LANDRELL, (Online) Available at http://www.architen.com/technical/articles/etfe-foil-a-guide-to-design.
-  JEFF BARBIAN (2008).
-  A. LeCuyer, ETFE – Technology and Design, vol. XXXIII, no. 2. 2012.
-  B. G. Morris, “Building Component,” US6860069 B2, 2005.  S. Chiu, D. Noble, E. Valmont, Acoustics in architectural fabric structures: the case of ETFE pillows, in: Fabric Structures in Architecture, 2015, pp. 241–256.
- ETFE Market Available at: www.marketsandmarkets.com (Accessed: 8th August 2018).