Glass for Use in Glass-Enclosed Elevators

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 dis-

comfort 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 enter- Glass-Intensive Elevators
ing through the glass. Even glass elevators operating

inside a building’s interior are often subjected to exces- For existing elevators experiencing problems

sive solar energy transmitted through multistory glass from heat through glass, the most expensive option

lobbies and atria. Stopping heat “at the glass” is the most is to replace existing glass and frames with new

effective means of maintaining a comfortable cabin tem- components designed to block heat and deal with

perature, downsizing or even eliminating an elevator’s energy performance needs. Keeping existing frames

air-conditioning (AC) system and reducing AC operating and replacing only the glass is less expensive. In ei-

cost. ther case, building owners may understandably be

Choosing the Right Glass reluctant to replace existing elevator glass, the per-

Choosing the right glass for an elevator application formance of which is generally adequate (though

can be a challenge. Some glass may reflect unwanted not optimum) in the case of blocking unwanted

heat, but in doing so, reduces desired natural light and heat. For all existing glass elevators, heat-reflecting

occupant visibility. To avoid the wrong glass choice, sev- applied window film is the least expensive and pre-

eral significant factors – especially the safety and security ferred solution to mitigate the impact of too much

of elevator passengers – for glass-elevator design should solar heat entering elevator glass.

be reviewed when making the choice. Building codes, For more information on applied films, see “Over-

insurance requirements and good sense dictate that tem- heating of Glass-Enclosed Elevators” (ELEVATOR

pered or laminated safety glass be used in any glass- WORLD, February 2005). For more information on

intensive elevator installation. applied security film, see “Security Window Film

Tempered glass is four to five times stronger than an- Can Enhance Safety in Glass-Enclosed Elevators”

nealed glass and is designed to break into small, harm- (ELEVATOR WORLD, February 2007). Both articles

less pieces when shattered. While eliminating the risk of are by Marty Watts, President and CEO of V-Kool,

being cut by sharp fragments, such shattering compromises Inc. in Houston, Texas.

occupant safety by leaving a large opening through which

an occupant can fall. Continued

December 2009 | ELEVATOR WORLD | 47

Evaluating

Heat-

Glass for Use in Glass-Enclosed Elevators

Continued

Laminated safety glass addresses this concern by

bonding a polyvinyl butyral (PVB) plastic interlayer be-

tween 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

Highly reflective glass (courtesy of Sydneywebcam.smugmug.com) be the foundation upon which other glass options are
added when designing a glass-intensive elevator. Sec-

ond 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 re-

ducing the ability of onsite security personnel and first responders to

clearly identify elevator passengers.

Heat-Reflective Coatings

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 re-

quirements?

Fortunately, advances in glass technology have made clear, heat-

blocking 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

Tempered glass after impact (courtesy of Tint- be difficult and expensive to manufacture. This effect is exacerbated in
works.net) curved laminated glass, which places more strain on the coatings and

48 | WWW.ELEVATOR-WORLD.COM | December 2009

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 sus-
pended-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 se-
lection 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 view-
ing, 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 in-

creased 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, Exterior

“lamination-friendly” technology ideal for glass-enclosed elevator design. Interior
XIR® Heat-Reflective Film

Fortunately, there is a superior alternative to coated monolithic PVB Interlayer

glass that provides higher heat rejection, more desirable aesthetics and Laminated glass construction (courtesy of
Guardian Sunguard Glass)
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 sand-

wiched between two PVB interlayers that are then bonded into a

monolithic glass unit. Continued

XIR Film

Heat-reflective-coated XIR film sandwiched inside laminated monolithic glass (source: Southwall Technologies, Inc.)

December 2009 | ELEVATOR WORLD | 49

Evaluating

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Glass for Use in Glass-Enclosed Elevators

Continued

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 alterna-

tive 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 conven-

tional 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 main-

taining 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

In climates in which it is necessary to reflect solar energy and insulate
Insulating glass (source: Efficient Windows against heat loss, even the most heat-reflective monolithic glass will not keep

Collaborative)

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 im-

pede 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 provid-

ing a much more robust level of insulating performance. If used in a glass-

intensive elevator, both reduced AC requirements and warm cabin wintertime

temperatures can be achieved.

Unfortunately, in those cold climates in which monolithic glass is not ac-

ceptable 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

50 | WWW.ELEVATOR-WORLD.COM | December 2009

Impact of Glass Slope on Insulating Glass Performance: Low-E reference example & SC 75 film

Glass Gas Glass Thickness Outside U-Factor Performance Solar Gain Performance Visible Performance
Dimension IP Units SI Units SC SHGC Tsol Tvis Rvis Ext. Rvis Int. Tuv
Product Configuration Fill IP SI
1in
LowE reference glass (Viracon) 1in

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
0.44 0.38 0.33 70.00 0.11 0.12 0.100
VE12M LowE/Clear Air 1/4” 6mm 0.42 2.37
20 degree slope -0.13 -45% 0.43 0.37 0.33

VE12M LowE/Clear Argon 1/4” 6mm 0.24 1.39 70.00 0.11 0.12 0.100

VE12M LowE/Clear Argon 1/4” 6mm 1in 0.35 2.01 0.43 0.37 0.33 70.00 0.11 0.12 0.100
20 degree slope -0.11 -46%

Representative Heat Mirror Insulating Glass (SC 75)

HM SC76 Clear/Clear Air 1/4” 6mm 1in 0.30 1.70 0.41 0.36 0.28 0.61 0.23 0.21 0.010

HM SC76 Clear/Clear Air 1/4” 6mm 1in 0.31 1.74 0.42 0.36 0.28 0.61 0.23 0.21 0.010
20 degree slope -0.01 -3%

HM SC76 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 Argon 1/4” 6mm 1in 0.26 1.41 0.41 0.35 0.28 0.61 0.23 0.21 0.010
20 degree slope -0.01 -4%

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 incor-
porate 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 oper-
ation 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 suspend-
ing a low-e and solar-heat-reflective Heat Mirror film mounted inside

Continued

December 2009 | ELEVATOR WORLD | 51

Evaluating

Heat-
Glass for Use in Glass-Enclosed Elevators

Continued

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 equiv-

alent of an insulated wall. These statistics make it no wonder that sus-

pended-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):

U-value R-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

(three films, quad-cavity, xenon) 0.05 20

Dual-cavity, tri-cavity and quad-cavity Heat Mirror suspended-film insulting glass (source: Southwall Technologies, Inc.)

52 | WWW.ELEVATOR-WORLD.COM | December 2009

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 perform-

ance, 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 con-

struction 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 ex-

cessive water leaks into the elevator cabin during normal

precipitation. Both vinyl and fiberglass are excellent insu- Installations of XIR Laminated Glass
lators, and fiberglass provides the strength and durability
necessary to accommodate all glass suitable for elevator Multi-Building Sports Complex (Shenzhen, China)
use.
Summing It Up Over 400,000 square feet of Southwall’s XIR glass
is being installed in a sports stadium, swimming
Heat-reflective-coated film, whether used as an inter- complex and indoor arena, to be the site of the
layer in laminated monolithic glass or as suspended-film World University Games in August 2011. This is one
insulating glass, offers increased energy conservation of the largest XIR glass installations in the world.
options never before available. Outstanding solar-heat- Magnocentro26, Shopping Mall (Mexico City)
reflection and insulating performance in a variety of
glass configurations allows designers of glass-intensive Nearly 35,000 square feet of XIR glass allowed for
elevators to specifically address: downsizing of the heating, ventilation and air-con-
N Unique requirements of the elevator to be designed ditioning system in this 246,000-square-foot mall.
N Geographic orientation Sahara Centre (Dubai)
N Location within the building
N Performance to match the available design budget 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)

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 un-

derground Metro, buses and taxis, while maintain-

Bruce Lang is vice president of Marketing & Business ing the aesthetics of the original train station.
Development at Southwall Technologies, Inc. in Palo Alto, Florida Aquarium (Tampa, Florida)

California. He can be reached at blang@southwall This massive glass structure designed for high

.com. light transmission and effective heat reflection used

XIR glass throughout.

December 2009 | ELEVATOR WORLD | 53