The architecture of American embassies has been stuck lately in a predictable tug-of-war between a desire to express openness and an obsession, in an age of terrorism, with security. The design for the new U.S. Embassy in London, released Tuesday morning by the State Department, finds a novel way to move past that split and take diplomatic architecture into fresh territory.

Designed by the Philadelphia firm KieranTimberlake, the proposed building makes an argument that an American embassy should do more than simply symbolize transparency, which all too often means a facility wrapped in glass but secluded deep inside an impenetrable suburban compound. Instead it aspires to a different and broader set of values, primarily having to do with ecological responsibility and neighborliness within a tight urban fabric.

The design suggests that, rather than standing in for certain American virtues, what a contemporary U.S. embassy should be doing is behaving virtuously. KieranTimberlake, in a written description of its concept, refers to the range of positive ways in which the building will "perform," both as an example of sustainable architecture and as a piece of urban design.

Even as the design itself, for all its airiness and crisp confidence, is hardly radical from a formal point of view -- it consists of a cube sheathed in a shimmering polymer scrim and resting on a ground-floor colonnade of concrete pillars -- it represents a major shift in how we think about the role of U.S. government architecture, both at home and abroad. It suggests putting an emphasis on action instead of values, measurable behavior rather than symbolic gestures.

Because this is the first major embassy design to emerge from Barack Obama's year-old administration, of course, it is tempting to see signs in the architecture of his own political priorities. As it turns out -- even though the embassy plans were first sketched out before Obama was sworn in, and the building won't be opened until he is out of office -- they are stamped all over the KieranTimberlake proposal.

Fans of the president may admire the embassy's forthrightness, its cosmopolitanism and its willingness to admit, and address, environmental and other problems. His critics may complain that the building, by taking such pains to be restrained, polite and energy-efficient, seems eager to apologize for the aggression or wastefulness of American culture.

Whatever side of that debate one comes down on, it seems clear that it was KieranTimberlake's careful attempt to balance formal, urban and environmental priorities that helped it prevail in a high-profile design competition for the embassy, which was organized by the State Department's Bureau of Overseas Buildings Operations. The runners-up were Thom Mayne and his Santa Monica firm, Morphosis; Richard Meier & Partners; and Pei Cobb Freed, the office founded by I.M. Pei.

The KieranTimberlake building will fill a roughly five-acre site along the southern edge of the Thames, in a district called the Nine Elms opportunity area, and replace Eero Saarinen's 1960 embassy in Grosvenor Square, which preservationists in London have been fighting to save for a post-diplomatic life. The State Department hopes to break ground on the new embassy in 2013, with a planned opening date of 2017.

At ground level, the architects, working with landscape architect Laurie Olin, have tried to engage the neighborhood despite security guidelines that require the building to sit back within a circular zone of blast protection. A park will wind like a corkscrew from the riverfront onto the embassy grounds and into the building itself, which will feature a number of interior sky gardens, including a two-level Ambassador's Garden on the upper floors. A formal plaza leading to an entrance for embassy employees and dignitaries faces east, and the general public will enter through a curving path lined on both sides by greenery.

The embassy will sit safely on its northern edge, where it faces the river, behind a protective semicircular pond. On the other sides, parking and some meeting rooms are tucked away securely under undulating landscaped mounds designed to do double duty as green space and protective barriers. Although this proposal represents a move away from the bunker mentality that has marked so many recent U.S. embassies, it will likely be a stiff challenge to keep the building from looking armored at pedestrian level. The move from Grosvenor Square to the new location, after all, was in large part driven by a desire to build a more easily protected facility.

Whether the new embassy seems open to the city around it may be a matter of perspective. For Londoners who are accustomed to the accessibility of Saarinen's building and who may even remember the days when it was possible to walk almost effortlessly inside, the new facility may appear hardened against attack. But compared with recent U.S. embassies in other cities -- in Beijing and Baghdad, to name two -- this one makes a noticeable effort not to turn away from urban life.

The embassy's cubic form will be wrapped on three sides in ETFE -- a transparent polymer and the same flexible material used on the outside of the Water Cube swimming arena for the 2008 Beijing Olympics -- that will not only shade the building but also hold flexible solar panels. (The north side, facing the river, is left uncovered.) The roof, meanwhile, will be covered with an array of solar panels that will hide mechanical equipment while producing electricity.

In a phone interview Monday, KieranTimberlake's James Timberlake said the goal is a building that creates more energy on balance than it uses.

Ultimately, the embassy's success as a piece of architecture will largely depend on how the ETFE scrim operates visually. If the architects can manipulate it to curl or bend dramatically away from the boxy form underneath or to reflect light in novel ways, the building will likely carry a sculptural power to go with its operating efficiency. If not, it may fade into the skyline, resembling a well-tailored but conventional office building.

The designs by the other finalists in the competition -- single images of which also were released Tuesday by the State Department -- suggest that the jury likely had a relatively clear choice in the end. The Meier proposal, with its oddly slumping, humpbacked profile, and the one by Pei Cobb Freed, calling for an oval form wrapped in a crosshatch window pattern and sitting atop a squat pedestal, both lack the crisp, light-on-its-feet appeal of the winning design.

The entry by Mayne and Morphosis, on the other hand, has the coiled, contorted power of the firm's most provocative work, calling for an undulating but also sagging tower rising unsteadily atop skinny pillars next to a series of jagged underground spaces that suggest military bunkers.

If conservative critics may find certain elements to dislike in the KieranTimberlake design, they would have positively howled over Mayne's design, which suggests not an embassy stoutly fortified against possible attack but one that already has absorbed some major blows. Still, I hope the State Department will allow his firm to publish the entire design in the coming days and not just the single rendering released Tuesday. It certainly says something about the way the selection process for embassies has evolved that Mayne's proposal -- the inspired, tightly wound work of an architect who has often used his buildings to advance a sharp critique of bureaucratic power -- made it to the final round.

-- Christopher Hawthorne

Images: Embassy renderings. Credit: U.S. State Department / Bureau of Overseas Buildings Operations