BOSTON--Green building professionals can point to dozens of houses that are so efficient they consume less than half the energy of an average American home. A low-income housing development here is bringing those same efficiency techniques to a block-long apartment building.

The Castle Square Apartments in Boston's South End neighborhood is in the midst of a project organizers say will be one of the largest deep-energy retrofits done in the U.S., with a projected 72 percent cut in energy use. Last Thursday, the state- and federally-funded development held a groundbreaking attended by Boston Mayor Thomas Menino, U.S. Secretary of Housing and Urban Development Shaun Donovan, and other officials.

The development itself is not unlike many low-income urban projects built in the 1960s, with a plain brick fa§ade and a total of 500 apartments. The experience of tenants is typical of many city apartments, too: residents say that it's often either too hot or too cold and complain of health problems from the bad air quality.

Almost three years ago, the building became eligible for a renovation and the Castle Square Tenant Organization, which is part-owner of the complex, decided it wanted to "go green." The problem was that "going green" wasn't well defined and renovation discussions focused on air quality, controlling pests, a leaking roof, and temperature, said Deborah Backus, the executive director of the Tenant Organization. "The basic necessities were always on the top priorities," she said.

One of the main practices of a deep-energy retrofit is to create a tight envelope around the living space to prevent air leakage and to add a thick layer of insulation, sometimes on the exterior of building. Good air quality is ensured by circulating in outside air. By lowering the heating and cooling needs, architects can choose smaller and less costly heating and cooling equipment.

During the renovation talks, some of the project developers proposed using those practices at Castle Square. Pushing the envelope on efficiency would allow the development to score significant efficiency improvements and address tenants' concerns over air quality and indoor temperature.

"In doing a lot of renovations on existing buildings, we found that there's this glass ceiling of improving efficiency by about 20 percent. It's almost like you're stuck," said Heather Clark, one of the project developers and technical consultant. "And with energy prices rising, we found people said they weren't doing that much better money-wise."

Taking a more aggressive approach led to the idea of a deep-energy retrofit at a large scale, which has been done in Europe but not in the U.S., Clark said. Because it addressed residents' heath and comfort concerns and the desire to go green, residents were quickly on board, said Backus.

Fitting in with the neighborhood Because Castle Square is a low-income housing development, the renovation would be funded from usual government agencies. But the stimulus program in 2009 and a $4.4 million state grant from the Massachusetts Department of Energy Resources opened up the possibility of being more ambitious, said Backus.

The total cost of the project is in the range of $50 million for all 500 units, with 192 units in a seven-story building getting the insulation on the exterior of the building and a solar hot water heating system, according to Clark. Some of the items, such as triple-pane fiberglass frame windows, will add to the cost of a typical project. But since it's a renovation, many new items, such as energy-efficient appliances and lighting, needed to be acquired anyway, she said.

The Castle Square Apartments are typical 1960s era low-income housing project units. A renovation now underway is designed to slash energy use, improve air quality, and give the development a new insulating facade.

Many of the techniques used during construction are similar to what weatherization contractors use in residential buildings, such as sealing cracks with foam and caulk. In the current construction, there are big gaps around pipes or gas lines between apartments. That means as air flows from the bottom of the building toward the top in what's called the stack effect, the source of indoor air is other apartments or hallways, rather than fresh outdoor air.

The ventilation is being addressed in a somewhat unique way. In a very tight construction, buildings have a mechanical ventilation system, such as a heat recovery ventilator that heats incoming air and ensures air exchange with the outside. In this case, each apartment will have a pressurized vent, called a constant air regulator, which will pull in outdoor air and provide even ventilation throughout the building. Also, contractors sealed up the ducts that feed into apartments by adding a thin film sprayed in from the roof that will adhere to the interior of the ducts, explained Clark.

The outer shell that's being put on one of the Castle Square buildings will make the exterior airtight and add insulation with an R-40 thermal resistance value to the brick walls and roof. The white-colored reflective roof will lower the amount of heat the building absorbs, which will lower its cooling needs.

The group of companies behind the project, which includes an architect, building manager, and energy efficiency services company, decided to use solar thermal, rather than solar electric panels because of the location. The shading on the roof would significantly affect the performance of solar photovoltaics, whereas the solar thermal system would not be affected as much and would still lower hot water heating needs by about 35 percent.

Lowering the heating and cooling load means that three wall-hung, high-efficiency boilers will be able to serve all the residents in 48 apartments, rather than inefficient boilers that take up an entire room. "We've done all the right things," said Clark. "It's a dream project for all of us involved." The project, which is expected to be finished next year, will apply for LEED certification from the U.S. Green Building Council.

The multi-layer insulating shell will have the advantage of changing the fa§ade from the current brick to a metal surface once the project is completed. "This won't have the look of a project," said Backus. "Our living environment will fit in with the rest of the South End."