A Case History Review of ETFE on Today’s Current Projects

This article is republished from the proceedings
of RCI’s Building Envelope Technology
Symposium, held October 17-18, 2016, in
Houston, Texas.
ABSTR ACT
ETFE, the fluorocarbon-based polymer
ethylene tetrafluoroethylene, is quickly
gaining popularity in North America and
being used on some of the continent’s most
prominent projects. ETFE was developed for
architectural purposes in the 1970s, and
since that time, mainstream use of ETFE
in construction projects has been largely
limited to Europe. The material has many
attractive attributes that provide not only
a new aesthetic quality, but also potential
cost savings. Weighing in at roughly one
percent of the weight of glass, significant
reductions in structural costs are made
possible by employing ETFE. Despite these
great potential benefits, the material is
not an equal substitution to glass or other
roofing systems in many respects. Through
review of material characteristics, performance
modeling, and multiple case studies
of current ETFE installations, the authors
will discuss lessons learned, limitations,
and the benefits of the material from the
perspective of building science implications.
ETFE History
In the late 1940s, DuPont developed
ETFE and worked to define an appropriate
end use for the material. Not surprisingly,
architecture did not get the first look. One
of the first explored applications was insulation
material for electrical wire, which
needed to be resistant to friction and abrasion
and immune to hostile environments
such as radiation exposure and extreme
temperatures. The product also found a
specialized use in greenhouse applications
and proved itself as a robust and stable
material, resistant to tear and puncture,
as well as the negative effects of UV, while
transmitting the needed spectrum of light
for plant growth.
In the 1980s, Stephan Lehnert, a
mechanical engineer by trade, first investigated
the use of the material as a ship sail
material. After determining the ETFE foil
was not an improvement to sail technology
of the time, he explored its use as an
architectural cladding and roofing material.
Lehnert later went on to found Vector
Foiltec in 1982, a design-build provider of
ETFE systems worldwide. The first project
with ETFE was a pavilion at a zoo located in
Arnheim, Holland.
ETFE is considered to be in the family,
or at least a distant relative, of tensile
fabrics used in tensioned fabric structures.
Unlike its tensile fabric cousins, it is neither
a coated fabric nor a mesh fabric, but
lends itself to many of the same design
considerations. In the late 1950s, a man
from New York by the name of Walter Bird
formed a company called Birdair and began
his pursuit of designing and constructing
some of the world’s most impressive tensioned
fabric structures. As ETFE became
a relevant material, it was quickly adapted
into Birdair’s wheelhouse, and Birdair has
continued to grow, with subsidiaries around
the world.
Today, Vector Foiltec and Birdair are
considered two of the largest ETFE designbuild
specialty contractors in the world.
With ETFE increasingly being specified on
a wide range of projects—from schools and
offices to government buildings and sports
facilities—the number of other competitors
is rapidly proliferating. This increased
competition has affected the manufacturing
of the material, as well, and DuPont is no
longer the only manufacturer of ETFE. The
most well-known brand names of ETFE
include Tefzel® by DuPont, Fluon® by Asahi
Glass Company, and Neoflon® ETFE by
Daikin, among others.
Foils to Cushion
ETFE has changed considerably from its
first use as an electrical insulator material
and takes on a much more eye-catching
form in architectural settings. To make the
material useful architecturally, the ETFE
is extruded into thin sheets, referred to
as foils. The thicknesses of individual foils
can vary, but are typically between 2 and
12 mils (0.002 to 0.012 in.), depending on
the performance requirements for given
loading conditions. In multilayered applications,
individual foils are perimeter-welded
together and inflated to become a cushion.
The most common applications in North
America have included two- and three-foil
No v e m b e r 2 0 1 6 I n t e r f a c e • 2 9
cushions, but single-layer installations do
exist as well as high performance systems
with up to five foils (Figures 1, 2, and 3). For
a simplified comparison, the number of foils
can be loosely compared to single-, double-,
and triple-lite glazing units. In fact, ETFE
systems are very similar in performance to
glazed systems and are serving as an alternative
option for these systems.
Insulation
Much like a glazed system, increased
thermal performance is possible with a multilayered
approach. When foils are formed
into cushions, the pressurized air serves
to stabilize the film and provides structural
performance in addition to the thermal performance
of the system. In a single-layered
application, ETFE will achieve an approximate
R-value of less than 1. A two-layer
system will reach approximately R-2.0, and
a three-layer ETFE system will have an
R-value of approximately 2.9.
Transparency
ETFE films can be up to 95% transparent
and allow for the passing of ultraviolet
light, which is responsible for promotion
of photosynthesis and facilitating plant
growth. The amount of solar shading and
transparency can be changed by adjusting
the translucency, density, and number of
layers, as well as the use of frit patterns.
While colors can be introduced to provide
a unique look for the ETFE, the ETFE is
generally left transparent, with only a slight
gray or white hue. In the end, it is no wonder
why this material was first used as a
greenhouse enclosure. With this new attribute,
the potential for stadiums to be fully
enclosed but have stationary natural turf
becomes much more feasible.
Solar Control
ETFE foil systems can incorporate a
number of frit patterns on one or multiple
layers to alter their solar transmission performance.
To achieve this effect, foils are
printed with various standard or custom
patterns and can provide 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). Much like solar shading
outside of a glazed window system, the heat
gain can be suppressed in the summer
months and allowed in the winter months.
In more advanced systems, pressurization
of chambers can be raised and lowered
3 0 • I n t e r f a c e N o v e m b e r 2 0 1 6
Figure 1– A sketch diagram of a one-foil, cable-supported ETFE system.
Figure 2 – A sketch diagram of a three-foil ETFE cushion.
Figure 3 – 3-D model of generic ETFE rail and cushion system.
Figure 4 – Diagram sketch of a four-foil ETFE cushion (fritted foils shown as dashed lines)
with lower pressurization between the second and third foil, closing the frit pattern and
limiting solar access during the summer months.
Figure 5 – Diagram sketch of a four-foil ETFE cushion (fritted foils shown as dashed lines)
with higher pressurization between the second and third foil opening the frit pattern,
allowing solar access during the winter months.
in order to move the internal foils, which
essentially opens or closes the frit patterns
based on operational needs.
Weight
Arguably the most economically desirable
property of an ETFE foil system is its
weight. Although you can barely measure
the thickness of a single ETFE foil without
a specialized instrument, when the material
is built into a cushion, it expands to create
a structural barrier several feet in depth.
Weighing in at roughly 1% of the weight of
glass, it can reduce the cost of the structural
support system significantly. Even
with the addition of the extra foil layers
to produce an inflated cushion, aluminum
extruded components, flashings, and an
inflation tubing system, roof weights are
often reported to be considerably lighter
when compared to a glazed system.
Fire
Like many of the other plastic-type
building materials, there is always a concern
for how the material will behave
in the event of a fire. The National Fire
Protection Association’s NFPA 285 has literally
changed the direction of many projects,
limiting material and system selections
based on flame spread and surface burning
characteristics. ETFE has undergone the
full gamut of testing and has been rated
under different national and international
standards as self-extinguishing, with no
melting or dripping of molten, burning
material. When exposed to fire or temperatures
above 500°F, the film simply melts
away. The ETFE material is classified under
several standards:
• ASTM E84, Standard Test Method
for Surface Burning Characteristics of
Building Materials, Class A
• UL 94VTM, Tests for Flammability of
Plastic Materials for Parts in Devices
and Appliances–Thin Material
Burning Test, Class 0
• EN 13501-1, Fire Classification of
Construction Products and Building
Elements–Part 1: Classification Using
Data From Reaction to Fire Tests,
Class B-s1-d0
• NFPA 701, Standard Methods of
Fire Tests for Flame Propagation of
Textiles and Films
When ETFE is exposed to fire, it only
melts and pulls away in locations where
flame is in direct contact, which reduces the
risk of a fire spreading across the material
or to other adjacent materials. In an atrium
space, rather than containing and feeding a
fire like a traditional roof system or even a
glazed system, ETFE has the unique ability
to self-vent the products of combustion
to the atmosphere. Under fire conditions,
any hot gases impinging on the cushions
will cause the foil to soften, lose strength,
and melt. In sample tests, it was observed
that when exposed to flame, the ETFE will
shrink back from the plume and disintegrate,
venting the fire to the atmosphere.
As the quantity of material used in the roof
is so small and the ETFE is self-extinguishing,
any material that falls from the roof or
is swept upward will not burn occupants,
first responders, or other materials, should
it come into contact with them. This selfventing
and self-extinguishing feature of
ETFE prevents the buildup of high temperatures
under the roof and can prevent catastrophic
structural collapse of the primary
structure.
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Acoustics
ETFE film also has about 70% acoustic
transmission, making it ideal for projects
in which loud noises are expected.
During design development, sound transmission
should be considered, as it will
indeed transmit sound beyond the ETFE
system and to the exterior of the building.
Additionally, any sounds originating outside
of the structure can be translated inward. In
the case of rain on the roof ETFE assembly,
it has the potential to sound more like a
metal roof than a fabric roof.
Safety/Security
ETFE has a high resistance to deformation,
but when that resistance is overcome,
it has a high elasticity, which makes it an
ideal building component where sudden
extreme loads, such as earthquakes or
blasts, could occur. Much like safety films
that are placed on glazing, the ETFE is itself
a standalone film that cannot shatter. The
worst possible damage that could occur to
ETFE under a shock-load situation would
be a tear or a hole. ETFE will either deflect
under load or—in the case of tearing—is
unlikely to cause any major damage to other
building components, property, or people.
In laboratory testing, ETFE has proven
to be surprisingly puncture resistant.
Projectile and “missile” testing of a three-foil
cushion showed that a 2- x 4-in. stud traveling
at 60 miles per hour rarely penetrated
completely through after being shot at the
cushion by an air cannon. Despite excellent
performance under these conditions, ETFE
is not overly resistant to being cut, and it
is not recommended as vertical railing and
should not be used at street or pedestrian
levels, as it cannot prevent intrusion.
Additionally, any ETFE installation has the
potential for damage and, like any roof, will
need general ongoing maintenance. Access,
repair, and maintenance are not routine, due
to the unique characteristics and damage
risks of the ETFE system. Because of this,
the specialty design-build contractor will
usually sign on for an extended warranty/
maintenance period in order to perform this
work.
Design Process
ETFE structures are generally specified
as design-build projects or a subcontracted
portion of a design-build project (delegated
design) due to the unique characteristics of
the system and the need for highly specialized
and experienced designers. Throughout
the design-build process, coordination is
critical to the system’s overall aesthetics
and performance. In general, the basic
enclosure performance of the ETFE system
is much like a curtainwall glazing system.
Performance: Air and Liquid Moisture
From a building science perspective,
technical performance of materials, assemblies,
and systems is mainly concerned with
the control of four elements: 1) heat, 2) air,
3) moisture liquid, and 4) moisture vapor
(known within the building science community
as HAMM).
With today’s typical ETFE cushion and
rail systems, much like a curtainwall glazed
system, the main strategy for management
of liquid moisture and air is a pressure seal
created by the extruded cap plate and silicone
gasket placed between ETFE cushions.
In laboratory testing, as well as in-field testing,
this general assembly has proven to be
effective in creating an effective barrier for
air and water (moisture liquid). As with all
systems of this type, workmanship is paramount
for performance.
Performance: Heat
In general, the thermal performance of
an ETFE system can be simplified to be
approximately as good as assemblies used
in similar situations. A cushion system with
three layers of ETFE foil will achieve approximately
R-2.9, while a five-layer system will
achieve approximately R-4.8. This is similar
to a thermally broken, glazed system;
however, some key differences do exist. In
a typical glazed system, the lites remain
parallel and are accepted by the frame carrying
the same R-value to the perimeter of
the unit. In the ETFE system, the cushion is
most thermally efficient at the center of the
cushion and less thermally efficient closer
to the edge of the cushion. As the cushion
pinches into the extruded frame, the air
space between the foils becomes smaller
and smaller, until eventually, the separation
between exterior and interior is simply the
thickness of the number of foils included in
the cushion. Without the air between the
foils, the R-value for the system is minimal.
3-D thermal modeling was performed
on a generic ETFE system to better understand
the risk of interior condensation when
utilizing a three-foil cushion in northern
climates at various temperatures that could
be experienced across the United States
and Canada. (See Figure 6.) As noted, the
least thermally insulating part of an ETFE
assembly is near the cushion-to-frame
interface, which corresponds to the location
of highest condensation risk. As such, the
team chose to model a typical section cut
through a cushion-to-frame interface near
a corner. The modelled section covers the
intermediate mullion between ETFE cushions
and the transition to the traditional
single-ply roofing system.
Modeling was performed using the Nx
software package from Siemens, which is
a general-purpose computer-aided design
(CAD) and finite-element analysis (FEA)
package. The thermal solver and modeling
procedures used for this study
were extensively calibrated and validated
for ASHRAE Research Project 1365-RP,
“Thermal Performance of Building Envelope
Details for Mid- and High-Rise Construction
(1365-RP)” and guarded hotbox measurements.
The thermal analysis utilized steadystate
conditions and published thermal
properties of materials. Glazing air cavities
3 2 • I n t e r f a c e N o v e m b e r 2 0 1 6
Figure 6 – Cross section of rail and cushion showing thermal gradient.
and film coefficients were based on ISO
10077-2:2003 (E), “Thermal Performance of
Windows, Doors and Shutters – Calculation
of Thermal Transmittance – Part 2:
Numerical Method for Frames.” Boundary
conditions were modeled using heat transfer
coefficients for convection (i.e., film coefficients).
Radiation, to the interior and exterior,
was directly simulated using assumed
view factors and emissivity for the system.
The model was analyzed for four different
exterior temperatures: 32°F, 14°F, 0°F, and
-22°F (0°C, -10°C, -17.8°C, and -30°C). The
interior temperature modeled was 68.9°F
(20.5°C), which represents a conservative
air temperature. For all temperature conditions,
the exterior wind speed was modelled
at 15 mph and set to nighttime conditions.
This was taken as a “typical worst-case” set
of wintertime conditions that do not include
any influence from solar heating.
Performance: Moisture Vapor
The materials that are used in the ETFE
assembly are designated as Class I Vapor
Retarders—essentially impermeable to
vapor. Simply put, they have the potential to
drastically slow vapor movement, and if the
temperature of the
material reaches
the dew point,
condensation may
occur. To understand
the potential
for condensation
due to surface
temperatures, the
average steadystate
conductive
heat flow in three
dimensions was
analyzed.
It must be recognized
that the
objective of this
analysis was not
to predict in-service surface temperatures
subject to variable conditions and/or heating
systems. In-service surface temperatures
of glazing systems are highly dependent
on variable surface resistances and,
as such, will vary from system to system. In
contrast, the condensation risk was evaluated
by determining surface temperatures
subject to standard constant surface resistances
for steady-state conditions.
Interior surface temperatures at key
areas for evaluating the risk of condensation
are highlighted in Figure 7. These
areas include the coldest surface temperatures
and locations with the greatest risk
of condensation. The color isothermal plots
illustrate the variation of the temperature
viewed from the interior. T1 and T3 are
along the intermediate mullion section,
between two ETFE pillows, while T2 and
No v e m b e r 2 0 1 6 I n t e r f a c e • 3 3
Figure 7 – 3-D thermal model of three-foil ETFE cushion in a northern
climate at -22°F (-30°C).
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T4 are at the transition between the ETFE
and the metal deck roofing. T1 and T2 temperatures
were taken directly on the ETFE
pillow, 1 in. away from the framing.
Figure 8 summarizes the simulated surface
temperatures at key locations for varying
exterior temperatures that could be
experienced in multiple locations and indicates
the indoor relative humidity (RH) level
at which condensation may occur (“Max
Allowable RH%”). The table contains all the
temperature scenario conditions described
in the modelling procedures.
Morrison Hershfield (MH) found that in
northern climates across North America,
the likelihood of condensation occurring
exists with a generic three-foil ETFE cushion
system but will depend on not only what
the expected interior humidity and heating
conditions are but also convective or forced
airflow patterns. During winter conditions,
the humidity contribution from the exterior
air used for interior ventilation may be low
if there is not any additional moisture generation
(humidification or occupant load).
In this generic scenario, if the internal conditions
for the ETFE atrium space are at or
above 25% RH (depending on the expected
moisture generation from occupancy), there
may be some risks of interior frost formation
when the exterior temperatures are below
14°F (-10°C). The extent of this formation
will also depend on the length of time the
exterior temperatures remain below 14°F.
MH also found the adjacent assembly
can impact the ETFE system’s ability to
resist condensation, and this should be
addressed on a case-by-case basis. As
with many low R-value fenestration options,
there are several strategies that could be
further explored to reduce the risk of condensation,
including using additional insulation
to increase surface temperatures,
increasing airflow at the ETFE system level
using blowers, or using radiant heaters.
Wind, Rain, and Snow Management
As with any roof, a formal review of
wind loads or wind study specific to the
case roof’s geometries should be completed.
The strength of the ETFE system will be
designed and built appropriately to handle
design pressures, but the structural support
system must also be carefully designed.
In many cases, the support system has been
fine-tuned for weight consideration, and
this must be balanced with the given loads
for each area of the roof.
The roofing or cladding system
needs not only to deflect the rain
(and in some climate zones, snow),
but the runoff or accumulation
also needs to be managed. In typical
roofing scenarios, perimeter
drains or in-roof drains are often
used. However, in ETFE systems,
water drainage and management
strategies are often much more
complicated, as drains are unable
to be hidden within the structure,
and transitions to gutters can present challenges.
In the case of a northern climate, certain
complications exist with the formation of
ice and accumulation of snow on the roof
(Figure 9). While the cushions are pressurized
according to the assumed design
loads, in extreme snow and ice accumulation
events, many times the design weight
can overwhelm the cushion pressurization,
and localized cushion deflation can occur.
Due to this fact, the major manufacturers
of ETFE systems have incorporated snow
cables as a structural backup plan (Figure
10). These cables are designed into the system
based on load potential at any given
location, and are intended to support the
intact but partially deflated cushion if the
structural air within the system becomes
overwhelmed. In the event of such a partial
deflation, a second safety mechanism
begins to take effect. While snow and ice
can build up in a bowl-shaped, deflated
3 4 • I n t e r f a c e N o v e m b e r 2 0 1 6
Figure 8 – Simulated surface temperatures at key locations.
Figure 9 – Manual removal of snow from an
ETFE system. Photo credit: SurfaceDesign.
Figure 10 – An ETFE system in a deflated state, supported by snow cables, with snow
load. Photo by SurfaceDesign.
cushion, as the cushion deflates, it loses
its thickness, structural air, and insulating
value, and the interior heat of the building
will begin to melt the ice or snow. Once
melted, water will simply run off at the rim
or low end of the perimeter extrusion. In
this way, the system can react to localized
areas of ice and snow buildup by lowering
its R-value and melting the snow. The water
is then easily drained, or a thin film of water
beneath will better mobilize ice and snow
off the roof. However, a heated interior and
loss of heat are required for this mechanism
to be effective. Once the high loads are
relieved, the system automatically reinflates
any affected cushions, restoring the system’s
insulating value.
Project-Specific Detailing
and Holistic Process
In all cases, ETFE systems should be
considered custom design-build aspects of
the project. While the major manufacturers,
designers, and contractors are beginning to
understand efficiencies of reusing similar
rail and cushion systems, the ETFE only
serves as a portion of the enclosure; interfaces
with adjoining materials, assemblies,
and systems will always occur. To date,
this proves to be the biggest hurdle to overcome
when working with an ETFE system.
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. In the case of an aluminum
extrusion rail with a pressure cap, the
adjacent material may lend itself to be fed
directly under the cap plate of the system
(for example, a single-ply roof membrane).
In other cases, a more elaborate transition
assembly may need to be developed in order
to allow for movement and provide a traffic
walkway. These interface details need to be
fully coordinated with the complete team to
minimize the risk potential.
Project-specific detailing will need to
be approached in schematic design (SD),
design development (DD), in the production
of final contract documents (CD), and in
shop drawings. As early as the SD phase, a
formal wind study (and snow and ice, if in
a northern climate) should be completed to
understand the feasibility of ETFE roofing/
cladding on the project, as well as to help
guide project-specific detailing. Also in SD,
the functional performance of the proposed
ETFE assembly should be reviewed and
tested, if warranted. Often, laboratory testing
(air, water, wind, condensation, blast,
missile, etc.) of the proposed system has
been completed in the past, and the results
can be applied to a new project with review
of laboratory testing reports. In the case of
a custom fit, full assembly mock-ups should
be constructed in order to demonstrate
performance in laboratory testing. Ideally,
any issues or concerns defined at this stage
should allow ample time for adjustments
or a significant change to assemblies. By
the DD phase, the final ETFE assembly
selection should be finalized, as well as
the adjacent systems to which the ETFE
will interface. Once involved with CDs, it is
recommended that the design team—often
in conjunction with an enclosure consultant
familiar with ETFE—bring the project-
specific details to a final point, taking
into account all of the conditions where the
ETFE will interface with other assemblies.
When CD detailing is not coordinated, the
various trades’ shop drawings often differ
in means and methods, resulting in incompatible
connections and/or scope gaps.
In the absence of good detailing ahead of
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time, coordination efforts increase dramatically as construction
gets underway, resulting in requests for information
(RFIs) and change orders that could have been avoided.
Once the coordination of the ETFE system is as complete
as possible, it is recommended that on-site visual and performance
standalone mock-ups be constructed that incorporate
the interface conditions (already-completed laboratory
mock-ups may suffice). These mock-ups help to further
define coordination and sequencing issues, as this oftenunfamiliar
system is tied together with more typical systems.
On-site mock-ups should include the exact materials that
will be installed in the field, and should be installed by the same subcontractor personnel who will
be working on the project. Performance testing should also be completed at this stage, primarily
focused on air leakage and water penetration. To better permit testing and detailed visual review,
standalone mock-ups are recommended instead of in-situ mock-ups, as access is usually very limited
in the locations ETFE systems are installed.
Lastly, it is highly recommended that a careful quality control (QC) program be implemented
by the design-build contractor and the third-party enclosure consultant while the ETFE assembly
is installed. Well-implemented and understood QC programs have been shown to have substantial
positive impacts on workmanship quality.
3 6 • I n t e r f a c e N o v e m b e r 2 0 1 6
Figure 11 – ETFE
roof spanning the
playing field.
Figure 12 –
A blend of
synthetic and
natural turf.
CASE STUDIES
Forsyth Barr Stadium
Known locally as the “Glass House,” the giant roof of the Forsyth Barr
Stadium in Dunedin, New Zealand, is the world’s first stadium with a nonoperable
roof to boast a stationary blended natural and synthetic turf (Figure 11). This is only
possible due to the high transparency of ETFE in all light wavelengths used for photosynthesis.
A total of 220,660 sq. ft. of transparent ETFE cover the field area. Supporting the nearly 300
double-layered cushions are five external arch trusses that span 345 ft. from the tops of stadium seats.
Though the roof system has an internal clearance of 121 ft. and a maximum height of 154 ft., its light weight
makes it possible to be supported by only 33-ft.-tall external arch trusses. Between each of the arches is a series
of flat trusses that support four long, inflated ETFE cushions. Although the turf is buffered with a synthetic component
to assist with the heavy impact traffic, it is still highly sought
after for its natural feel and play (Figure 12).
ETFE: Vector Foiltec
Architect: Populous – Jasmax
Owner: Carisbrook Stadium Trust
Engineer: Grayson Engineering Ltd.
General Contractor: Hawkins Construction
Completion Date: 2011
Center Parcs
Center Parcs, in Vienne, France, is a waterpark that
is open to the public all year long, thanks to its ETFE roof
(Figure 13). The ETFE roof covers 63,000 sq. ft., with threefoil
ETFE cushions. This project emphasizes the unparalleled
design flexibility with its unique shapes and artistic form
(Figures 14 and 15). Additionally, the occupants, including
the plant life, are treated to the full spectrum of needed light
with the ETFE being permeable to both natural light and UV
rays (Figure 16).
No v e m b e r 2 0 1 6 I n t e r f a c e • 3 7
Figure 13 – Indoor aquatic park. Photo by Birdair.
Figure 14 – ETFE roof system from exterior. Photo by Birdair.
U.S. Bank Stadium
The new U.S. Bank Stadium in
Minneapolis, Minnesota, is one of the first
major sporting venues in the United States
to incorporate ETFE (Figure 17). The roof
of the new home for the
NFL’s Minnesota Vikings is
fixed, but 60% of it was constructed using
240,000 sq. ft. of ETFE. The ETFE roof
is comprised of 75 cushions, the longest
measuring over 300 ft.
in length (Figures 18 and
19). To date, U.S. Bank
Stadium is the largest
ETFE installation in North America and is
the only stadium in the nation with a clear
ETFE roof. However, the decision was still
made to proceed with an artificial field turf
instead of a natural one, due to the multiple
event venues that the stadium will host.
Because of the slopes of the roof, ETFE
3 8 • I n t e r f a c e N o v e m b e r 2 0 1 6
Figure 16 – ETFE roof system from interior.
Photo by Birdair.
Figure 15 – Installation of ETFE
cushion. Photo by Birdair.
Figure 17 – Stadium at night with interior lighting coloring the
ETFE and curtainwall assemblies. Photo by www.Vikings.com
Figure 18 – View of ETFE roof
system (left) and single-ply
membrane (right).
Figure 19 – Installation of
ETFE cushions.
material on the south side makes up
60% of the entire roof, while a traditional
single-ply roof over metal decking
accounts for the remaining 40% on
the north side where the solar exposure
benefits are reduced (Figure 20).
Though the ETFE doesn’t cover the
entire field, the angle of the roof allows
sunlight over its entirety (Figure 21).
Even on a cloudy day, the interior of
the stadium is well lit without the assistance
of artificial lighting. Rain and
snow are managed with a sloped roof
strategy that empties into a large snow
gutter system encircling the building.
In the case of snow, this gutter has an
ice/snow melt system that will melt and
drain it away. Having already been in
place through one winter season while
under construction, the ETFE roof was
observed to shed snow better than the
single-ply membrane roof on the opposing
slope.
ETFE: Vector Foiltec
Architect: HKS
Owner: Minnesota Sports Authority
Owners Rep: Hammes Co.
Engineer: Thornton Tomasetti
General Contractor:
Mortenson – Thor JV
Completion Date: 2016
Mercedes Benz Stadium
The NFL Atlanta Falcons will enjoy
ETFE accents on their new $1.8 billon
stadium, slated for completion in 2017
No v e m b e r 2 0 1 6 I n t e r f a c e • 3 9
Figure 20 – Installation of ETFE cushions.
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(Figures 22 and 23).
The stadium’s operable
roof consists of
three-layered ETFE
cushions on eight
“petals” that retract
radially, similar to
a camera’s aperture
diaphragm. Additionally, the façade will
feature a single-foil ETFE skin supported
by a cable net system. The design requires
approximately 135,000 sq. ft. of triplelayered
ETFE pillows with an air inflation
system for the roof, and approximately
165,000 sq. ft. of vertical, single-layered
ETFE film and cable net for the vertical
portion. Even when closed, the combination
of the ETFE roof and wall areas will create
an outdoor feel, allowing in natural sunlight
when the weather is clear, and protecting
players and fans during inclement weather.
In addition to the transparency benefits,
ETFE was a good match for the operable
mechanisms employed in the center
portion of the roof as opposed to a glazed
assembly. It became clear during the design
process that the stresses of operational
movement posed a risk of broken glazed
units and negative impacts on the seals if
the team went with a traditional
glazed system. With
the lightweight and flexible
nature of the ETFE petals,
the Atlanta Falcons’ operable
roof is expected to be an
engineering marvel.
ETFE: Birdair
Architect: HOK & tvsdesign
Owner: AMB Sports & Entertainment
Owners Rep: Darden and Company
Engineer: Buro Happold
General Contractor: Holder/Hunt/
Russell/Moody – a joint venture
Completion Date: 2017
Detroit Entertainment and Event Center
The main roof area of the new Detroit
Entertainment and Event Center, home to
the NHL’s Red Wings, and currently under
construction (Figure 24), will be built using
traditional low-sloped roof assemblies. The
street-like walkway between the arena and
surrounding structures, however, referred
to as the “Via,” will be covered with a threefoil
ETFE cushion system (Figure 25). This
lightweight and translucent structure will
provide a cost-effective and aesthetically
pleasing enclosure for a space that is
designed to feel like an outdoor street yearround.
Even as visitors enter from the exterior
and through the perimeter building into
the Via, they will be able to look up and see
the impressive arena rising overhead while
in the comfort of an indoor environment.
4 0 • I n t e r f a c e N o v e m b e r 2 0 1 6
Figure 22 – Rendering of the Mercedes Benz
Stadium showing ETFE roof system and ETFE
cladding system. Photo by HOK.
Figure 23 – Current construction of stadium. Photo by
www.11Alive.com.
4 2 • I n t e r f a c e N o v e m b e r 2 0 1 6
ETFE: Vector Foiltec
Architect: HOK
Owner: The District
Detroit
Owners Rep: Hines
Engineer: Magnusson
Klemencic Associates
General Contractor:
Barton Malow – Hunt
– White JV
Completion Date: 2017
CON CLUSION
ETFE has made an
impressive impact around the
world and recently in the North American
building community, and with some of the
pinnacle structures of this generation being
crowned by the unique material, its popularity
is sure to grow. The attributes of being
lightweight, durable, and transparent set
ETFE apart from other industry-standard
materials and assemblies. Add to that the
aesthetic factor of providing a near invisible
separation from the exterior, letting in
almost uninhibited natural light, and ETFE
becomes a very attractive solution.
As with all systems, there remain potential
risks. The potential for condensation
under certain interior conditioning loads
in northern climates needs to be carefully
analyzed and addressed to minimize risk.
Interfaces with surrounding materials and
systems must be carefully designed and
diligently coordinated during construction
to ensure total enclosure performance. The
unique characteristics of this material and
the assemblies in which it is used are a
clear example of the importance of employing
a holistic process of design, construction,
and performance testing to ensure
success.
Lee Durston is a
senior building
science consultant
with Morrison
Hershfield’s St.
Paul, Minnesota,
office with over
16 years of building
science experience.
He holds a
BS in microbiology
from North Park
University and is a
member of the National Institute of Building
Sciences and the Air Barrier Association
of America. Durston holds a certification in
Building Science Thermography (CBST). His
current work includes many prominent ETFE
installations, including the largest installation
of ETFE in North America.
Lee Durston
Shawn Robinson, a
department manager
and senior building
science consultant
at Morrison
H e r s h f i e l d ’ s
Atlanta office, has
over ten years’
experience with a
variety of project
types, including
sporting venues,
high-rise, military/
government, higher education, data centers,
hospitality, and medical facilities. Robinson’s
experience includes project management,
building envelope design, field review assignments,
and condition assessments. Robinson
recently received certification as a Building
Envelope Commissioning Process Provider
(BECxP).
Shawn Robinson
Figure 25 – Current construction of
arena and surrounding structures.
Figure 24 – Rendering of the Detroit
Entertainment and Event Center.
Photo by the District Detroit/HOK.
The U.S. House of Representatives voted September 29 to stall the Labor Department’s overtime rule that would increase wages for an
estimated 4.2 million Americans, set to take effect December 1. The House vote comes on the heels of a lawsuit by 21 states against the
Obama administration, claiming the overtime rule would place a heavy burden on state budgets and force layoffs. The Associated Builders
and Contractors (ABC), along with a coalition of other groups, also filed a federal lawsuit seeking to overturn the rule.
Labor Secretary Thomas Perez has claimed the legal challenges are attempts to deprive workers of fair pay. “The same interests that
have stood in the way of middle-class Americans getting paid when they work extra are continuing their obstructionist tactics,” he told
Bloomberg. President Barack Obama has said he would veto the measure, which passed the House 246 to 177, if it was presented to him.
Overtime protections require employers to pay one-and-a-half times an employee’s regular rate of pay for any work past 40 hours a
week. The final rule raises the salary threshold for overtime eligibility from $455 per week ($23,660 per year) to $913 per week ($47,476
per year). The new rule also updates the total annual compensation level above which workers are ineligible for overtime from the current
level of $100,000 per year to $134,004 per year. The salary thresholds will automatically update every three years under the new rule.
Overtime W age I ncrease Fought