Double-skin facades have long been hailed as a way for glass buildings to provide great daylight and air quality without sacrificing energy performance. One commercial example is the world’s first Passive House office tower, RHW.2. But the common wisdom is that double wall facades aren’t suitable for New York City (the New York Times building’s 52-story climbing wall not excepted). Weill Cornell Medical College must have had a thick skin to ignore criticism of this approach, since their new LEED Gold certified Belfer Research Building discussed at Urban Green’s recent case study successfully boasts a double wall façade among many other green features.
The Belfer façade has an outer aluminum and laminated and fritted glass curtain wall, a chambered air gap with a maintenance catwalk, and an inner aluminum and insulated glass curtain wall, according to Lois Mate (Ennead). In hot weather, air movement in the gap cools the inner skin, while the outer wall provides additional cooling by shading the inner wall. Bret Mantyk (Atelier Ten) said the glass frits are white on the outside and black on the inside to maximize cooling without creating a distracting view for people looking out the window. The air gap is compartmentalized, with rising hot air ejected one-third and two-thirds of the way up the façade, so that hot air doesn’t heat the upper floors on a sunny day.
Annual energy cost savings from the façade are expected to be about $15,000. It will probably take a while to recoup the cost of the low e vision glass and the complex, 3-D outer skin. But that savings doesn’t include the value of productivity of the occupants of the lab building, who will have great views and good daylight without overheating from the windows.
The main reasons I’ve heard for the lack of interest in double wall facades in NYC are concerns about maintenance and loss of rentable square footage. How did Weill Cornell deal with these issues? On the maintenance front, the façade has absolutely no moving parts. Mate noted that movable dampers or motorized controls in the façade would have marginally improved shading or ventilation control, but the ongoing cost, complexity, and possibility for improper operation steered the project towards fixed façade elements. Since a façade is a very durable item that will last for decades, this seems wise. In my experience, it’s easy for a year’s worth of savings from fancy system doodads to be lost in a few days through something as simple as an incorrect system setting. A system that’s carefully designed so building operators can “set it and forget it” may be most likely to provide stable energy savings over time.
As for giving up some valuable Upper East Side real estate to the air gap, campus architect William Cunningham (Weill Cornell) admitted the thick skin was a tradeoff from potentially usable floor area. On the other hand, the building’s floor plate is still deep enough to be space-efficient for the lab, and the college felt the efficiency, productivity, and occupant comfort benefits were worth it.
Weill Cornell is an institution that expects to own the building over the decades it may take to pay back the extra cost of the façade, and has a high commitment to the building occupants, so this may have influenced its decision differently from a traditional commercial real estate setting. (It’s no coincidence that the other high-profile double wall façade in NYC is at New York Presbyterian Hospital.) In addition, Cunningham said the final building design approved by the city more or less maximized the building’s usable square footage anyway, so it may have not been much of a sacrifice.
Still think a double-wall façade is impossible in NYC? Take a close look at Belfer before coming to a final judgment.