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Curtain call: The new technology on display at the CPL is sure to inspire encore building performances

The Cambridge Public Library (CPL) won over the 2010 Harleston Parker Medal jury for a host of reasons. But of particular interest to the local building community is that the library is the first U.S. building to incorporate the three key features of advanced European double-skin curtainwall technology: a 3-foot-deep air space, a multistory thermal flue and 12-inch movable sun shades.

Clifford Gayley AIA, LEED AP, principal at William Rawn Associates, speaks about what that all means, particularly for this notable civic project.

What design challenge were you trying to meet with this new technology?

With the double-skin curtainwall, we were interested in exploring the library as a new type of civic building. When the historic Van Brunt & Howe library was built in 1889, it was the epitome of the Victorian-era “library in the park”—a solid and commanding presence, surrounded by green space with a grand stair leading to the front door. We wanted to create a contemporary vision for a library in the park: a decidedly transparent, welcoming and accessible place that brings the experience of a park into the library—a new kind of civic building.

The challenge was achieving maximum transparency in an environmentally appropriate way. Conventional methods are limiting. External louvers must be big and heavy enough to withstand wind and snow loads. Tinted glass, which cuts down the solar heat gain on people and books inside, also makes the building appear opaque from the outside. We wanted the glass surface to appear transparent and shimmering, not reflective and dark. We needed a new solution.

How did you decide to go with a double-skin curtainwall?

The decision involved significant research. Double-skin curtainwalls are relatively new in the United States, but they have been used in Europe for decades. We were able to tap into that experience and quickly educate ourselves. Principal Bill Rawn, project architect Kevin Bergeron and I took a trip to Germany and London with our façade consultant, Arup, for a tour of about a dozen buildings with different types of double-skin curtainwalls. Some were quite simple, and some were very intricate. It was a great opportunity to see firsthand the benefits of the technology as well as its potential issues.

During our trip we observed how effectively double-skin walls maximize transparency and daylight while controlling building temperature. Because we visited these buildings during the dead of winter, we were able to directly experience the temperature difference between the inside and outside glass. The inside glass felt as warm as an interior wall. It was amazing. We also observed examples that made the case for keeping things simple: one building had a highly interactive louver system that raised and lowered the louvers each time light sensors detected sun or clouds, requiring a full-time maintenance staff, which our public project could not offer. From these precedents, we were able to learn valuable lessons for deploying double-skin curtainwall technology, and we knew we needed to go with a version that had few moving pieces and, therefore, lower maintenance requirements.

How does the curtainwall at the CPL work?

The library’s double-skin curtainwall is its main façade (180 feet long, 42 feet high), celebrating both visual transparency and thermal comfort. Two layers of glass define a continuous 3-foot air space and a multistory flue with operable vents, top and bottom. During colder months, these vents are closed, allowing solar-heated air to create a thermal blanket. During warmer months, these vents are opened, creating a natural airflow that draws heat up and out of the wall cavity.

Inside the 3-foot cavity, 12-inch horizontal adjustable louvers provide solar control. They are positioned to block direct solar heat from entering the building, trapping the heat in the cavity wall, where it can be used or vented. They also are slightly curved to redirect daylight into the building interior. Reflecting the lesson of keeping things simple, these louvers are always deployed (only retracted for annual window washing) and are limited to two positions: horizontal and canted 45 degrees forward. For half the year, the louvers do not move—remaining horizontal during the summer and 45 degrees during the winter. For the other half-year, the two “shoulder” seasons, the louvers rotate only once each day, from horizontal in the morning to 45 degrees in the afternoon.

Was it difficult to employ something so cutting-edge in an addition to a historic building?

Throughout the design process, the City of Cambridge was highly supportive of a very contemporary building and a historic building coming together to form the new library. We and our colleagues, Ann Beha Architects (who led the effort on the historic building), worked closely with Charles Sullivan of the Cambridge Historical Commission and with the Mid-Cambridge Neighborhood Conservation District Commission, who urged us to celebrate innovation in our building, pointing out that, in its day, the historic building had been quite innovative as a library with its state-of-the-art book-delivery system. We believe that the project benefited from the city’s nuanced understanding of history, which steered the project away from trying to replicate the historic architecture. Instead, old and new together stand shoulder to shoulder and define a new civic park.

Genevieve Rajewski is a Boston-based freelance writer who covers science, nature, animal issues, travel, food and passionate people for acclaimed publications such as Smithsonian, Washington Post Magazine, and The Boston Globe. Her website is

Top: 1, Exterior Glazing. 2, Interior Glazing. 3, Structural Frame. 4, Operable Sun Shade. 5, Sun Shade Canopy. 6, Lower Operable Ventilation. 7, Upper Operable Ventilation. Image by William Rawn Associates.