The bright, lime-green roof that helps define the Anacostia Neighborhood Public Library in Washington, D.C., serves as both an iconic visual form and a sun control device. The perforated, corrugated metal plane wraps down over the building's western glass wall—opaquely reflective during the day, glowing transparently at night. From the library's interior, it offers views to an adjacent park while providing controlled, filtered daylight, which is key to making the library a healthy, sustainable building.

It's also a testament to the power of teamwork. Maximizing the effectiveness of the shade, as well as the daylighting potential of the library itself, was the result of an integrated, collaborative, and cooperative effort between the architect, the Freelon Group, and lighting design firm Horton Lees Brogden Lighting Design (HLB), says Hayden McKay, HLB's principal in charge of the building's daylighting design.

Welcoming to the Neighborhood

The team's focus on daylighting reflects the D.C. Public Library system's desire to make its new Anacostia branch an appealing, transparent space. "They wanted to really make this feel welcoming and inviting and bring in the community," McKay says. "So the issue of transparency, being able to see inside, being able to know that it's open, making it feel exciting—that led to a lot of glass."

Project architect Michael Rantilla, associate principal at the Freelon Group, also envisioned the library as a beacon in the historic, but traditionally underserved, Anacostia neighborhood. "We had a fixed, somewhat modest footprint to work with," he says, "but we still wanted to bring this excitement, and grand space, and views and light, and just make it a very airy experience that people would want to be in."

With the library mandated by the client to achieve LEED Silver certification, creating a healthy and sustainable environment was also a vital goal. The focus on daylighting helped the building to achieve its sustainable mission on multiple fronts: lowering the luminaire's electrical usage while also reducing the heat load they generate, and providing views and daylight for a more cheerful indoor setting. By using daylight as the primary source for ambient illumination, the team reduced the interior lighting system's power usage by at least 50 percent over an equivalent electric lighting system, and the library surpassed the client's expectations by achieving LEED Gold certification.

Sunny Disposition

A highlight of the team approach, McKay says, was a productive, two-day charrette in New York that included the Freelon Group, McKay, and Teal Brogden, the HLB principal who led the project's electric lighting design. The Freelon Group included sun control in the initial forms and space planning of the library design, and McKay says her team wanted to make sure the building optimized daylighting and connection to the outdoors.

"We spent two days looking at every possible option," she says. Afterwards, the two teams continued to have regular, remote meetings to discuss details like materials, color, reflectivity, and finishes. HLB's daylight analyses helped guide a number of design decisions, including details about the windows, the design and location of the skylights, and the shape, materiality, and geometry of the roof shade.

The roof, for instance, had to address the building's north–south orientation and shape—an unusual parallelogram angled away from the true cardinal directions. The original plan was designed with an overhang above the southern plaza to shade the summer sun, but HLB recommended an asymmetrical shape, coming to a point in the southwest corner, to extend shade coverage in the afternoon. The team studied the climate and used a sun chart, plotting the "overheated period"—the times of day and year when the outdoor temperature is above 70 F—for Washington, D.C. "The objective is to keep the sun off of the exterior of the glass as much as possible in that overheated period," McKay says.

HLB also studied the color, shape, and perforation pattern of the screen on the western side to balance views and avoid distracting sun patterns. The screen functions like a theatrical scrim, becoming transparent or opaque depending on the direction of the light. The architect had originally looked at having very large (6-to-10-inch) perforations, but McKay says her team was concerned that the light from those large circles would create strong, high-contrast patterns that would be visually distracting for readers. HLB recommended smaller perforations that would still allow a view of the park to the west. The design team also considered using a more extreme corrugation for the material or fins instead of perforations—a strategy they eventually employed at a sister library in Tenleytown—but for Anacostia, the perforated screen provided a solution that balanced aesthetics and cost.

The building's shaded glass structure was supplemented with four skylights evenly spaced throughout the reading room to bring light deep within the large space, and clerestories and a light-shelf on the eastern wall. The clerestories bounce light up against the green roof to provide reflected light and a bright, playful "main street" area that borders the stacks and reading room.