PLAY The building can be an energy source instead
of a liability. (Rendering courtesy of Kiss + Cathcart
building owners and designers are taking a new look at high-performance
glass facades as key in creating buildings with improved access
to daylight, better indoor air quality and improved energy
efficiency. But a facade that helps improve a building's interior
environment and limits its loss of energy is not enough, say
some sources. A building's skin should be a power generator
rather than an energy liability.
"Although glass curtain walls are
becoming more and more thermally efficient," says Gregory
Kiss, a founder of Kiss + Cathcart Architects, Brooklyn, the
goal of creating an energy-conserving facade is one of "diminishing
returns." His firm has designed a hypothetical 150-story tower
to be built in New York City in 2020 with a skin that would
be a source of power. The structure, clad entirely in photovoltaic
(PV) panels, would generate 60% of the building's electricity
requirements. Wind turbines enclosed in PV louvers would supply
Although the 2020 tower may seem
more like a fantastic vision, Kiss maintains that photovoltaic
technology "is ready for prime time." Even now, the least-expensive
building-integrated panels cost about $12 per sq ft. "There
are many more expensive cladding materials," he says. And
despite the fact that many recent high-profile projects use
PVs to generate only a small portion of their total power
requirements, building-integrated photovoltaics could be common-place
in three to five years, he predicts. "The number-one limitation
is education," Kiss says.
Click here to view rendering.
Scientists and manufacturers are
also looking beyond photovoltaics to the next generation of
variable materials. These so-called "smart" glazings dynamically
respond to exterior conditions to control daylighting and
solar heat gain. The most promising of such switchable technologies
for use in buildings is electrochromic glazing, which undergoes
a reversible change in optical properties when exposed to
light, according to Stephen Selkowitz, head of the building
technologies department at the Lawrence Berkeley National
Laboratory, Berkeley, Calif. The lab is currently using room
mock-ups to discover how best to optimize lighting levels
under various conditions.
But an energy-efficient building
envelope isn't just about the materials. "Limiting solar and
thermal transfer requires integrating the facade system with
the lighting, mechanical, heating and cooling systems," says
facade engineer Maurya McClintock, an Arup associate principal
in San Francisco. "If designers look solely to curtain walls
as a solution for comfort and energy consumption, they will
never achieve the synergy that is possible with integrated
design," says Stephen Lee, a professor at the Center for Building
Performance and Diagnostics at Carnegie Mellon University,
The U.S. lags Europe in the development of advanced building
systems and high-performance facades, due in part to lower
energy costs. Curtain wall technology in the U.S. "is about
10 to 15 years behind," says Alistair Lazenby, technical director
of curtain wall contractor Schmidlin A.G., Aesch, Switzerland.
Recent legislation, aimed at cutting
greenhouse gas emissions in line with commitments to the Kyoto
Accord, will likely increase scrutiny of building envelope
performance in Europe, says Cameron Johnstone, a lecturer
at the Energy Systems Research Unit at Scotland's Strathclyde
University. The directive, enacted last month, applies to
renovations and new construction and requires governments
to introduce standardized methods for calculating building
energy use by 2006. With buildings said to account for a third
of energy use by European Union countries, immediate and full
implementation of the legislation would allow the EU to achieve,
within 10 years, its full Kyoto goal of cutting emissions
by 8% below 1990 levels, according to some estimates.
Double-skin facades are a popular
approach to all-glass building envelopes in Europe, especially
in Germany, where regulations mandate that office workers
have access to daylight and fresh air, according to John Durbrow,
senior vice president of architect Murphy/Jahn. The Chicago-based
firm designed a 40-story, double-skin headquarters building
for Deutsche Post in Bonn that was occupied in late December.
The building's envelope consists
of an outer layer of laminated glass and an interior layer
of double-glazed glass with operable windows, separated by
a 1.7-meter gap. Blinds between the two layers, controlled
by a building management system, provide protection from solar
gain. During the cooling season, warm air is drawn into the
gap and purged through outlets located every nine stories.
Cooler air on the inside of the blinds is drawn into the office
space through fan-coil boxes that further lower the air temperature.
Radiant cooling in the floor slab helps lower the room temperature
more before the air is exhausted into an atrium space. During
the heating season, the system acts as a buffer, tempering
the outdoor air before it is drawn into the offices.
Deutsche Post's double-skin system,
along with its integrated approach to heating and cooling,
could cut energy use by 50% compared to a building with a
single facade and central air conditioning, says Matthias
Schuler, principal of Transsolar Energietechnik GmbH, the
project's Stuttgart-based climate consultant.
Despite the expected effectiveness
of this deep double-skin system, it soon could be replaced
by one that is less real estate and capital intensive. Murphy/Jahn
plans to use a double-skin system that is only 26 cm deep
for a speculative 200-m-tall office tower in Frankfurt set
to begin construction in 2005. The goal is to achieve "the
same efficiency in a thinner package," says Durbrow.
There are signs that such systems
have a future in the U.S., even though low energy costs seem
to provide little incentive for owners. In Philadelphia, contractors
are nearing completion of a $16.5-million building for the
University of Pennsylvania's School of Engineering and Applied
Science that is clad with a double-skin system that has a
4-in.-wide cavity between an external double-glazed unit and
an interior single-glazed unit.
The architects chose the system
in order to maximize the penetration of natural light into
the 48,000-sq-ft, six-story building located on a tight site
surrounded by several existing structures, while maintaining
the thermal comfort of the users. "The university didn't want
to fry the occupants in the summer or freeze them in the winter,"
says Richard Maimon, an associate at KieranTimberlake Associates
The unitized wall is tied to the
building's mechanical system and helps maintain comfort by
using the cavity between the inner and outer glazing as a
plenum through which return room air is circulated. This keeps
the building's perimeter warm in the winter and cool in the
summer. The cavity also houses adjustable blinds that keep
out solar radiation.
Permasteelisa SpA, Treviso, Italy,
the manufacturer and installer of the panels used at the University
of Pennsylvania, sees an expanding market in the U.S. The
company will start installing a similar panel system later
this month at a Skidmore Owings & Merrill-designed 14-story
office building in Boston that will serve as the U.S. headquarters
for Toronto-based Manulife Financial, says Roberto Bicchiarelli,
executive vice president of Permasteelisa Cladding Technologies,
Windsor, Conn. Permasteelisa does not expect much demand for
this product from developers or speculative builders since
it costs about 20% more than a high-quality standard curtain
wall system. "The wall system has a medium- to long-term payback,"
says Bicchiarelli. which works for owners who occupy buildings
for many years.
LEEDING THE WAY. One factor
nurturing this embryonic interest in high-performance facades
is the Leadership in Energy & Environmental Design rating
system, known as LEED. The performance-based system, devised
by the U.S. Green Buildings Council, Washington, D.C., is
designed to evaluate the environmental impact of commercial
and institutional buildings and provide a standard for what
constitutes a sustainable building.
SCREEN Mockups of the New York Times building's
ceramic rod "veil" informed the curtain wall bid documents.
(Photo courtesy of Benson)
Seattle is one of several municipal
and federal owners that require some level of LEED certification.
Because of this mandate, convincing city officials to sacrifice
the space required for a 30-in.-deep, 12-story, double-skin
facade at the recently completed $70-million Seattle Justice
Center was not difficult, according to designers. Although
it cost roughly $320,000 more than a traditional facade, "it
was an easy sell because the city was motivated by LEED,"
says Seattle-based Rick Zieve, design principal at architecture
firm NBBJ. The cavity, which is independent of the building's
mechanical system, helps maintain comfortable perimeter temperatures
by trapping solar heat and expelling it through louvers at
the top of the wall.
courtesy of Renzo Piano Building Workshop/ Fox & Fowle
Little post-construction data is
available to demonstrate how well double-skin facades work
in conserving energy.
Anecdotal evidence suggests they
work well. In 1999, Telus, a Canadian telecommunications firm,
wrapped an existing concrete framed building in downtown Vancouver,
B.C., in a new double-glazed facade as part of a $13-million
renovation designed by local architecture firm Busby &
Associates. The 42-in. gap between the new "jacket" and the
existing building acts as a thermal buffer in much the same
way as the double-skin facade at the Seattle Justice Center.
On one recent 15°F day, air was evacuated from the top
of the cavity at 85°F. "We were throwing energy away
because we were creating too much," says Doug Green, special
projects manager for Telus.
courtesy of Renzo Piano Building Workshop/ Fox & Fowle
Sources stress the importance of
having the curtain wall contractor's input early in a building
project. "The supplier will have a good handle on the manufacturing
process as well as an understanding of what can and cannot
be achieved," says Arup's McClintock. Although she cautions
that the contractor should not be considered a substitute
for a facade engineer who can provide an impartial view of
components and systems.
As the complexity of wall systems
escalates, unusual procurement methods intended to help the
owner obtain engineering feedback from the curtain wall contractor
are becoming more common. For challenging projects, award
of the curtain wall contract before hiring the general contractor
is not unheard of, says Lou Niles, president of Benson, a
Portland, Ore.-based curtain wall contractor.
Benson is competing for the 51-story
New York Times building. Its skin will include a screen of
horizontal 11Ú2-in.-dia ceramic rods that will start at the
second floor and extend beyond the top floor. This "veil"
is intended to reflect atmospheric conditions but will also
act as a shading device, says Dan Kaplan, principal of Fox
& Fowle Architects, New York.
This past summer, the New York
Times project team gave four curtain wall contractors a stipend
of $50,000 each to build a mockup of the facade. The mockup
process allowed designers to incorporate the contractors'
knowledge into the bid documents. "It gave us the benefit
of a lot of different thinking and made us feel more certain
about the budget," says Bob Sanna, executive vice president
of Forest City Ratner Cos., the project's developer, along
with the New York Times Co. The project team is now evaluating
bids and expects to award the contract in March, in advance
of selecting the construction manager, says Sanna.
The growing complexity of curtain
walls is revealing the limitations of modeling tools used
to optimize design and demonstrate code compliance. "It took
us one month to show compliance with the energy code," says
Gary Pomerantz, senior vice president, Flack & Kurtz,
New York City. The firm is mechanical engineer for the New
York Times project. "The three-dimensional skin [with its]
moving and overlapping shadows made modeling difficult," he
Existing tools work well for modeling
complex but conventional curtain walls, says Lawrence Berkeley's
Selkowitz. In other words, curtain walls with many layers
of glass and several coatings, use laminates or gas fills.
But the tools do not reliably predict the performance of dynamic
materials such as those that transmit or reflect light in
a non-linear manner or systems that have between-pane air
cavities. "When you introduce a prismatic daylight control
layer or an automated venetian blind with a special reflective
surface or a double facade with a complex air flow pattern,
these systems are more difficult to model," says Selkowitz.
His lab, which has developed much of the software that is
in the public domain, is working to add these capabilities.
Stricter U.S. building codes could
push more high-performance facades. In 1999, the Building
Energy Standard for Buildings Except Low-Rise Residential
Buildings, known as ASHRAE 90.1, was overhauled for the first
time in a decade. It was revised again in 2001. So far, 13
states have adopted energy standards equal to or stricter
than the 1999 version, says Joseph Derringer, chair of the
American Society of Heating, Refrigerating and Air-Conditioning
Engineers Inc.'s building envelope subcommittee.
For envelope performance, the main
difference between the most recent standards and the 1989
version is more rigorous code language, says Derringer. Although
some observers believe that adopting the latest versions of
90.1 will discourage construction of glass buildings, he says
the tougher standard "should instead mean more buildings with
advanced glass systems."