POWER PLAY The building can be an energy source instead of a liability. (Rendering courtesy of Kiss + Cathcart Architects)

Product developers, 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 the rest.

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, Pittsburgh.

LAGTIME. 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 LLP, Philadelphia.

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.

ELABORATE 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.

(Photo courtesy of Renzo Piano Building Workshop/ Fox & Fowle Architects)

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.

(Drawing courtesy of Renzo Piano Building Workshop/ Fox & Fowle Architects)

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 says.

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."