Tim Toben is worried about sustainability. “We’re going to build our population by 50 percent in the next 10 years,” he says. “Where are these people all going to go? Are we going to keep building these Southern Village developments, or are we going to utilize the city spaces we have?

“This is something we have to pay attention to, or we’re going to find ourselves looking like Houston, Texaswhich nobody wants.”

Toben is doing more than just worrying. As one of the partners behind Greenbridge, a $30 million pair of mixed-use towers planned for West Rosemary Street at the border between Chapel Hill and Carrboro, he is hard at work providing space for at least some of the Triangle’s future residents. When constructed, Greenbridgewith 100 condominium units and more than 25,000 square feet of retail spacewill boast the tallest building in Chapel Hill at 135 feet, or 10 stories.

Before the Chapel Hill town council approved the project for construction last month, the maximum allowable building height was 90 feet.

But it’s not only its size that makes the project stand out. Once built, Greenbridge will be poised to become the first development in North Carolina to receive gold certification in the Leadership in Energy and Environmental Design (LEED) standard, a voluntary national program for energy-efficient, environmentally conscious architecture. The development will “set a new standard in our state for environmentally conscious development,” according to Larry Shirley, director of the North Carolina State Energy Office. “Ten years from now, our dream is that there are not one, but 1,000 such developments in the making across our state.”

Greenbridge is being designed by William McDonough and Partners. McDonough, profiled in 1999 in TIME magazine as a “hero of the planet” for his work in sustainable architectural development, designs green buildings all over the world, and recently signed a contract to build the new Google corporate campus in California. The firm was enticed to Chapel Hill by the dedication of the Greenbridge partners and McDonough’s personal friendship with Toben.

“In this case, the initiative is real,” says Josh Gurlitz of GGA Architects, the architect of record for the project. “I’m a skeptic sometimes, but [Tobin and his partners] convinced me, a long time ago, that they’re for real. These partners have been very single-minded in the area of sustainabilitynot only energy efficiency and greenness, but also social equity and the other issues that make buildings perform in terms of their social function, not just their bricks and mortar.”

But what makes Greenbridge so green? Every aspect of the project has been fine-tuned to ensure a balance with what Toben calls “the triple bottom line”: environmental sensitivity, social equity and economic viability. “If you have those three things, you have a sustainable project. And we think we have those things.”

Consider Greenbridge’s shape. Most buildings of its sort would be rectangular, straight and flat, with everything at 90-degree angles. Greenbridge is irregular and angled, and split down the middle into two separate unitsa unique shape that will allow 90 percent of the units to be day-lit.

The roofs of the buildings will be vegetative, literally gardens. This is not just for appearances; the sod roof cools the building below, reducing energy costs, as well as reducing rainwater run-off in the storm drains and releasing oxygen into the atmosphere. “In general, our firm’s designs seek to create rooftops that are productive in some way. All the rooftops in Greenbridge are doing something,” says Mark Rylander, a director at William McDonough. “It’s not just a roof membrane; it’s valuable real estate.”

Other roof spaces, and possibly the walls of the building itself, will be paneled with solar photovoltaic and solar thermal collectors, further reducing the building’s net energy expenditure. A cistern will collect rainwater not absorbed by the garden roofs for use in the building’s cooling system.

In the units themselves, energy-efficient lighting and Energy Star appliances, as well as low-e (emissivity) windows, low-flow toilets and waterless urinals, and a top of the line fresh air delivery and heat recovery system, will further save on energy and resource costs. Air quality in particular is something the designers have worked to improve. “One of the real problems in our society today is that the air quality indoors is very often poorer than the air quality outdoors,” Toben says. “Since human beings today spend 80-90 percent of their time indoors, this is something you have to pay attention to.”

The construction materials themselves are being chosen in accordance with what McDonough calls his “cradle to cradle” philosophy. Green building materials have been used wherever possible, including sustainably harvested or reclaimed renewable woods, such as cork in the floors and bamboo in the cabinetry. Moreover, Tobin promises, the Greenbridge developers will be assiduous in recycling their construction waste: “A number of materials that are put into landfills still have utilitythe landfill is just more convenient. Rather than going into the landfill, we’ll be recovering between 80-90 percent of the material for use in building.”

Even the building itself has been designed with an eye to its eventual demolition. “We try to use simple materials in a pure state rather than composites made of incompatible material. The best materials are those that return to nature safely or can be remade into new products perpetually as part of the technical cycle. We detail materials to be disassembled and re-used at some point in the future,” Rylander says. Where adhesives and paints must be used, great care is being taken to use materials that have low VOC (volatile organic components)looking for the healthiest possible materials and installation techniques.

One of the most innovative aspects of the Greenbridge development is the attention being given to changing the way its residents actually live. The designers have been looking at ways to help keep residents engaged with their energy use and carbon footprint, and are thinking of putting displays into each of the units that inform the owners just how much energy they’re using, compared to last year or some other baseline. “We’re looking for an interface that gives residents the information to make the right decisions,” Rylander says. “We want to help the building grow smarter over time.”

But the single largest environmental feature of Greenbridge is its location. Rather than building out in the surrounding rural bufferwhere new infrastructure would have to be put in, trees cut down, roads paved, power lines raised and pipe laidGreenbridge is making use of underutilized land in downtown itself. “This is a classic case of contrasting with suburban sprawl,” Toben says. “Typically, if you look at the average amount of land used in a typical sprawl development, you’re looking at between one and 10 acres per residence. Greenbridge is 100 residences on an acre and a half of land.”

The benefits don’t end there. “In many ways the location, density and underground parking are the most significant, sustainable features that tend to be taken for granted, or not even counted, in the LEED rating system,” Rylander says. “The number of car trips that are reduced by committing to build inside a town has environmental benefits that aren’t as easy to calculate as reducing the energy use, but they really need to be counted. If you can walk to work, or to dinner, and not be getting in your car to do every last thing, the environmental externalities of your living arrangement are completely changed.”

Even if it weren’t LEED Gold, even if all the other innovations were taken off the table, Greenbridge (with 128 planned bicycle parking spaces, and showers and lockers for non-residents who want to bike in to work) would be environmentally friendly just by getting cars off the road, Toben says. “I could probably swap out half of these green features and it would still be a great environmental contribution, just for being in town.”

Of course, there have been controversies. There has been much concern about the plan for the building, especially its proposed height. Some Chapel Hill residents worry that Greenbridge will change the character of the surrounding neighborhood, and perhaps the character of Chapel Hill as a whole. And compromises have already been made as a resultgetting back, once again, to Toben’s triple bottom line.

“One big architectural move was to place the buildings running in a north-south direction,” Rylander says. “If we had run them east-west there would be a huge shadow across the courtyard and across Rosemary, all day long. There may have been some tradeoff in energy efficiency, but we decided it would be best for the building and the community to break the building in that way.”

Other compromises in pure environmental efficiency have come from the dreaded third rung of the triple bottom line, economic viability. “The problem with being first and early is that there aren’t many of [these sorts of buildings], so the cost of each of these green features is high,” Toben says. “Once these features and attributes catch on, and they’re in the mainstream and widely accepted, the cost of these things will drop precipitously. But this is the first in the state.”

“We’re paying a premium for these features, and the market will only bear a certain amount of price.”

But, he says, it’s worth it. “The greatest injustice in the world today, in my view, is that 5 percent of the world’s populationAmericanscontribute 30 percent of the world’s greenhouse gases. If you look at the effect on the world as a whole, the consequences are immense.” The sort of people who will buy into Greenbridge, he predicts, are “environmentally and socially conscious buyers who are willing to pay a premium to reduce their carbon footprint. The early adopters are people who really value walking lightly on the earth.”

And of course the question mark hanging over all this is the future cost of energy. “When energy is four times the cost, anything you invest in as a first-cost strategy is paying for itself four times faster,” Rylander points out. If the cost of energy goes up, as it is likely to, investment in ecologically sustainable developments like Greenbridge makes more and more sense.

For now, though, environmental sensitivity and economic viability often find themselves in conflict. “I think this project demonstrates the difficulties of this sort of environmentally minded development,” Carrboro alderman Dan Coleman says. “Because of the costs involved, they’ve had to restrict this building to a very high-end marketI think the challenge for builders and for government will be to distribution these sorts of innovations and features more broadly.”

As a condition of zoning approval, the Greenbridge developers have promised to subsidize 15 of the units for the benefit of families earning less than the median income in Chapel Hill. However, the market rate for the remaining 85 units will start between $225,000 and $350,000with the largest and most expensive units going past $1 million.

Despite the current high cost of environmental living, Josh Gurlitz hopes that Greenbridge won’t be alone for long. “I certainly hope that this is going to be a leader and a pathmaker for the way projects can be done. That’s what we’re all hoping for, that this will be a great building in itself and a demonstration of a variety of strategies that other people can adapt.”

Believe it or not, so far has been the easy part. “We’ve only gotten so far as the permits to build the thing,” Toben says, with what sounds like a 50-50 mix of enthusiasm and exhaustion. “Now we’ve got to build it.”