“More Than Skin Deep”

An integrated facade strategy helps designers create more comfortable and better-performing glass buildings.

By Joann Gonchar, AIA

At least since Modernism emerged in the early 20th century, and perhaps as early as the mid-19th century, when Joseph Paxton built the Crystal Palace in London, architects have been fascinated with glass. They’ve exploited the material’s properties to make buildings that almost disappear, to create a glowing effect at night, and to enhance connectivity with the outdoors. And of course, many have chosen to clad their structures in glass because of its associations with openness and honesty. But a mostly transparent building can be at odds with sustainability. Depending on how it is designed and built, an all-glass-clad structure is prone to heat gains and losses, resulting in interiors that are too hot or too cold and creating the need for bigger mechanical systems.

One response to the competing demands of energy efficiency and transparency is a double-skin facade — a cladding assembly made up of two surfaces or walls of glass separated by an air cavity ventilated either by mechanical or natural means. This intervening air space, which acts like a climatic buffer, often encloses shading devices and can be as narrow as a few inches, but is typically 3 or more feet deep to allow access for maintenance. In cold seasons, the air within the cavity can be distributed to the building to help fulfill heating needs, and in warm weather it can be vented to lessen cooling loads.

The approach does have drawbacks, including the loss of usable floor space and the cost of an extra skin and its supporting structure. A double wall also adds a level of complexity requiring a whole building approach that closely coordinates several disciplines, including mechanical and structural engineering, thermal analysis, and lighting design.

The just-completed 400,000-square-foot, $85 million expansion of the Frankfurt, Germany, headquarters of KfW Bankengruppe is one project that is the product of this kind of tight integration. Berlin-based Sauerbruch Hutton won the commission in 2004 with a proposal for a 184-foot-tall glass-clad building that includes a 10-floor office tower, shaped like an airfoil in plan, on top of a curvy four-story podium.

The form was the outcome of an effort to preserve views and daylight for the occupants of the bank’s cluster of existing buildings. At the same time, the designers hoped to reinforce the street edge and draw an adjacent botanical garden into the rear of the site.

This configuration, especially the tower’s winglike shape, was also ideal for an unusual type of double-skin facade that takes advantage of the prevailing wind direction and should allow offices in the new KfW tower to rely on natural ventilation for several months of the year. “The urban concept and the ventilation strategy fit well together,” says Tom Geister, Sauerbruch Hutton project architect.

The facade, along with several other coordinated features, including radiant slabs and geothermal heating and cooling, is expected to help the building meet an ambitious operating target of consuming no more than 27 kBtu per square foot of primary energy per year, if calculated in accordance with the U.S. standard ASHRAE 90.1 — about half the needs of a standard German office building. The goal was important to the client, since much of its lending activity supports energy-efficient housing and the development of sustainable technologies.

Depending on conditions, the building management system (BMS) opens or closes flaps to introduce fresh air and create a zone of consistent pressure surrounding the curtain wall’s inner skin while simultaneously producing a slight pressure differential between the cavity and the building’s interior. This air is then drawn into offices through floor vents near the perimeter, or through the occupant-controlled windows. It is subsequently exhausted naturally to the negatively pressurized corridor, and ultimately through the building core.

Auer expects that the building will operate in this mode — with the mechanical systems for heating and cooling the offices off — during much of the spring and fall. During the winter and summer, the offices will be supplied with fresh outdoor air through a duct buried underneath a below-grade parking garage. It will carry the air from an intake louver located at the site’s edge near the botanical garden and temper it with the constant temperature of the earth before delivering it to the work areas from a plenum below their raised floors. In winter, the air will be further warmed by a recovery system that captures heat from exhaust air and from the data center. And during the summer, radiant ceilings will absorb heat.The KfW envelope system, which designers have dubbed a “pressure ring,” consists of an encircling sawtooth-shaped cavity, 28 inches wide at its deepest point, that encloses automated blinds to block solar gain and control glare. The space is defined on the exterior by a skin made up of fixed, tempered-glass panels and colorful ventilation flaps, and on the interior by alternating operable and fixed argon-filled insulated glazing units (IGU) incorporating a low-E coating.

This double-wall assembly will extend the number of days each year that natural ventilation is practical, maximizing air quality, but not at the expense of energy conservation, according to Stuttgart-based Thomas Auer, managing director of Transsolar KlimaEngineering, the project’s energy consultant. In high-rise buildings with operable windows, pressure differences on the windward and leeward faces can produce too much cross ventilation, causing unwanted heat loss, he explains. But at KfW, the pressure ring should keep the cross ventilation and associated heating loss in check.

The building has a roof-mounted weather station that monitors wind direction and speed, among other factors, and controls the ventilation flaps in the facade’s outer shell.

Frankfurt has a mild climate, with long, benign shoulder seasons, making it well suited for such an approach. But a double-skin can also be incorporated into a coordinated strategy for energy conservation and occupant comfort in buildings in more extreme environments, as illustrated by the $271 million headquarters for public utility Manitoba Hydro. The 700,000-square-foot building opened in September in downtown Winnipeg, Canada — a city with short and humid summers and long and brutal winters. It has the dubious distinction of being the coldest city on the planet with a population of 600,000 or more.

Despite the harsh conditions, the client and its design team, led by Toronto-based Kuwabara Payne McKenna Blumberg Architects (KPMB), opted for a highly transparent structure. But by deploying a host of tightly coordinated features, including a double-skin, they’ve created a tower expected to consume only a third of the energy of a code-compliant office building.

Manitoba Hydro’s massing, the product of in-depth site analysis, includes two 18-story office blocks separated by a service core on top of a three-story podium. The blocks are set at angles to one another, forming the long legs of a triangle, with dominant exposures to the west and east-northeast. To the north, at the triangle’s apex, is a finlike solar chimney that extends several stories beyond the roof. And at the opposite end, forming the triangle’s base, are three stacked atrium spaces, or winter gardens, each six stories tall.

This configuration, and especially the south-facing atria, allow the building to make the most of Winnipeg’s unique atmospheric conditions: Although frigid in winter, the city’s skies are among the clearest in Canada. “Even when it is cold, it is almost always sunny,” points out Transsolar’s Auer, whose firm also served as this project’s environmental consultant.

The winter gardens were conceived to take advantage of this free solar energy. The 90-foot-long and 30-foot-wide space acts almost like an expanded double skin, providing a chamber for preconditioning outdoor air before it is distributed to the office areas through an underfloor displacement ventilation system.

Fresh air enters each atrium through the louvers in the south-facing insulated glazing. During the winter, it is warmed by the sun and humidified by 80-foot-tall fountains made of tensioned mylar ribbons that carry water along their length. In the summer, chilled water runs along the ribbons, helping remove humidity from the air. The winter gardens are the building’s “lungs,” says Bruce Kuwabara, KPMB principal.

Once the air is introduced into the offices, heat is added or absorbed, depending on the season, by radiant ceilings. During the summer, the stack effect draws the air upward through the solar chimney and out of the building. But in the winter, the heat in the air is recovered and used to warm a below-grade parking garage.

The long, exposed faces of the office blocks are clad with a more typically dimensioned double skin. The system includes a 49-inch-deep cavity enclosed by an IGU on the exterior and a single lite on the interior. Both inner and outer skins are of low-iron glass, incorporating low-E coatings, but of differing performance levels: Somewhat counterintuitively, the outer skin’s coating allows much of the sun’s radiant energy to pass through the glass into the cavity. However, the inner skin includes a higher-performing pyrolytic, or baked-on, coating. It reflects a large portion of the solar radiation back into the cavity while helping maintain comfortable temperatures for office areas immediately adjacent to the curtain wall. “The goal was to collect as much heat [in the cavity] as possible,” explains John Peterson, KPMB project architect.

The combination of coatings is so effective that excess heat often builds between the inner and outer curtain-wall layers whenever outdoor temperatures rise above 41 degrees. But at those times, the BMS opens operable windows in the outer skin to vent the cavity. The system also controls automated blinds to further block unwanted solar heat gain and control glare. Occupants can open windows, as well, on the curtain wall’s interior skin to introduce more fresh air if they desire.

Luminous library

Given all the variables and components, optimizing the performance of a double-skin facade is not as straightforward as that for a standard curtain-wall assembly. Energy modeling of a double-skin curtain wall involves not only thermal analysis of the complete assembly, but also analysis of the contribution of dynamic components, such as blinds and vents, explains Andrew Hall, a director in the London office of Arup. A double-skin facade “is not a static system,” he says. Hall’s firm served as facade consultant for the new central branch of the Cambridge Public Library, in Cambridge, Massachusetts.

Despite the inherent complexities, the library’s designers saw a double-skin as the perfect solution for the building’s main facade. They desired a transparent expression, but a typical single-wall curtain wall was impractical because of the southwest exposure and the associated heat gain and potential for glare. “We wanted the building to be welcoming from the outside, luminous at night, and not intimidating,” says Clifford Gayley, a principal at Boston-based William Rawn Associates, the project’s lead architect. In addition, the architects sought to establish a relationship between the library’s interior and the 4-acre city park that surrounds it. And they hoped to avoid dwarfing the much smaller original library — a late-19th-century masonry building by Van Brunt & Howe restored as part of the $70 million project. The new, 76,700-square-foot structure is connected to the 27,200-square-foot historic building, quadrupling the size of the library.

The team developed a double-wall assembly, 180 feet long and 42 feet tall, with an outer skin of 1⁄2-inch tempered low-iron glass and an inner, thermally broken skin of 1-inch IGUs. The two layers define a 3-foot-wide, two-story cavity that serves as a thermal flue: Depending on the season, louvers at the top and bottom of the wall can be opened or closed, to vent or to warm the air within.

Because the connection between indoors and out was such an important part of the concept, the project team worked hard to limit the visual obstructions between the library interior and the park. Their first move was to cantilever the strip of floor slab immediately behind the double skin from a row of columns 15 feet away, creating a zone free of large vertical elements at the building’s edge.

To support the curtain wall, the team devised a framing system that was as minimal as possible but still able to withstand the necessary loads. The structure includes 33 vertically oriented Vierendeel trusses spaced 5 feet 6 inches apart and connected by catwalk grilles and steel angles. Because the vertical trusses contain no diagonals and because the horizontal members are placed above or below occupants’ sight lines, views through the facade, even at oblique angles, are relatively unimpeded, explains Hall.

Sunshades within the cavity for controlling direct sunlight penetration are always extended and are set to one of two possible angles, depending on the season or time of day. But in keeping with the design mandate for unobstructed views, the shades shield only the upper portion of the two floors behind the curtain wall. The first 8 feet of these floors are instead protected by laminated-glass visors that project from the building face and have a slight gray tint. “It was important that they cut the transmission of light but still be read as glass,” explains Gayley.

The product of all of these carefully considered design decisions is a crisply detailed crystalline facade optimized for its orientation. “A double skin is not the only way of achieving a green facade,” says Arup’s Hall. But, he adds, it makes sense where daylighting, protected shading, and transparency are desired.

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