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Technology
Evaluating
Heat-
Glass for Use in Glass-Enclosed Elevators
by Bruce Lang
Though elevators amount to only a small percentage of a structure’s total floor space, glass-intensive
elevators on a building’s exterior are often dramatic focal points in an office complex or hotel, attracting
the attention and engaging the interest of visitors and building occupants alike. Therefore, it is important
to ensure that elevator passengers enjoy a positive experience with comfortable cabin temperatures
throughout the year, along with a clear exterior view.
Unfortunately, sun exposure can cause glass-enclosed elevators to overheat, leading to passenger discomfort that can negatively impact guest occupancy, tenant retention and value of leased office and retail
space. To avoid such an outcome, prudent decision makers should exercise due diligence in evaluating and
selecting the optimum glass for glass-intensive elevators in either
new construction or as part of a major building renovation.
The cooling requirements for an external, glass-enclosed
Overheating of Existing
elevator are driven by the amount of solar energy enterGlass-Intensive Elevators
ing through the glass. Even glass elevators operating
For existing elevators experiencing problems
inside a building’s interior are often subjected to excesfrom heat through glass, the most expensive option
sive solar energy transmitted through multistory glass
is to replace existing glass and frames with new
lobbies and atria. Stopping heat “at the glass” is the most
components designed to block heat and deal with
effective means of maintaining a comfortable cabin temenergy performance needs. Keeping existing frames
perature, downsizing or even eliminating an elevator’s
and replacing only the glass is less expensive. In eiair-conditioning (AC) system and reducing AC operating
ther case, building owners may understandably be
cost.
reluctant to replace existing elevator glass, the perChoosing the Right Glass
formance of which is generally adequate (though
Choosing the right glass for an elevator application
not optimum) in the case of blocking unwanted
can be a challenge. Some glass may reflect unwanted
heat. For all existing glass elevators, heat-reflecting
heat, but in doing so, reduces desired natural light and
applied window film is the least expensive and preoccupant visibility. To avoid the wrong glass choice, sevferred solution to mitigate the impact of too much
eral significant factors – especially the safety and security
solar heat entering elevator glass.
of elevator passengers – for glass-elevator design should
For more information on applied films, see “Overbe reviewed when making the choice. Building codes,
heating of Glass-Enclosed Elevators” (ELEVATOR
insurance requirements and good sense dictate that temWORLD, February 2005). For more information on
pered or laminated safety glass be used in any glassapplied security film, see “Security Window Film
intensive elevator installation.
Can Enhance Safety in Glass-Enclosed Elevators”
Tempered glass is four to five times stronger than an(ELEVATOR WORLD, February 2007). Both articles
nealed glass and is designed to break into small, harmare by Marty Watts, President and CEO of V-Kool,
less pieces when shattered. While eliminating the risk of
Inc. in Houston, Texas.
being cut by sharp fragments, such shattering compromises
occupant safety by leaving a large opening through which
an occupant can fall.
Continued
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Evaluating
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Continued
Laminated safety glass addresses this concern by
bonding a polyvinyl butyral (PVB) plastic interlayer between two pieces of glass. The interlayer is designed to
flex but not break, providing greater resistance to impact
and maintaining an impenetrable, protective barrier for
elevator occupants.
For maximum occupant safety, laminated glass should
be the foundation upon which other glass options are
Highly reflective glass (courtesy of Sydneywebcam.smugmug.com)
added when designing a glass-intensive elevator. Second only to safety is the need to maintain a comfortable temperature
without sacrificing visible light or occupant view (the key reasons why
a glass elevator was chosen in the first place). Fortunately, there are
several types of heat-blocking glass available that can be used in a
glass-enclosed elevator design. However, many consist of glass that is
either darkly tinted or highly reflective (mirrored).
While somewhat effective at blocking heat, mirrored and tinted
glass not only produce undesirable aesthetics, but also make it difficult
to see inside or outside of the elevator, particularly at night. This can
present a safety and security issue in the event of an emergency by reducing the ability of onsite security personnel and first responders to
clearly identify elevator passengers.
Heat-Reflective Coatings
Tempered glass after impact (courtesy of Tintworks.net)
48
In addition to requiring a laminated interlayer for safety, effective
glass-enclosed elevator designs also require glass that maintains a
cool cabin, while transmitting optimal amounts of visible light. But
how does clear, colorless glass reflect the sun’s heat and reduce AC requirements?
Fortunately, advances in glass technology have made clear, heatblocking glass possible. Transparent low-emissivity (low-e) coatings
are tuned to transmit or reflect specific wavelengths of light. These
coatings simultaneously transmit desirable visible light, while reflecting
unwanted infrared heat and reducing the transfer of heat through the
glass. “Emissivity” is defined as the ability of a surface to radiate energy.
Low-e coatings are designed specifically to reject the sun’s heat and
damaging solar rays. The solar-heat gain coefficient (SHGC) is the key
measure of solar-heat reflection performance. The lower the SHGC, the
better the glass reflects solar energy. Equally important is visible light
transmission (Tvis) and, more specifically, the tradeoff between SHGC
and Tvis.
While low-e coatings have seen dramatic performance improvements
over the past decade, it’s important to note that these heat-reflective
coatings do not always work well on laminated glass. Such advanced
coatings are quite delicate and designed to be protected by the air cavity
in an insulating-glass construction. Consequently, they were not designed
to withstand the high temperatures and pressures used in the glass
lamination process, and high-performance, low-e laminated glass can
be difficult and expensive to manufacture. This effect is exacerbated in
curved laminated glass, which places more strain on the coatings and
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Installations of Heat Mirror
Suspended-Film Insulating Glass
284 Barracks Fort Wainwright (Fairbanks, Alaska)
One hundred and fifty windows containing Argon gas and equipped with over 3,000 square feet of suspended-film insulating glass were installed in this U.S. Army facility in 2008.
Museum of Flight (Seattle)
This six-story glass-and-steel exhibit hall located adjacent to historic Boeing Field houses 30 full-sized
aircraft, many of which are visible through 90,000 square feet of suspended-film insulating glass. Glass selection played an important role in the building’s compliance with stringent energy code requirements.
Mount Rushmore Visitor Center (South Dakota)
Seeking to preserve the majesty of what has been called the greatest mountain carving in the world, the
National Park Service selected 8,200 square feet of suspended-film insulating glass to provide maximum
insulation in South Dakota’s harsh winter. The technology’s limited light reflectivity optimizes visitor viewing, especially at night when the mountain is illuminated.
Hyatt Regency (Cambridge, Massachusetts)
Turning the top floor of a parking garage into a glass-enclosed swimming pool for guests and swim-club
members demanded a glass that would tame the chill of New England winters. The choice: 4,500 square
feet of suspended-film insulating glass.
Princess Elisabeth Research Station (Utsteinen, Antarctica)
Open for research duties between November and February, the International Polar Foundation facility is
home to as many as 20 researchers, whose comfort, well-being, and productivity is enhanced by daylight
via 35 fixed suspended-film insulating glass units. As the first zero-emissions polar research station, it is
20 times more energy efficient than typical facilities in the Antarctic.
makes low-e-coated glass cost prohibitive for curved-glass applications.
Such applications include dramatic designs that architects are likely to
select for glass-intensive elevators.
Despite production challenges, application limitations and increased cost, lower-performing coatings are still commonly used to
create heat-reflective laminated glass with an SHGC rating of about
0.59. While it’s true that this level of heat rejection is much better than
that of uncoated glass (with an SHGC of 0.8), there is a much superior,
“lamination-friendly” technology ideal for glass-enclosed elevator design.
XIR®
Exterior
Interior
Heat-Reflective Film
Fortunately, there is a superior alternative to coated monolithic
glass that provides higher heat rejection, more desirable aesthetics and
the ability to work well in curved-glass applications. Instead of laminating
a lite of heat-reflective coated glass, as is the case with conventional
monolithic glass options, this alternative uses an infrared (IR) reflective
coated film, such as XIR film from Southwall Technologies. XIR film
was developed specifically for laminated glass, so it’s “process-friendly”
and can integrate into a glass manufacturer’s standard manufacturing
process. In a laminated-film-based construction, the XIR film is sandwiched between two PVB interlayers that are then bonded into a
monolithic glass unit.
Continued
PVB Interlayer
Laminated glass construction (courtesy of
Guardian Sunguard Glass)
XIR Film
Heat-reflective-coated XIR film sandwiched inside laminated monolithic glass (source: Southwall Technologies, Inc.)
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Evaluating
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In this configuration, the heat-reflective coating does not reside on the glass
itself, but on the film interlayer. Such a construction not only works better in
curved-glass designs, but with an SHGC rating of 0.41, the coated film alternative is 30% more effective at blocking heat than conventional glass-coated
monolithic options. In addition, laminated glass incorporating coated film exhibits
a more neutral color (less color variation) when viewed at an angle than conventional monolithic glass. Monolithic laminated glass heat rejection performance is
as follows:
SHGC
Tvis
Clear laminated glass
0.80
89%
Low-e-coated laminated glass
0.59
78%
Low-e-coated film laminated glass
0.41
72%
In summary, for glass elevators with extensive sun exposure in which maintaining a comfortable cabin and lowering AC operating costs are the primary
concerns, laminated monolithic glass consisting of heat-reflective XIR film offers
optimum performance and maximum design options.
When Monolithic Glass Won’t Do
Insulating glass (source: Efficient Windows
Collaborative)
50
In climates in which it is necessary to reflect solar energy and insulate
against heat loss, even the most heat-reflective monolithic glass will not keep
elevator cabin temperatures warm on cold nights or during the winter. The
ability for glass to impede heat flow and insulate against heat loss is a function
of reducing heat transfer due to conduction and convection.
Reducing conduction and convection is primarily achieved by introducing
an air space between two pieces of glass. Though patented in 1865, insulating
glass – two panes of glass separated by a sealed air space – was not widely
adopted until the middle of the 20th century. The sealed air space is critical to
reducing heat transfer and insulating against heat loss. In addition, inert
gasses such as argon or krypton can be used to fill the airspace to further impede heat transfer. Insulating performance is measured as U-value. The lower
the U-value, the better the glass insulates.
Monolithic glass, for all its strength and ability to incorporate heat-reflective
coatings, contains no air spaces and, therefore, does not have the ability to
effectively insulate. However, insulating glass equipped with low-e coatings
can both reject the sun’s heat in summer and prevent heat from escaping in the
winter. Generic low-e insulating glass, consisting of a piece of clear glass and
a piece of low-e-coated glass separated by a sealed cavity filled with krypton,
can achieve a U-value and SHGC as low as 0.25 and 0.27, respectively.
Insulating glass can provide heat-reflective performance superior to that of
the best heat-reflective laminated monolithic glass option, while also providing a much more robust level of insulating performance. If used in a glassintensive elevator, both reduced AC requirements and warm cabin wintertime
temperatures can be achieved.
Unfortunately, in those cold climates in which monolithic glass is not acceptable for external elevator applications, insulating glass may also present
challenges to optimum performance. Of primary concern is the additional
weight of insulating glass, which becomes heavier when constructed with a
lite of laminated safety glass in an elevator application. In addition to weight,
conventional low-e insulating glass, when used in a sloped or horizontal
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Impact of Glass Slope on Insulating Glass Performance: Low-E reference example & SC 75 film
Product
Glass
Configuration
Gas
Fill
Glass Thickness
IP
SI
Outside
Dimension
U-Factor Performance
IP Units
SI Units
Solar Gain Performance
SC
SHGC
Tsol
Visible Performance
Tvis Rvis Ext. Rvis Int.
Tuv
LowE reference glass (Viracon)
VE12M
LowE/Clear
Air
1/4”
6mm
1in
0.29
1.65
0.44
0.38
0.33
70.00
0.11
0.12
0.100
VE12M
LowE/Clear
20 degree slope
Air
1/4”
6mm
1in
0.42
-0.13
2.37
-45%
0.44
0.38
0.33
70.00
0.11
0.12
0.100
6mm
1in
0.24
1.39
0.43
0.37
0.33
70.00
0.11
0.12
0.100
6mm
1in
0.35
-0.11
2.01
-46%
0.43
0.37
0.33
70.00
0.11
0.12
0.100
0.30
1.70
0.41
0.36
0.28
0.61
0.23
0.21
0.010
0.42
0.36
0.28
0.61
0.23
0.21
0.010
VE12M
LowE/Clear
Argon
VE12M
LowE/Clear
20 degree slope
Argon
1/4”
1/4”
Representative Heat Mirror Insulating Glass (SC 75)
HM SC76
Clear/Clear
Air
1/4”
6mm
1in
HM SC76
Clear/Clear
Air
1/4”
6mm
1in
20 degree slope
HM SC76
0.31
1.74
-0.01
-3%
Clear/Clear
Argon
1/4”
6mm
1in
0.24
1.39
0.41
0.35
0.28
0.61
0.23
0.21
0.010
HM SC76
Clear/Clear
20 degree slope
Argon
1/4”
6mm
1in
0.26
-0.01
1.41
-4%
0.41
0.35
0.28
0.61
0.23
0.21
0.010
Sloped Table Notes:
Italicized values designate performance of glass unit at a standard NFRC 20 degree slope.
Performance values for other Heat Mirror film units reflect similar performance and are available upon request.
General Table Notes:
All performance data is calculated using Window 5.2, developed by Lawrence Berkeley Laboratories, with funding from the U.S. Dept. of Energy, copyright, Regents of the University of California,
California. All simulations run utilizing standard NFRC environmental conditions.
Spectral data for clear and tinted glass is based upon PPG spectral data for clear, Solarbronze®, Solargray®, Azuria®, Solargreen® as appropriate. Low-e spectral data based upon Guardian
7138. Performance values may vary slightly for Heat Mirror units constructed with glass from different glass manufacturers. IP=Imperial unit, SI=System International (Metric)
For performance values with glass products not shown in these tables, please contact the Southwall Product Services Department at (800) 366-8794, or a SouthWall Technologies authorized
Heat Mirror film insulating glass fabricator. Heat Mirror film is a registered trademark of Southwall Technologies, 3799 Fabian Way, Palo Alto, CA 94303, (650) 962-9111.
Product Designations:
HM=Heat Mirror, a single film insulating glass unit, with two cavities and one suspended film. HM 88 has two cavities and one suspended HM 88 film.
88, 77, 66 - Heat Mirror film types. Numbering is based upon the percentage of light transmitted through the coated film, not the finished IGU - 88 film has = 88% light transmittance.
TC88 - Twin coat - TC88 film has coatings on both sides of the film. 88 refers to the type of coating on each side of the film.
SC76 Solar Control - SC76 has different combinations of cooling layers than other HM films. 76 refers to the light transmittance through the coated film, not the finished IGU.
SGQ - Superglass Quad - A double film insulating glass unit with three cavities and two suspended films. SGQ TC88 - has two suspended TC 88 films.
HPR - High performance Reflective - A single film insulating glass unit with two cavities and one highly reflective Southwall film
The insulating performance of low-e glass versus Heat Mirror suspended-film insulating glass in sloped-glass applications
orientation typical in glass elevators, can lose up to 40% of its ability to
insulate against convective heat transfer – a direct result of shortening
the convective airflow paths when not mounted vertically. To address
this sloped glazing performance limitation, insulating glass can incorporate a third lite of glass to create a second sealed airspace. However,
such triple-paned configurations can be prohibitive for an elevator
application with a lift system that is likely not designed to support the
additional weight.
Suspended Film Insulating Glass
While at first glance it may seem that glass-intensive elevator operation in cold climates may be problematic due to the limitations of both
laminated monolithic glass and generic low-e insulating glass, a better
insulating glass alternative does exist. This glass consists of suspending a low-e and solar-heat-reflective Heat Mirror film mounted inside
Continued
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of an insulating glass unit. One or more Heat Mirror films can create
two, three or even four insulating cavities inside the sealed airspace
without adding weight.
Adding a heat-impeding gas to the internal cavities can achieve
insulating performance ranging from a 0.16 to 0.05 U-value, the equivalent of an insulated wall. These statistics make it no wonder that suspended-film insulating glass was chosen for use in the newly opened
Princess Elisabeth Research Station in Utsteinen, Antarctica, where
winter temperatures routinely fall as low as -80°F. Note that because of
the multiple air cavities inside the insulating glass, the entire glass unit
performs equally well, whether its orientation is vertical, sloped or
horizontal, eliminating the performance problem of conventional
low-e-coated glass options in non-vertical applications.
Such internally mounted low-e-coated films do not replace low-e
glass. Rather, they leverage the benefits of film-based coatings and
glass-based coatings to create a lightweight, multi-cavity insulating
glass that offers a new level of performance. Most insulating glass
incorporating suspended film utilizes low-e coated glass to minimize
solar heat while using coated film to maximize insulation performance.
Here’s a look at the U-values of the insulating glass referenced above
(R-value = 1/U-value and is included for reference):
R-value
U-value
Low-e-coated
insulating glass (IG)
0.25
4
Low-e coated IG (triple pane,
dual-cavity, krypton)
0.11
9
Low-e-coated suspended-film IG
(one film, dual-cavity, krypton)
0.10
10
Low-e-coated suspended-film IG
(two films, tri-cavity, krypton)
0.08
13
Low-e-coated suspended-film IG
0.05
20
(three films, quad-cavity, xenon)
Dual-cavity, tri-cavity and quad-cavity Heat Mirror suspended-film insulting glass (source: Southwall Technologies, Inc.)
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Performance at the edge of the insulating glass unit (where insulation is
traditionally least effective as compared to the center of the glass) is improved
by using thermally insulated spacer materials to separate the glass, often referred
to as “warm edge” construction. In addition to superior insulating performance, suspended-film insulating glass blocks ultraviolet radiation, reduces
noise and increases occupant comfort more effectively than low-e glass
alone. To comply with all safety codes, suspended-film insulating glass can
be used in conjunction with tempered or laminated glass.
Frames
Glass exists within frames designed to be attached to the exterior construction of glass-enclosed elevators. Consideration must be paid to both
the strength and the energy conservation performance of the frames. It
makes no sense to select energy-conserving glass if that glass is used in
metal frames that do not provide adequate insulation.
Ideal frame choices for use in elevators include thermally broken aluminum
in which the interior of the aluminum frame consists of a non-conducting
material such as plastic or rubber. Vinyl and fiberglass frames eliminate the
risk of plastic or rubber shrinkage due to extreme exterior frame surface
temperatures. Such heat-induced shrinkage can result in excessive water leaks into the elevator cabin during normal
precipitation. Both vinyl and fiberglass are excellent insuInstallations of XIR Laminated Glass
lators, and fiberglass provides the strength and durability
Multi-Building Sports Complex (Shenzhen, China)
necessary to accommodate all glass suitable for elevator
Over 400,000 square feet of Southwall’s XIR glass
use.
is being installed in a sports stadium, swimming
Summing It Up
Heat-reflective-coated film, whether used as an interlayer in laminated monolithic glass or as suspended-film
insulating glass, offers increased energy conservation
options never before available. Outstanding solar-heatreflection and insulating performance in a variety of
glass configurations allows designers of glass-intensive
elevators to specifically address:
◆ Unique requirements of the elevator to be designed
◆ Geographic orientation
◆ Location within the building
◆ Performance to match the available design budget
complex and indoor arena, to be the site of the
World University Games in August 2011. This is one
of the largest XIR glass installations in the world.
Magnocentro26, Shopping Mall (Mexico City)
Nearly 35,000 square feet of XIR glass allowed for
downsizing of the heating, ventilation and air-conditioning system in this 246,000-square-foot mall.
Sahara Centre (Dubai)
This multilevel, 500,000-square-foot shopping
mall used XIR glass to create open, light-intensive
ambience for over 150 retail shops and 34 fast-food
and fine-dining establishments.
Historic Railroad Station (Strasbourg, France)
Bruce Lang is vice president of Marketing & Business
Development at Southwall Technologies, Inc. in Palo Alto,
California. He can be reached at blang@southwall
.com.
Sixy-four thousand square feet of XIR glass was
used in a transparent pedestrian foyer designed to
facilitate access to high-speed trains, trams, the underground Metro, buses and taxis, while maintaining the aesthetics of the original train station.
Florida Aquarium (Tampa, Florida)
This massive glass structure designed for high
light transmission and effective heat reflection used
XIR glass throughout.
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