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ETFE moves mainstream
By Edward M Peck, AIA, LEED AP
What is ETFE?
ETFE was pioneered by DuPont more than 40 years prior to its architectural debut in Europe in the early 1980s. DuPont developed the fluorocarbon-based polymer to have high corrosion and chemical resistance, structural strength with low propagation of tear and integrity over a wide temperature range. ETFE is a copolymer of ethylene and tetrafluoroethylene and is known as a “tough polymer.”
The material is extruded from a resin into highly transparent, strong and lightweight sheets commonly called foil. The foil’s surface is nonporous and has a low coefficient of friction, which allows the material to resist atmospheric pollution and the buildup of pollutants, dust or dirt particles. In addition, the material is unaffected by UV light and doesn’t break down, discolor or weaken structurally over time. At the end of the material’s fabricated use, it can be recaptured and fully recycled.
ETFE in architecture
ETFE has been used for more than 25 years for transparent roofs and facade systems around the world, but only recently entered onto the world stage with the debut of the Allianz Arena for the 2006 FIFA World Cup in Germany and with the light-filled National Aquatics Center at the 2008 Beijing Olympic Games, both constructed of air-filled ETFE pillows. Now the material —with its diverse aesthetic and performance capabilities— is considered a material of choice for both traditional skylights and innovative building skins. Few other materials utilized as a building skin match its transparency, durability and strength.
Prior to these two projects, the most critical project relating to the technology’s development was the Chelsea and Westminster Hospital in London, England, designed by Sheppard Robson and completed in 1986. The Chelsea Hospital was instrumental in transitioning the use of ETFE from ephemeral programs —like exhibition, aquatic centers and zoo pavilions— to program types that had life-safety, performance and longevity concerns at the center of the system’s evaluation. Additional performance testing and technical reports were commissioned during the system evaluation for this project. The success of these tests and the endorsements by leading experts that followed began to validate the use of ETFE within mainstream architecture, leading to further acceptance and awareness of the material and technology.
Today ETFE foils have penetrated the entire range of architecture programs, from hospitals to stadia and corporate complexes to highly secure governmental headquarters. Currently in North America several projects on the drawing boards will integrate ETFE foils, including a sculptural skylight designed to depict a DNA molecule at a major university medical complex and a prototype for a rooftop greenhouse that could be used on urban rooftops around the world.
System overview
ETFE systems have a carbon footprint that is approximately 80 times lower than comparable transparent systems and they weigh as little as 1-3% of traditional cladding systems. These factors, combined with the system’s life expectancy and its capacity to be completely recycled makes the system one of the most sustainable building products available. In addition to its minimal weight and mass the material’s superior strength allows individual panels to be much larger than comparative glazing units. Beyond the lightness of the skin itself these characteristics allow for the development of lighter and more efficient structural support systems while maximizing the overall transparency. The system’s impact resistance and strength, its capacity to accept large deflections and its inert properties makes it one of the safest materials in severe conditions like terrorist threats, extreme climatic events and earthquake activities.
There are two main concepts for integrating ETFE foils into architectural cladding systems:
- Replace fabric within traditional tensile membranes with ETFE foils. This concept can be problematic because of its basic structural properties. Specifically, the elongation and potential creep of ETFE makes it difficult to stretch without long-term deformations. If a single layer tensile membrane is to be constructed with ETFE, close attention needs to be given to the engineering and management of the stresses within the foil. This concept also develops a system that is limited to no thermal performance and should not be considered a viable thermal envelope in most applications.
- Utilize ETFE within a pneumatic system. This second and more common method comprises multiple layers of foils welded into panels that are inflated with air at a low pressure to stabilize the foil and provide superior thermal performance. The panels must have at least two foils; however, more foils can be added to enhance the cladding insulation properties. In addition, each foil can be modified with a variety of treatments to control and manipulate the aesthetic qualities, the visual transparency and the level of solar gain/transmittance.
Each panel is restrained at its perimeter by aluminum extrusions, which are fastened to a support structure. Most aluminum extrusions incorporate secondary drainage channels and high-quality EPDM (ethylenepropylene diene) gaskets to ensure the waterproof integrity of the cladding over the life of the building.
The systems can be fabricated in any size or shape, limited only by the external loads that the cladding has to resist, such as wind and snow loads. Structurally these configurations are either considered a one-way-span or two-way-span. A one-way solution would develop panels that are long and slender with the short width governed by the load conditions and the long dimension by fabrication and erection issues rather than structural restrictions. In a two-way system, load transfer within each panel occurs in multiple directions, transferring the loads efficiently in all directions. This concept allows panels to be wider in both directions than the narrow dimension of one-way systems, although both dimensions are governed by the external loading so the total area of the panel may be smaller than what could be engineered utilizing the one-way concept. Any shape or geometry can be created by following either of these two principles.
Foil options / system make-up
In its basic form, ETFE is highly transparent across the entire light spectrum, providing superior natural day lighting. Though many programs and energy modeling call for complete transparency —capitalizing on the sun for day lighting and to drive natural ventilation through the generated stack effect— many specific conditions require the ability and capacity to create localized shading, either to control heat-gain or to shade occupants.
To accomplish this, ETFE has two forms of treatment to alter its transparency, solar control and/or skin aesthetics. One treatment involves printing on the foil itself as a “frit” pattern; various standard and custom patterns can be applied with a wider range of possibilities than those available from the glass industry. The other treatment is the embodiment of color in the ETFE resin prior to the extrusion process. This treatment can provide consistent tints to the foil, rendering them translucent in an array of colors.
These treatments provide single-layer foil options, but one can also utilize various foils within multilayer systems in unique combinations to provide greater flexibility for both aesthetic and performance conditions. This variation of foil types and prints can be taken one step further through activation of the middle foil by utilizing the existing air supply of the system to change the pressures between the foils, actively changing their position within the system to the top or bottom. Because the top and middle foils of the system would be printed with positive and negative patterns, as the middle foil moves closer to the top foil, the patterns begin to align, allowing the system to block more light. (See Diagrams A & B in the box above) By moving the middle foil away from the top foil, the patterns begin to separate, creating more gaps that allow the system to become more transparent. This concept can be used to adjust both performance and aesthetics of the system in real time, contributing to either energy efficiency for the building or accommodating flexibility in space programming.
Thermal performance
The pneumatic system has an extreme advantage utilizing the air that stabilizes the foil as an insulator. The simple addition of layers of foils will increase the thermal performance without increasing the volume of air within the system. The typical system has three layers of foils, which is a good balance between thermal performance and optical transparency of the system, but some systems have incorporated up to five layers of foil, which almost doubles the performance from the standard three-layer system. The three-layer system has a R-value of approximately 3.2 ft2•°F•h/Btu which is superior to other transparent materials and cladding. In addition to adding layers of foil to increase performance internal blankets of Nanogel® can be integrated to substantially increase the system’s thermal properties. The additional performance would be based on the thickness of the added blanket of Nanogel. Estimates of this solution would be in the range of 5–15 ft2•°F•h/Btu. Thus the system can be designed to meet specific thermal parameters dictated by energy modeling for ultimate building performance.
Structural properties and impact strength of ETFE
The system’s minimal weight, superior structural capacity and ability to accept movement under deflection are critical aspects that guide the development of efficient long span and light-weight structures utilizing ETFE. Much of the cost efficiencies of ETFE systems and sustainable benefits are derived indirectly because the lighter structure means using less material from the roof to the foundations.
ETFE foil is extremely strong and systems integrating ETFE can be engineered to meet any specific load criteria by modifying the foil thickness, panel size or panel cambers. Designing within these variables, the system has been successfully engineered in both high wind/coastal zones and in high or heavy snow areas.
Although the air pressure within the pneumatic systems is not structural the internal pressure does resist flogging under typical wind loads. The panels are structurally sound in a deflated state, however the potential of water ponding should be analyzed. This analysis of geometry and slope could determine localized areas where a secondary support system could be used to prevent extreme elongation of the ETFE foil and avoid excessive ponding under water or snow loading. A secondary support system could be as simple as a series of small cables integrated just below the bottom foil.
In addition to the foil’s strength under loading, it is extremely resistant to natural impacts. This durability and strength is proven with successfull tested of both the small and large “missile tests” (representing hurricane conditions) and blast performance testing.
Both the load capacity and impact strength of the foil have been contributing factors for use in high security projects by the U.S. and British governments because foil systems do not contribute to dangerous conditions during blast events.
Sustainability/energy efficiency
ETFE systems are composed of “green” materials with low embodied energy and are demountable and recyclable. However, the most sustainable attribute is the system’s ability to become a catalyst for holistic sustainable design, driving building performance and creating facilities that are environmentally significant and conscious. This result and the ultimate building performance has a greater ecological effect than the sum of its parts. Areas that drive these holistic sustainable opportunities include natural ventilation, reduction of structural tonnage and controlled light transmittance for daylighting.
Conclusion
The technology is not static. Specialty design fabricators continue to push new advancements and architects develop innovative concepts for system integration that drives building performance, sustainability and aesthetics. LED integration within ETFE systems is similar to any other cladding system in that the lighting systems are secondary to the cladding system itself lighting the skin for graphic or informative display. What is unique about ETFE is that several foil types are very good with absorbing or transmitting the light that is projected onto it with minimal projections and without hot spots.
In the future, LED will be fully integrated within the skin itself. This type of system—digital skins transmitting digital images from one side of the skin while presenting themselves as transparent skins from the other side—is in the prototype phase.
The specifics of these developments and the design possibilities are not known, but one thing for sure is that the future of this technology is limitless.
Case studies
Click here to view case studies using ETFE systems.