One distinction, however, must be made between historic buildings and national landmarks, Frey notes. "No one is going to put solar panels on Mount Vernon," says Carl Elefante, a principal of Quinn Evans/Architects--a Washington, D.C., and Ann Arbor, MI-based firm with a specialty in historic buildings. Perhaps not, "but in the vast majority of buildings, such as Trust buildings, a lot can be done," Frey adds.

To some degree, preserving an existing structure is already "green building"--the mere act of not tearing down and rebuilding saves energy. "If you've been into historic preservation, you've been green for years," says John D. Lesak, a principal at the Los Angeles office of Page & Turnbull, an architecture firm with a specialty in historic preservation.

[IMGCAP(2)]And many old buildings, in fact, are more "green" than their newer counterparts, using native materials and plants. But new technology such as energy modeling can determine exactly where an existing building is not maximizing its energy and help designers decide where new systems can be used. "As technologies get better and cheaper, with energy modeling and life cycle analysis, we simply have better information to make decisions with," Elefante said.

Some methods do not conflict with aesthetics at all. The key, Lesak says, is to assess what elements define the character of the building, and thus must be left alone, and what elements can be worked on.

Temperature control (and a corresponding reduction in HVAC-related energy consumption), for example, can be as simple as opening a window, a capacity far more likely in an older building than a post-War skyscraper. Unobtrusive high-tech sensors can be located in buildings to monitor room temperature and notify tenants to open a window or transom. Other technologies also can help.

"There are sophisticated lighting and building controls," says Alan Scott, a principal at Portland, OR-based consultancy Green Building Services. "and with the growth of wireless technology, we can have a lot of low-voltage lighting without wires snaking around."

Replacing eroded window seals, but not the windows themselves (which often are more energy-efficient than newer models), also is a simple process that does not affect the nature or integrity of the building. Other possibilities are thermal films that can easily peel off a window, or indoor storm windows that can removed, Lesak says.

Water usage also can be reduced in older buildings, though many already are fairly sustainable. Unlike new construction, older buildings never utilized asphalt. Cobblestone, for example, is an old form of pervious pavement. "If there are flush-type toilets [already] in the building, there are low-flow units that are compatible," Scott said.

Energy production also is possible, particularly as solar panels have come down in price, Rive said. Solar panels are possible on a flat roof, or situated so they are not visible from the street. Some entry canopies can integrate photovoltaics. While the roofs of newer buildings may not be strong enough to hold the weight of panels, older building construction may be able to support them.

Renewable energy isn't limited to solar. Quinn/Evans' rehabilitation of the Brown vs. Board of Education National Historic Site in Topeka, KS, added renewable energy through a new geothermal heating and cooling system on an adjacent playground, with 81 geothermal bores transferring and collecting heat from the earth. "We were able to use something there because it was fairly easily done," Elefante said. "It produced a renewable source of heating and cooling."

[IMGCAP(3)]Geothermal energy also is being used by Page & Turnbull to upgrade the Antelope Valley Indian Museum in Lancaster, Calif., built in 1927 by a Hollywood set designer. "The Indian artifacts housed in the building are undergoing degradation due to the radical temperature swings," Lesak noted.

A series of geothermal wells will be constructed in the parking lot, while rigid, tapered insulation will be added to the roofs to maintain the eave line and preserve the murals painted on the ceilings. Weather-stripping and sun control film also will be added to the extant windows. Nor is the trend limited to the United States. Last year, Pope Benedict XVI announced plans to place solar roofs on several Vatican buildings.

Clearly, then, the trend isn't going away, as sustainability becomes ever more important, and existing stock continues to age. The key is to find the balance between those who advocate to "do what you want -- if it's compatible, it fits" and strict preservationists who won't change a building at all, Elefante said. "I describe it as 'do what you can.'"