Riverhouse – Double Vision

When designing Riverhouse, a new 32-story condo located in Battery Park City, designer Todd Schliemann of Polshek Partnership Architects had two main goals that, at first glance, appeared to be irreconcilable: he wanted to maximize views of the Hudson River for occupants, while at the same time he was charged with creating the first LEED Platinum building in New York City, meaning energy conservation would limit glazing. These design aspirations were further challenged by the fact that the Battery Park City Authority requires its buildings to feature 60/40 glass/masonry ratios. Not to be undone, and famous for award-winning solutions, the architect prevailed by cladding Riverhouse’s west elevation in a double glass curtain wall, increasing the transparency of the envelope for views and daylight while simultaneously offering greater insulation values than its punched-windowed neighbors.

Double curtain walls, in which two layers of glass create a pocket of insulating air, have been common in Europe for years, but have taken longer to infiltrate the American market. “Due to cost and unfamil­iarity, there has been resistance from developers and clients,” explains Bob Heintges, principal of facade consulting firm R. A. Heintges & Associates. “We’ve worked with architects to develop double-skin designs only to have them value-engineered out.” So when Polshek Partnership approached Heintges to deploy a double curtain wall for Riverhouse, Heintges half expected to see the idea pulled. But by designing the facade as a panelized system, which saves money during construction owing to its quick and easy installation, the team was able to justify the approach to both the developer and regulating authority, Battery Park City.

Schliemann and his design team had previously designed a double curtain wall in 2006. At that time the firm completed the University of Michigan Biomedical Science Research Building, a 472,000­ square-foot facility located in Ann Arbor. For this double curtain wall, three feet of air space was used to separate the two layers of glass, a depth creating an enormous cavity for insulating air, convection cool­ing, and accommodating catwalks for maintenance access. Vents are located at the bottom and top of the cavity, together with manually operable shades installed within the cavity for controlling daylight admission and solar heat gain absorption, which allows occu­pants to regulate the heat load on individual rooms, as well as the building as a whole. The shades function as a heat absorption device in the air space: “The shades absorb solar heat gain,” explains Schliemann. “They heat up independently of the envelope, and cool air entering from the bottom vents passes over them, cooling them. Through natural convection the air then exits through the top vents.” In summer, the vents are open all the way, allowing fresh air to constantly whisk heat away. In winter, the vents are closed halfway to wrap the building in the heated air. (They are closed only halfway because the build up of heat would be extreme if closed completely). According to computer models run by engineering firm Buro Happold, the glass-and-air sandwich allows the office ribbon of the lab building to achieve a 25 percent energy savings over a typical glass curtain wall.

The Riverhouse system, which was engineered and fabricated by Minneapolis-based Enclos Corp., approximates the University of Michigan design, writ small. Just as the condo building is made up of individual apartments, so the double curtain wall here is divided into panelized units measuring 10-½ feet tall by 5 feet or 2½ feet wide. Each panel is only 9 inches deep. The air cavity measures slightly under 5 inches and con¬tains a manually operable shading system.

“The interior glass of the double wall is a 1-inch insulated glass unit comprised of a quarter-inch-thick clear heat-strengthened outer lite with a low-E coating on the number-two surface, a half-inch air space filled with argon gas, and a quarter-inch-thick clear heat-strengthened inner lite,” explains Enclos engineering manager John Lusch. The outer glass of the double wall is a single pane. Small operable windows in the system include their own dampers that bypass the hot air in the cavity, and redirect outdoor air into the apart¬ment. Each overall unit includes its own vents, and the whole composition is framed in extruded aluminum that has a Polyvinylidene Fluoride two-coat silver metallic finish. Interior aluminum compo¬nents are finished in one coat of acrylic resin that matches the facade’s appearance.

Installation is rather painless, Schliemann says. “These unitized systems travel up the elevator, are set on the floor, and put on the building from the inside. They basically get slid out and hoisted into position.” Because the building is constructed of poured concrete, famous for its imprecise and un­even floor plates, Enclos designed a multi-component anchorage system for securing each panel. “It accommodates specified building tolerance and dynamic building movements, in addition to with­standing the loads imposed on the curtain wall by wind, snow, and seismic movements,” Lusch says.

Now in place, the double skin behaves very much like its University of Michigan predecessor. Air travels from lower vents into the cavity, picks up heat from the  internal blinds, and then passes through upper vents. Computa­tional fluid dynamic modeling has ensured that the cavity does not get excessively hot, which would cause the air within to move too rapidly and, in passing over the blinds, cause them to flutter. And whereas the biomedical science research building’s double curtain wall included maintenance catwalks, the operable Riverhouse windows may be cleaned from in­side. In fall and spring, Riverhouse’s window washers will be charged with reaching inside the sandwich and closing or opening the vents to ready the wall for the change in seasons.

“Even if the shades aren’t drawn, the cavity still performs,”  Schliemann says of the innovative system, which, by all projections, should help Riverhouse exceed New York’s energy code requirements by 25 percent. The condo’s developer, the Sheldrake Organization, is also putting together an extensive tenant guide that educates apartment dwellers about the simple tasks that can improve Riverhouse’s green mission even more. As for Polshek Partnership, the architecture firm continues to seek innovative solutions, conceiv­ing double-curtain alternatives for other projects, such as one that substitutes vents and blinds with ceramic frit and different ventilation openings. As Schliemann says, “There’s more than one way to make a double-wall system.”  

PUBLISHED IN: Metals in Construction DATE: Fall 2008