The words “modern” and “historic” are opposites, and yet they both describe the renovated City Hall Annex in Cambridge, Mass. Prompted by the necessity to remediate the building — mostly because of mold contamination — city leaders decided to use modern “green” technology to rebuild their historic annex, which was built in 1871.

Working with Somerville, Mass.-based HKT Architects and Milford, Mass.-based Consigli Construction, the city began an overhaul of the four-story, 33,216-square-foot brick-and-brownstone structure in October 2002. City leaders wanted the renovation to strike a balance between economic limitations, workplace needs and the preservation of natural resources.

To that end, designers specified energy-efficient materials for the project, including new insulated windows and walls, high-efficiency lighting systems, a ground-source heat pump and photovoltaic solar panels that supply 10 percent of the energy for the building. Maximizing natural light in the building with windows and skylights also was key because it creates a more work-friendly environment for employees — 90 percent of offices have outside views — and the light triggers sensors, which adjust artificial light levels in each room during the day. Designers also chose to use double-glazed windows that minimize heat loss and gain, carpet made from recycled content and water-efficient landscaping to reduce water use by 50 percent.

To incorporate the new materials, the team opted to replicate most of the historical elements of the annex rather than to preserve the originals. The team worked from old photographs to recreate wood trim on doors and windows, paneling, brick parapets and lighting fixtures. The exterior brick and one stairwell are all that remain unchanged after the renovation.

Difficulties arose when the Mid-Cambridge Neighborhood Historic District Commission voiced concerns about the appearance of the photovoltaic panels on the roof and the new windows. In response, the project team hid the solar panels in a steel frame and replicated original mullions to surround the windows.

Displaced since 2000, when the annex closed because of mold, five city departments moved back into the building in early February. The Traffic, Parking and Transportation Department; the Community Development Department; the Cambridge Arts Council (CAC); the Animal Commission; and the Conservation Commission now occupy the city's first “green” building. In addition to city offices, the building includes CAC's art gallery, media-ready public meeting rooms and a two-story atrium that features murals of office and outdoor scenes. The annex now is completely handicap accessible, as well. The project cost $9.5 million and was partially covered by a $337,500 grant from the Renewable Energy Trust Fund of the Massachusetts Technology Collaborative.

The city has applied for Silver Leadership in Energy and Environmental Design certification, which is a rating for sustainable buildings granted by the U.S. Green Building Council. Overall, the city expects the annex's energy consumption to be approximately 50 percent less than the consumption of a conventionally constructed building.

The city posted displays throughout the annex to describe the renewable energy technologies that were used in the renovation. Even the hidden elements, such as the ground-source heat pump, feature visible displays that explain how the building operates. City leaders recommend that visitors car pool or take public transit when traveling to the annex because the building is designed with only one parking space for every four employees — just one more sustainable design feature.