Introduction:
ETFE or ethylene Tetra Fluoro Ethylene is turning one of the most interesting materials in today’s modern design industry and architects, contractors and clients are becoming increasingly excited of its potential.
Originally invented by DuPont as an insulation material for the aeronautics industry and semiconductors, ETFE was not initially considered as a main stream building material, its principle use being as an upgrade for the polythene sheet commonly used for green house polytunnels. It wasn’t until the early 1980s, when a German innovator investigated it in his quest for new and exciting sail materials, that its use was reconsidered. Although discounted for the original purpose, he saw its strength, high light transmission and structural properties as an advantage to the construction industry and started to develop the systems we see today.
Over the past thirty years, there has been an increased awareness of the material and its uses and it is now specified by architects and designers across the world on a wide range of projects – from schools, shopping malls and offices, to government buildings and sports facilities.
ETFE is under the architectural spotlight and intends to shine.
An extremely thin membrane, ETFE copolymer is extruded into thin films (or foils) which are used in two ways; as a single layer membrane supported by a cable system or as multi-layer cushions supported in an aluminum perimeter extrusion which, in turn, is supported by the main building frame.
ETFE cushions are kept continually pressurized by a small inflation unit which maintains a constant pressure and gives the foil structural stability and some insulation properties.
Insulation (U Value):
While a single ply ETFE membrane has an approximate U value of 5.6 W/m²K and provides little more than a barrier to the outside world, a multi-layer ETFE cushion can offer a good level of insulation and therefore is often used as part of the architecture of mainstream spaces.
The thermal qualities of ETFE cushions can be improved by the addition of more layers of foil to create additional pockets of air as follows:
-2 layer gives 2.7 W/m2.K
-3 layers gives 1.9 W/m2.K
-4 layers gives 1.4 W/m2.K
Light Transmission:
ETFE foil is a naturally translucent material and transmits light across the entire visible light region (380-780nm). A single layer of medium weight ETFE has a light transmission of approximately 90-95% light transmission, with only a small reduction when multiple layers are added.
Transmission across the ultraviolet range (320- 380nm) is also very good (approx. 83-88%) and therefore provides an environment where plants and vegetation can thrive. It is also important to note that the film absorbs a large proportion of infra-red light transmitted, a quality which can be exploited to improve buildings’ energy consumption.
Solar Heat Gain (G Value):
As a naturally translucent material, when used in a multi-layer cushion an ETFE roof can contribute to natural solar gain within the space below.
G value is the term used to describe the amount of solar energy entering an enveloped space due to sunlight/solar radiation and conduction. This solar gain is generally desirable in winter, due to the passive heating effect the space will have as a result of the sun, and undesirable in summer due to overheating as the space will require additional cooling measures.
The G value of an installation is usually expressed as a percentage or a value between 0 & 1; the higher the number, the more energy is being transmitted through the ETFE foil and the more the building will heat up.
In order to reduce this and to control any glare within a space, ETFE foil can be treated in a number of ways to manipulate its light transmission properties and reduce solar gain. These include adding fritting to the top layer of the ETFE foil, using white or matt diffused ETFE foil or using tinted ETFE foil specifically designed to reduce solar gain.
Fritting:
Using a specialist printing method, the surface of the ETFE foil can be covered with a silver pattern to reflect light transmission and reduce solar gain while retaining visual translucency. By varying the percentage of coverage and density of the ink, the energy and light transmission can be altered. A standard range of fritting patterns to achieve this variety of light transmissions and for clients with very specific performance or aesthetic requirements is available, it’s also possible to have custom patterns.
Inflation:
ETFE cushion systems get their structural stability and ability to withstand weather conditions (i.e mainly wind load) through pneumatic pressure (around 250-300pa). As a result, every ETFE cushion system is continually connected to an air inflation unit from which air pipes network run to each individual cushion. As the cushions only need to maintain pressure and not generate air flow, the energy consumption used by these units is minimal. An entire roof is generally powered by a single air inflation unit; for large installations (2,500sqm and upwards) multiple air handling units are installed and networked together to allow load sharing.
Each air inflation unit contains 2 fans powered by electric motors; the fans run alternately, with only one fan running at any given time. In the event of a cushion failure, adverse weather conditions or a drop in cushion pressure, both fans will run simultaneously to maintain a steady pressure.
A typical air inflation unit has a footprint of 1m x 0.5m and is located near to the ETFE cushion system, internally or externally. The system requires a dedicated and secure 240v power supply. For huge roofs this may increase a bit and a use of 400v power might be needed.
As standard our systems are supplied with:
» Industrial PLC controller with touch screen interface, data logging and full diagnostics
» LCD panel & traffic light indicators for local status
» Dual fans for load-sharing & redundancy
» Dehumidifier
» BMS interface modem to replicate traffic light system status.
» Non-return valves to reduce air loss.
» Inverter-driven fans to optimize efficiency.
» Stock parts for quick replacement
Additional options include:
» Weather station for reactive pressure control
» Remote 3G monitoring via web interface.
» UPS battery backup for control system
» Remote LED mimic panel
» Remote LCD control panel
As standard, ETFE cushion roof systems are supplied with an advanced, active monitoring system. An LCD screen mounted on the inflation unit supplies real-time information on the pressure within the cushions and any faults within the operation of the system via a traffic light system:
The control system monitors activity automatically and can adapt to avoid potential issues.
In the event of a problem the control system will automatically try to compensate for the fault; for example, when a drop in pressure occurs the rate of airflow to the cushions will increase. In addition, the fault is highlighted on the LCD display.
As an extra option, we can offer remote diagnostics which allow ETFE supplier (or a designated user) to gain access to the control system remotely. The remote view lets a user see how the system is performing, assist with off-site updates or diagnostics in aid of fault finding and allow minor pressure and performance adjustments.
Repair and Replacement:
One of the outstanding characteristics of ETFE foil is its exceptional tear resistance, lack of notch weakness and stress crack concentration. Any cuts and scratches initially propagate but the material rapidly stretches and rounds out into a tough low radius area that dissipates the loads and prevents further tearing.
Minor repairs to the foil, such as a puncture hole, can be carried out in situ and within a relatively short timescale.
If an ETFE foil cushion becomes more significantly damaged, an individual cushion can be easily removed and replaced with minimal disruption to the installation. The outside surface of the ETFE cushion can be accessed, using rope access techniques, from the main structural support. This would require the rigging of working ropes and is routinely done under.IRATA/BS 7985:2009 guidelines.
Where remote diagnostics are included within the system, every effort will be made to diagnose potential problems remotely. Once the problem is identified, it can be fixed in one of two ways:
Remote repair (software problems)
On site (hardware problems). If on-site repair is required, the supplier or user need access to the building control room to resolve the problem or repair the cut.
Cushion Size:
ETFE foil cushions can be manufactured to any size and to fit any shape. Size is limited by the wind, snow and water bonding loading allowed for within the design and by the orientation of the cushions i.e. whether they are installed horizontally or vertically.
As a general design guideline, rectangular cushions can span up to 4.5m in one direction and as long as required in the other direction. For triangular cushions, the size can be greater than this. If design dictates that larger cushions are required, these can be created by reinforcing the internal and external layers of the cushion by cable restraints or by using thicker ETFE foil.
Life Expectancy:
ETFE foil has an excellent life expectancy as it is unaffected by UV light, atmospheric pollution, and other forms of environmental weathering.
While no ETFE structures have been in place for longer than 30 years to allow us to gain a true understanding of the aging process of the foil, the material has been extensively researched and tested in a laboratory environment and out in the field. These tests have concluded that no degradation or loss of strength has occurred and there is no sign that the material will become brittle or discolor over time. As a result, it is anticipated that the material has a life expectancy of more than 35 years.
Fire Performance:
ETFE Foil as a material has low flammability (320C) and is considered self-extinguishing. In the event of a fire, hot smoke will cause the foil to soften, evaporate and then shrink away from the fire source to create natural ventilation. The quantity of material used in the roof is not important in this situation – the foil will not create molten drips or any appreciable fumes.
ETFE foil has been comprehensively tested. This is a selection of the fire results:
DIN 4102 Class B1
EN 13501-1 Class B-s1,d0
NFP 92-505 M2
NFPA 701 Pass
Fire Performance
In some cases, it is not possible to guarantee that smoke will reach the ETFE at a temperature which will cause the cushions to evaporate/melt (i.e the temperature of smoke may stay below 320C; therefore, it is worth consulting the project fire expert to analyze the scenario of fire, predict the smoke temperature and then decide whether the installation of automatic actuators is necessary in order to ventilate the space of smoke or not(i.e ETFE will melt away and form natural smoke vent which is in the favor of more safety)
To know more about ETFE design, try out below free educational web application