ETFE Foil: A Guide to Design
A comprehensive introduction to the properties, qualities and ways to use ETFE foil.
Introduction
ETFE foil roofs can be supplied as a single layer membrane supported by a cable net system but generally consist of a series of pneumatic cushions made up of between two and five layers of a modified copolymer called Ethylene Tetra Flouro Ethylene. The ETFE Foil is extruded into thin films and supported in an aluminium perimeter extrusion which, in turn, is supported by the main building frame.
The cushions are kept continually pressurised by a small inflation unit which maintains the pressure at approx. 220 Pa and gives the foil a structural stability and the roof some insulation properties.
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Insulation
While a single ply ETFE membrane has a U value of 5.6 w/m²°K, a standard three layer cushion has a U value of 1.96 w/m²°K – a better insulation value than triple glazing when used horizontally (glazing manufacturers figures are for vertical glazing which considerably enhances the figures). The insulative qualities of ETFE cushions can also be improved by the addition of more layers of foil which can be treated with specialist coatings to enhance the thermal properties.
Transparency
ETFE Foil is a very transparent material and transmits light across the entire visible light region (380-780 nm) totalling approximately 85% of total light. Transmission across the ultraviolet range (320- 380nm) is also very good (approx 83-88%) and therefore allows plants and vegetation underneath to 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.
A standard single ply EFTE foil roof has a nominal G-value of 0.93.
Solar Control
As described above, the base material of the cushions is very transparent, the ETFE Foil can be treated in a number of different ways to manipulate its light transmission properties. These include:
- Printing: Also known as fritting, the surface of the foil is covered with a variety of patterns to reduce solar gain while retaining translucency. By varying the percentage of coverage and density of the ink, the energy transmission can be altered. Alternatively, the foil can be over printed with a number of treatments to affect transmission.
- Tinting: A wide variety of coloured foils are also available, although less readily than the standard clear foil. Coloured foils can be used alongside clear foil to incorporate branding and large scale imagery.
- Surface treatments: Surface treatments undertaken during the manufacturing process can vary the properties of the fabric and allow us to manipulate light transmission. These treatments render the foil matt in appearance and therefore provides an excellentprojection surface for light shows and images.
- Radiation: The foil can also be conditioned with a range of radiation treatments which can reduce the levels of IR and UV rays transmitting through the skin.
Adding additional layers of ETFE foil to a cushion also allows light transmission and solar gain to be controlled. Multi-layer cushions can also be constructed to incorporate movable layers and intelligent (offset) printing. By alternatively pressurising individual chambers within the cushion, we can achieve maximum shading and reduce shading when required. Essentially this means that it is possible to create a building skin which is reactive to the environment through changes in climate.
The G value of an ETFE cushion system can be reduced to as little as 0.48 by having a fritted surface and to 0.35 by using a 3 layer system. For comparison, standard glass is approx 0.88 whereas some specially treated glass may be as low as 0.46.
Life
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 more than ten years, 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 discolour over time. As a result, it is anticipated that the material has a life expectancy in excess of 30 years.
Fragility
ETFE foil cushion systems are certified as class C non fragile roof assembly in accordance with ACR(M)001:200 – test for fragility of roofing assemblies.
Class C is the lowest class of non-fragile assembly and, particularly if engineered to pass the test criteria, may be close to the boundary between fragile and non-fragile. Its classification and use therefore requires the following to be taken into account:
a) Normal industry-recommended best practice is that Class 'C' assemblies should never intentionally be walked upon and appropriate temporary access equipment, such as crawling boards, etc., should always be used. Note: Accidental damage to such assemblies might render the classification void.
b) A Class C assembly must be treated like any other safety critical item, e.g., a safety net. Therefore, any adverse occurrence that could affect its fitness for purpose should trigger an inspection. If an assembly has been subjected to an impact load (such as a trip or stumble), it can be treated as a fragile area and identified and protected accordingly, until it has been replaced and the adjoining fitted panels inspected by a competent person and replaced if necessary. Procedures to ensure this happens must be in place.
c) The workforce must be aware of these limitations, as required by Regulations 3 and 8 of the Managing Health and Safety at Work Regulations [MHSWR].
d) Any person falling on a class C assembly may make it fragile for subsequent loads. While persons may be capable of self-recovery from a fall or stumble, where they are unable to, the additional weight of a rescuer may cause the assembly to fail. And, because all non-fragility classifications depend on the fixings of assemblies, any adjoining assemblies may also have become fragile. In such situations the incident panel and all adjoining panels must be treated as fragile. This is a foreseeable risk of selecting Class C assemblies. Therefore, where class C assemblies are being used, rescue plans must be developed in advance of work starting. Again, in accordance with Reg. 5 and 8 of the MHSWR, the workforce needs to be aware of the Rescue Procedures.
Inflation Units
ETFE cushion systems are continually inflated by air inflation units located close to the cushions. 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 two or more fans powered by electric motors within each inflation unit. The fans are normally used alternately to maintain pressure with only one fan running at any given time and uses a similar amount of power to a light bulb. In the event of a cushion failure, both fans will be used to maintain a steady pressure.
A typical air inflation unit measures 1.2m x 1.2m x 0.9m and is located nearby to the ETFE cushion system, internally or externally. The system required a dedicated and secure power supply consisting of two 240V 13 amp electrical connections – as the ETFE foil roof is a live system, the cushions are permanently linked to the air inflation unit to ensure the pressure is maintained.
The inflation units are fitted as standard with dehumidifiers to dry the air being fed into the cushions
Control System
As standard our ETFE cushion roofs are supplied with a state of the art active monitoring system within the inflation unit. This continually supplies real-time information on the pressure within the cushions, the local weather conditions, temperature and humidity levels and any faults or changes within the operation of the system to a local LCD screen. It is also possible for Architen Landrell to gain access to the control system remotely which assists with off-site updates or diagnostics in aid of fault finding.
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 it will increase the rate of airflow to the cushions. In addition, the fault is logged and depending on the severity of the fault, one of the warning lights (red or amber) will be shown on the main controller and relevant air handling unit, and an automatic alarm email sent to the designated person. The alarm will be raised as soon as a fault is registered.
Continual monitoring allows the system to automatically adapt to avert potential problems. If humidity levels become too high, for example, the system will automatically start the built-in dehumidifier to dry the air. The system will also automatically adjust to cope with small punctures or adverse weather conditions and if necessary will notify the designated person automatically.
To allow simple diagnostics, maintenance staff can inspect the coloured indicators “traffic lights” on the front of the air handling unit. The traffic lights display the status of the inflation equipment:
Ø Green light – all working fine
Ø Green & amber lights – minor problem, needs attention
(for example, a partially blocked air filter)
Ø Red – Immediate attention required
Power Failure
In the unlikely case of a power failure, the ETFE cushion system will maintain it’s pressure for between 3 and 6 hours before deflating (dependant on weather conditions). This is due to the non-return values built into the air inflation units. After this time, there is a possibility that the roof will become damaged, therefore we recommend that there is either a standby generator or alternatively a cable bracing system installed to support the cushions in this situation. As well as this, we always recommend that the roof is closely monitored to avoid these problems occurring.
Safety/Explosion Risk
As a flexible material, ETFE Foil can take very high loadings for a short period of time which makes it an ideal material for use in locations where there is a risk of explosion. If vandalism is a threat, ETFE foil is also a useful material as the cushions will not break or fall from the extrusion frames if damaged.
Repair and Replacement
One of the outstanding characteristics of EFTE that allows it to be used for roofs is its exceptional tear resistance and lack of notch weakness and stress crack concentration problems that prevent us from using other foils like polyesters etc. Any cuts and scratches initially propagate but the material then rapidly stretches and rounds out into a tough low radius area that dissipates the loads preventing further tearing.
The most common threat of tearing is from birds. Excessive bird pecking can cause small punctures in the ETFE cushions which in general pose no threat to the stability of the cushion as a whole. Should a larger hole be made and pressure start to drop our system is supplied with a built in active monitoring system.
If an ETFE Foil cushion becomes damaged, the individual cushion can be easily removed and replaced with minimal disruption. The outside surface of the ETFE cushion can be accessed by technicians, using rope access techniques, from the main structural steel support. This would require the rigging of working ropes from the steel and is routinely done under the IRATA guidelines.
Minor repairs to the foil, such as a puncture hole, can be done in situ and within this time scale. Fritted material would be used to match existing fritted foil in order that repairs do not affect the aesthetics of the structure.
Should a problem occur with the control system, every effort will be made to diagnose the problem 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, we will only need access to the building control room to resolve the problem.
Fire
ETFE Foil as a material has low flammability (200C) and is considered self extinguishing. In the event of a fire, hot smoke will cause the foil to soften, fail and then shrink away from the fire source to create ventilation. The quantity of material used in the roof is not important in this situation – the foil will not create molten drips or any fumes.
In some cases, it is not possible to guarantee that smoke will reach the ETFE at a temperature which will cause the cushions to fail, therefore, it is possible to install a ‘hot wire’ system that can cause the cushions in part or whole to fail on demand (at the flick of a switch!)
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
For more information on specific fire tests, please contact Architen Landrell.
Acoustics
ETFE foil cushions are a relatively transparent form of roofing which means that there are minimal acoustic benefits. For more information on the acoustics of ETFE foil, please consult Architen Landrell’s ‘ETFE and Acoustics’ document.
Rain noise can be suppressed using a rain suppression layer added to the top surface of the cushions, however, we would advise that it is only necessary in exceptional circumstances. This can be retro fitted to the ETFE foil cushion system and therefore we recommend that rain noise is assessed prior to making a decision to install.
The raw material associated with ETFE is a class II substance admitted under the Montréal treaty. Unlike its class I counterparts it causes minimal damage to the ozone layer, none of which are used in the manufacturing process. The production of ETFE involves the transformation of the monomer TFE in to the polymer ETFE using polymerisation; no solvents are used in this water based procedure. The material is then extruded to varying thicknesses depending on application; this process uses minimal energy. Fabrication of the foil involves welding large sheets of the ETFE; this is relatively quick and again a low energy consumer.
ETFE can be recycled with ease, but due to its properties (does not degrade under UV light, sunlight, weather, pollution) it has a very long life which is estimated between 50-100years, making the need for recycling small. The aluminium frames do require a high level of energy for production, but they also have a long life and are readily recycled when they reach their end of life.
ETFE cushion systems offer both good insulation and translucency, due to the fact they trap a layer of air and can be adapted using dot matrix coatings to change the solar transmission. The weight and size of the EFTE has added benefits making it much more energy efficient than materials with the same desired architectural effect. For example, transportation of the material is much easier as it can be rolled, taking up less space, hence the need for less conveyance. The cleaning and maintenance of the ETFE is also small, the majority of the time water will wash off any dirt, this is due to the smoothness and anti adhesive properties of the material. If cleaning is needed then only light PH neutral detergents are used making the environmental impact minimal.
Unlike traditional fabric structures, ETFE Foil is an extruded material and therefore has a smooth surface. This smoothness reduces the amount of dirt retained on the ETFE foil surface and allows the rain to wash away the majority of bird droppings etc. As a result, we advise that ETFE foil cushions are cleaned externally every 2-3 years.
The cushions themselves also need to be cleaned internally, although far less often. Depending on the amount of dirt collected in the internal atmosphere, we would recommend they are cleaned every 5-10 years.
We recommend that the inflation units are serviced every year, however, an active monitoring system is incorporated in all our units which supplies continual information on performance.
Architen Landrell has a designated Maintenance Department which deals with planned and emergency works at sites across the world. Our maintenance personnel are based at our office in Chepstow and have a fleet of vehicles available for site visits. In the event of an emergency, we also have a 24/7 dedicated telephone line for call outs and advice.
Weight
ETFE Foil cushions are extremely light weight weighing only 2 - 3.5 kg/m².
Cushion Size
ETFE foil cushions can be manufactured to any size and to fit any shape. Size is limited by the wind and snow 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 3.5m 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.
Warranties
Warranty is available for all our ETFE foil cushion systems. We recommend that a maintenance agreement is also undertaken to ensure the foil and air inflation equipment are in full working order.
Material Strength
Based on 250 micron ETFE
| Test | Unit | Value | Test Method |
|
Tensile Strength |
M Pa | 50 | DIN EN ISO 527-1 |
|
Tensile Stress at 10% Strain |
M Pa | 21 | DIN EN ISO 527-1 |
| Tensile Strain at Break | % | 600 | DIN EN ISO 527-1 |
| Tear Resistance | N/mm | 500 | DIN 53 363 |
| Opacity | % | 7.5 | DIN 53 363 |
