From Rooftop to Alleyway, Chicago Fights Extreme Urban Heat With Greener Ideas

C40 Partners with Global Cool Cities Alliance to Tackle ‘Heat Island Effect’ in Cities

The urban ‘heat island effect’ is a term perhaps little known outside policy circles, but the phenomenon will be familiar to city dwellers the world over. Cities are up to eight degrees Celsius warmer than rural areas; this is because built environments absorb more of the sun’s heat, while also emitting heat from air conditioning and other building energy use. Warmer temperatures brought by climate change exacerbate the issue – and its knock-on effects on local air quality – posing a serious threat to the health and well-being of urban populations.

To tackle this issue, C40 has partnered with the Global Cool Cities Alliance to advance policies and actions in cities that increase the solar reflectance of building roofs and pavements. These measures tend to be low-cost, for example coating a roof surface white, but are highly effective, making them a viable solution for emerging cities in developing regions. By cooling cities, we not only reduce climate risks, but also cut greenhouse gas emissions through lower energy demand.

“C40 is excited about collaborating with the Global Cool Cities Alliance, a leader in its field, which brings a wealth of knowledge and experience, as well as a comprehensive tool-kit for tackling the heat island effect in global cities. Through our partnership, we will go far in helping C40 cities develop solution-oriented approaches to lowering urban temperatures”, said C40’s JT McLain who is leading the creation of a global network of C40 cities focused on this issue.

There is a great opportunity for knowledge sharing and collaboration among C40 cities – learning from the experiences of early-adopting cities and accelerating the worldwide dissemination of successful strategies to mitigate urban heat islands.

New York City has taken a comprehensive approach that includes volunteer efforts, a law requiring that new roofs be cool, and a program to monitor progress and results. To date, the city has coated more than 3.2M square feet of rooftops across 388 buildings and engaged nearly 4,000 volunteers. It estimates that every 1,000 square feet of coated rooftop reduces CO2 emissions by 1 ton; and these efforts could save residents $100 million per year in energy costs alone by reducing urban temperatures by as much as 1 degree. New Delhi recently announced that most new flat roofs would have to be reflective in order to qualify for a building permit. Other cities such as Houston, London, and Tokyo have also undertaken measures to reduce the impact of urban heat islands.

“These local efforts have a global impact,” said Kurt Shickman, Executive Director of the Global Cool Cities Alliance. The warming effect of 500 coal power plants-worth of carbon dioxide emissions can be cancelled by transitioning to white roofs where it makes economic sense to do so. Reducing urban heat with cool surfaces helps “These local efforts have a global impact,” said Kurt Shickman, Executive Director of the Global Cool Cities Alliance. “The warming effect of 500 coal power plants-worth of carbon dioxide emissions can be cancelled by transitioning to white roofs where it makes economic sense to do so. Reducing urban heat with cool surfaces helps mitigate the effects of climate change while making our cities more resilient, enjoyable, and healthy. We are very excited to support C40 cities’ efforts to deploy this strategy.”

The two organizations have already begun to work together. This week, they are co-presenting at a the Cool Buildings and Community Conference in New Delhi, India, hosted by the US Department of Energy and the Indian Bureau of Energy Efficiency, and drawing cities from the region to share best practices and experience. Going forward, C40 and the Global Cool Cities Alliance will provide interested C40 cities the tools and resources to assess the issue at the local level and design successful policies and programs.

http://newswatch.nationalgeographic.com/2012/10/04/c40-partners-with-global-cool-cities-alliance-to-tackle-heat-island-effect-in-cities/

Green Roofs in Big Cities Bring Relief From Above

MAY 23, 2012, 7:00 AM

By TINA ROSENBERG

New York Times

It’s spring — time to plant your roof. Roofs, like coffee, used to be black tar. Now both have gone gourmet:  for roofs, the choices are white, green, blue and solar-panel black.

All are green in one sense.  In different ways, each helps to solve serious environmental problems.  One issue is air pollution, which needs no introduction.The second is the urban heat island. Because cities have lots of dark surfaces that absorb heat and relatively little green cover, they tend to be hotter than surrounding areas — the average summer temperature in New York City is more than 7 degrees hotter than in the Westchester suburbs.  This leads to heavy air-conditioning use — not good — and makes city dwellers miserable. For a few people every year, the heat is more than a discomfort — it’s fatal.

The other problem is storm water runoff.  In New York, as in about a fifth of American cities, there is only one sewer system to conduct both rainwater and wastewater. About every other rainfall in New York, sewers flood and back up, discharging their mix of rainwater and wastewater into the city’s waterways. It doesn’t take much to overload New York’s sewers — it can take only 20 minutes of rainfall to start water from toilets flowing into Brooklyn’s waterways.  The water does more than flood streets.  It makes us sick — cases of diarrhea spike when sewers overflow. When sewers back up, polluted water runs into our lakes and oceans, closing beaches.

How can a new roof help?

At 1:45 in the afternoon on August 9, 2001, the temperature in Chicago was in the 90s. Eleven stories up, on the roof of City Hall, the surface temperature of the black tar measured 169 degrees. But Mayor Daley, environmental innovator — yes, that Mayor Daley — had done something interesting. The year before, a section of the City Hall roof had been painted white.  The surface temperature there was between 126 and 130 degrees. And much of the roof of the building, and the adjacent Cook County building, had become a garden — 20,000 plants in 150 varieties, chosen for their abilities to thrive without irrigation and stand up to Chicago’s notorious wind. The surface temperature of the green roof varied between 91 and 119 degrees.

So the difference between a black tar roof and a green roof was at minimum 50 degrees. And the green roof was able to retain 75 percent of a one-inch rainfall.  The two tasks go hand in hand — green roofs cool by capturing moisture and evaporating it.

Putting living vegetation on the roof is not a new idea.  For thousands of years people have made sod roofs to protect and insulate their houses, keeping them cooler in summer and warmer in winter.  The modern movement for green roofs began in the last 50 years in Europe.  Germany, where about 10 percent of roofs are green, is the leader; some parts of Germany require green roofs on all new buildings.

Greening a roof is not simple or cheap. Over a black roof — flat is easiest but sloped can work — goes insulation, then a waterproof membrane, then a barrier to keep roots from poking holes in the membrane.  On top of that there is a drainage layer, such as gravel or clay, then a mat to prevent erosion. Next is a lightweight soil (Chicago City Hall uses a blend of mulch, compost and spongy stuff) and finally, plants.

An extensive roof — less than 6 inches of soil planted with hardy cover such as sedum  — can cost $15 per square foot. An intensive roof — essentially a garden, with deeper soil and plants that require watering and weeding — can double that. But because the vegetation is thicker, it will do a better job of cooling a building and collecting rainwater. Plants reduce sewer discharge in two ways. They retain rainfall, and what does run off is delayed until after the waters have peaked.

A study conducted by Columbia University and City University of New York of three test roofs built by Con Edison in Queens found that the green roof — an extensive roof, planted with sedum — cut the rate of heat gained through the roof in summer by 84 percent, and the rate of heat lost through the roof in winter by 34 percent.

Another study (same researchers, same Con Ed test sites) found that green roofs are a very cost-effective way to reduce storm water runoff.  If New York has one billion square feet of possibly greenable roof, planting it all could retain 10 to 15 billion gallons of annual rainfall — which would cut a substantial amount of sewage overflow. “If you add in all the other green infrastructure, such as street trees, permeable pavement and ground collection pits, it might be possible to eliminate the combined sewage overflow without building specialized water detention tanks, which are hugely expensive,” said Stuart Gaffin, a research scientist at Columbia’s Center for Climate Systems Research, who co-authored both studies with colleagues from City College.

Green roofs have other advantages.They scrub the air: one square meter can absorb all the emissions from a car being driven 12,000 miles a year, said Amy Norquist, chief executive of Greensulate, which installs green roofs.And green roofs can provide the plants that animals, birds and bees need where parks are far apart.

White roofs are cheap and don’t require any engineering — just a layer of special paint. New York City is trying to coat a million square feet of roof a year. Building owners can do the work themselves, or they can engage CoolRoofs, a city initiative that promotes white roofs and organizes hundreds of volunteer painters. Since 2010, about 3,000 volunteers have coated 288 buildings.

But less investment buys less return. White roofs don’t catch rainwater, help biodiversity or clean the air. Gaffin’s group found that the white portion of the Con Ed roof averaged 43 degrees cooler than black at noon on summer days. That’s something, but it’s a smaller cooling effect than green roofs offer. Green roofs improve each year as vegetation becomes denser and taller.But after a few months, a white roof tends to look like city snow — covered with soot. As a white roof dirties, it loses a lot of its cooling ability.

The newest roof variation is a blue roof. It’s a roof covered by a waterproof membrane and gravel, with controlled-flow drains, and costs about $5 a square foot. Blue roofs don’t cool anything — they help only with storm water control by releasing water more gradually.  Despite the price, a blue roof is a hard sell — not everyone is comfortable with the idea of a pond on the roof.

How have New Jersey and California done it?  Private vendors install and maintain the solar panels, and are paid in future energy savings. Scott Stringer, the Manhattan borough president, argues that New York should use this system to put solar panels on the roof of every public school.  Stringer’s report says putting solar roofs on all available public schools would eliminate as much carbon emissions as planting 400,000 trees — eight times the number in Manhattan now.The fourth roof option doesn’t save energy — it creates it. New Jersey has installed 500 megawatts of solar power — enough to run half a million homes. California has installed double that. New York City?  So far, just 6.5 megawatts.

Public schools have become a testing ground for the new roofs. At the Robert Simon complex in the East Village, which houses three schools (my children attend two of those schools), work is beginning this summer on a farm. A committee at the Earth School was looking for green ideas that would go beyond recycling and create a curriculum.  Abbe Futterman, the science teacher, was already growing vegetables and fruit in sawed-off pickle barrels right outside her classroom window, using the garden to teach plant science and nutrition. The kids tend it, and use the produce to cook food from around the world.

The Fifth Street Farm will be a much larger vegetable and fruit garden in planters raised above the roof on steel girders — not a classic green roof.The money has come from various government offices — those of Stringer, State Senator Daniel Squadron and City Council member Rosie Mendez.  Douglas Fountain, the architect who is implementing construction (and a parent of a Tompkins Square Middle School student) said that it was designed to be easily replicable by other schools.

Is a green roof a good investment for a building owner? Perhaps, but the biggest reason might not be reduced energy costs — lots of factors affect a building’s energy use.  More savings come from the fact that temperature swings make a black tar roof expand and contract. The smaller the spread, the longer the roof life. Roanoke, Va., for example, just installed a green roof on its municipal building, at a cost of $123,000, adding anywhere from 20 to 60 years to the life of the current roof membrane. “I personally believe a green roof is the last roof you’ll have to put on,” said Gaffin.

But any changes to a black tar roof are undoubtedly good investments for cities — indeed, interest in green roofs is soaring largely because of the sewage problem and the costs of trying to solve it the old way.  New York City decided it was more cost effective to build green infrastructure, including green roofs, than to construct more sewer pipes or storage tanks, and it is spending $1.5 billion over the next 20 years on green projects that will reduce rainfall runoff.   The goal is to cut sewer outflows by 40 percent by 2030.

“The good news is that this is a ‘no harm’ intervention,” said Carter Strickland, the commissioner of the Department of Environmental Protection.  “People want it; there’s a lot of other benefits.  If at the end of the day it doesn’t do the full job, whatever you have to build on top will be much smaller and less expensive.”

To encourage building owners to install green roofs, New York City has a pilot program that will end next year offering a $4.50-per-square-foot tax abatement for green roofs that cover more than half a rooftop.  (There are also tax abatements for solar-panel roofs.)

Amy Norquist of Greensulate said this was less attractive than it sounded.“What you have to do to get that is quite onerous,” she said. “You need permits, filing fees, people have to sign off — it ends up being a lot of money.”

Strickland said that permits were required for a good reason.  “It requires you to do a structural analysis of the roof,” he said.  “You’re going to need that permit whether you build it with a tax abatement or alone.”

New York City was not one of the first American cities to promote green roofs. “But the city is doing quite well,” said Gaffin.  “The green infrastructure plan is very ambitious.”  The problem is that the little-by-little approach won’t produce real environmental benefits until they reach a critical mass, and that could take a long time.  “We get biodiversity benefits from small scale greening, and individual building owners will get an energy benefit,” said Gaffin. “ But to make a difference to the city’s climate or hydrology we’d have to get up to 30, 40 or 50 percent coverage. What we have now is a drop in the bucket.”

How to Build a Greener City

Bike lanes, micro wind turbines, pneumatic garbage collection—and other ways to make urban areas more environmentally friendly


Wall Street Journal

By MICHAEL TOTTY

Can cities be part of the environmental solution instead of part of the problem?

The question isn't an idle one. Urban populations around the world are expected to soar in the next 20 years, to five billion from more than three billion today. If the current rate of urbanization holds steady, cities will account for nearly three-quarters of the world's energy demand by 2030. Most of the increase will come in rapidly developing countries like China and India; China's cities alone will have to deliver water, housing, transportation and other services to 400 million additional urban dwellers by 2030.

"There's going to have to be new forms of energy, new ways of delivering energy and new forms of infrastructure," says Warren Karlenzig, president of Common Current, a consulting firm on sustainable cities based in San Anselmo, Calif. "All this will be necessary to allow cities to operate the way they do now."

So, cities aren't going to have be made a little greener; they're going to have to be rethought from the ground up. The goal: compact living environments that require less resources and that get the most out of the land, water and energy they do use.

It wasn't long ago that the idea of using "green" and "city" in the same sentence seemed absurd. Cities were considered a blight on the environment: energy-hogging, pollution-spewing, garbage-producing environmental hellholes. But in recent years, they've begun to be seen as models of green virtue. City dwellers tend to walk more and drive less than their suburban counterparts, and dense urban development encourages transit use. Apartment living generally means lower per-household energy use.

Building on these strengths, planners and developers are devising innovative solutions to meet urbanites' energy, water, transportation and sanitation needs well into the future.

Some improvements are fairly easy, such as switching to energy-efficient LED lighting in buildings and streetlights, or setting aside bike lanes and widening sidewalks to encourage alternatives to driving (although such moves aren't without political hazards, as a recent battle over bike lanes in New York shows). Others are more ambitious, requiring new construction or even an extensive rebuilding of city infrastructure—consider what is needed to add a second set of pipes for a water-reuse system.

Some of the most ambitious projects—and the greatest source of innovative ideas—are the dozens of "eco-city" developments in the works or on drawing boards around the world. Projects like the Songdo International Business District near Incheon, South Korea, are testing grounds for the latest in green technologies.

But green initiatives aren't just found on blueprints for new cities. Chicago, for example, has about 350 green-roof projects covering more than 4.5 million square feet.

Of course, many of these initiatives can be expensive, with high up-front costs. Urban planners say savings from lower energy bills and other operational efficiencies can more than cover the added expenses, but the break-even point can be years out. Still, cities—unlike the average homeowner considering rooftop solar panels—can take a long view and make investments with a decades-long payback.

So, how can cities—old or new—take green to a new level? Here's a look at some of the ways.

District Heating In a typical office building, heating and cooling account for nearly two-thirds of total energy use. So an alternative to traditional electricity or natural-gas HVAC systems can go a long way toward making cities greener. One solution: tapping the excess heat produced by nearby utilities or industry. A network of pipes distributes the heat, which can be used for hot water, space heating and in absorption chillers to provide air conditioning in the summer. These district heating systems are considerably more efficient—capturing up to 90% of the available energy—than in-building boilers. And they can tap any number of heat sources, including high-efficiency natural-gas turbines, large-scale solar thermal systems, biomass incinerators or furnaces in a steel mill. Common in Europe, high-efficiency district heating systems are being used in South Korea's Songdo IBD and are in the plans for other eco-city developments.

Micro Wind Turbines The giant windmills that dot the countryside aren't suitable for cities, where vibrations can rattle windows and the noise would be annoying. So developers are turning to microturbines. These small generators sit atop commercial or residential buildings and are designed to take advantage of the quirks of big-city wind patterns—lots of turbulence and frequent, sudden shifts in direction. The turbines are generally small, rated at one to three kilowatts each. But when installed in arrays and combined with high-efficiency solar panels, they can generate a large share of a building's energy needs, especially when the structure is equipped with a full set of energy-saving features. A handful of companies provide micro wind systems around the world, and the devices, while more expensive per kilowatt than bigger systems, have been installed at scores of locations, including PepsiCo Inc.'s Chicago office building.

Pumped Hydro Storage/Micro Hydropower Wind and solar power are notoriously fickle, producing more power than needed at some times and less than needed at others. A city that wants to rely on such intermittent sources needs to find a way to bank that power. One technique: pumped hydroelectric storage. When wind or solar power is plentiful, electricity is used to pump water to an upper reservoir; later, when power is needed, the water is allowed to flow downhill, turning turbines in the process. (The lakes have the added benefit as open-space landscaping.) Large-scale pumped-hydro systems are increasingly used for storing energy, and many isolated towns rely on small-scale micro hydro plants to generate electricity. Adding a pumped-storage capability isn't technically difficult, but it's expensive, especially on a small scale, and current technology generally requires a large "drop," or change in elevation to produce much power—though companies are working on lower-flow hydro turbines that can work in more level settings.

Walking and Biking When it comes to transportation, dense urban areas like Manhattan already have an advantage over suburbs: By packing people, jobs and services close together, they reduce the need for many car trips and provide the density to support bus and transit services. Green-city planners do even more, designing streets so that walking is safe, convenient and interesting—with wide sidewalks, landscaping and abundant crosswalks—and providing separate designated bicycle lanes. Songdo's 1,500 acres are designed so that most shops, parks and transit stops can be reached in less than a 15-minute walk, and the city also has a 15-mile network of bike lanes.

Personal Rapid Transit Not every urban trip can be made on foot, bicycle or public transit. Cities can encourage greener auto choices by providing electric-vehicle charging stations in parking garages. A futuristic solution: personal rapid transit, or PRT—pod-like, self-powered vehicles that can carry as many as six passengers. The vehicles can travel along dedicated roadways, like an automated airport transit system, or on streets equipped with buried magnets. There are no fixed schedules or routes; passengers pick their destinations, and a central computer guides the car without intermediate stops. Although still a novelty, PRTs are operating at Heathrow International Airport near London and at the Masdar Institute of Science and Technology in Masdar City, an eco-city development in Abu Dhabi. Masdar, however, has put on hold plans to deploy the pod cars throughout the entire planned two-square-mile development.

Pneumatic Garbage Collection Even the greenest cities produce lots of garbage, which creates two problems: collecting the trash and getting rid of it. On the collection side, a centralized waste system, using an underground network of pneumatic tubes, can replace the fleets of trucks that block traffic, tear up streets and burn fossil fuels. The tubes can collect garbage from both households and outdoor trash bins and carry it to a centralized collection and sorting facility. Though some systems handle only food waste, others are set up to handle separate streams for paper and other recyclable trash. The systems are used in scores of cities world-wide; a pneumatic trash-collection system on New York's Roosevelt Island has been in operation since 1975.

Waste to Resources Getting to zero waste is as important to cities as getting to zero carbon. This doesn't mean just encouraging residents to recycle—cities also can deploy technologies to tap the energy and other valuable resources buried in the trash. Advanced anaerobic digesters process organic garbage waste and the sludge left over from treating wastewater to produce biogas, which can be burned for energy; more common in Europe, the technology is just being deployed in the U.S. for handling municipal garbage. High-temperature plasma-arc gasifiers can consume nearly the entire waste stream, making a synthetic gas that is burned to produce electricity; the leftover slag can be used in building materials. One novel approach under consideration by the PlanIT Valley project, an eco-city development planned for northern Portugal: Aluminum cans are processed with water and energy, producing aluminum oxide and hydrogen, which can then be used to power fuel cells. But because aluminum oxide requires tremendous energy to make aluminum, it may be more economically feasible just to recycle aluminum containers.

Green Roofs Rooftops, which take up a fifth of urban surface area, can be used to support solar panels or wind turbines, but they're otherwise underutilized. Covering the tops of buildings with grasses, shrubs and other plants can deliver a host of benefits. Though often more costly than traditional coverings, green roofs can provide insulation and trim a building's heating and cooling needs. They absorb rainwater, reducing the load on storm-water systems, and filter what water does run off so it can be used for many domestic needs. They also filter air pollutants.

Mr. Totty is a news editor for The Journal Report in San Francisco. He can be reached at michael.totty@wsj.com.
http://online.wsj.com/article_email/SB10001424053111904009304576535113877346554-lMyQjAxMTAxMDEwMjExNDIyWj.html?mod=wsj_share_email#articleTabs%3Darticle

With cool hand, Mayor Bloomberg helps paint town white for energy efficiency

BY ADAM LISBERG AND BOB KAPPSTATTER
DAILY NEWS STAFF WRITERS

Thursday, October 14th 2010, 4:00 AM

Florescu for News

Volunteers from NYC Service cover E. 146th St. Bronx rooftop with a white coating to help reduce cooling costs and energy use.

But he was covering a rooftop and not a scandal. Hizzoner wielded a paintbrush to slap a reflective white coating to the 1 millionth square foot of rooftop as a part of the NYC °CoolRoofs program.Mayor Bloomberg found himself involved in a whitewash Wednesday.

Trading his business suit for sneakers, jeans and an orange T-shirt, Bloomberg and other officials went to the roof of theNew York City Housing Authority's Betances Development in the South Bronxto publicize the milestone.

With help from the Buildings Department, more than 1,500 volunteers organized by NYC Service - the mayor's volunteer initiative - have added the reflective coating to more than 340,000 square feet of government buildings and 70,000 square feet of NYCHA rooftops alone, to help reduce cooling costs, energy use and greenhouse gas emissions.

Officials predict the program will help the city's efforts to reduce greenhouse gas emissions by 30% by 2030, a prime goal of PlaNYC, the city's comprehensive sustainability plan.

"By simply applying a reflective, white coating, we can reduce rooftop temperature by up to 60 degrees, which translates into reduced cooling costs and reduced carbon emissions," said Bloomberg.

The mayor added that homeowners and landlords need to seriously think about the benefits - reducing their energy costs by 25% or more.

"Next time you get your Con Ed bill, say, 'Hey, wait, maybe I should do it. ... The landlords get paid back so quickly. This is in their interests. It's sort of a no-brainer."

A roof with reflective, white coating - known as a cool roof - absorbs 80% less heat than traditional dark colored roofs and can lower indoor temperatures by 10 to 20 degrees on hot days, reducing the need for air conditioning - and reducing the potential for brownouts and blackouts.

If all roofs were similarly coated, officials said it could drop the city's overall ambient air temperature by 1 degree.

That may sound small, but because of all its concrete and roadways and general lack of green space, officials said the city suffers from an urban heat island effect, with temperatures anywhere from 5 to 7 degrees higher on hot days than the surrounding region. In the South Bronx, it can stretch up to 10 degrees 

http://www.nydailynews.com/ny_local/bronx/2010/10/14/2010-10-14_cool_hand_mike_helps_paint_town_white_for_energy_efficiency.html

Wall Street Journal

Up on the Roof

Adding a layer of vegetation on the top of buildings can keep things cooler in the summer and warmer in the winter

By SHELLY BANJO

A growing number of urban residents are happy to have a green roof over their heads.

Buildings topped with ground cover, shrubs and other greenery are sprouting up across the country. Such roofs cost more to build than conventional roofs, because of the extra materials and labor.

But they also offer savings on energy, reductions in storm-water runoff and a possible way to lower urban temperatures in summer. So, many cities have started offering builders tax and other incentives to adopt the features.

Even without the incentives, the extra cost of building a green roof -- a 30% to 50% premium, compared with a conventional roof -- can be recouped within about five years, says Ed McMahon, senior resident fellow for sustainable development at the Urban Land Institute in Washington, D.C.

Green roofs provide a natural layer of insulation, keeping buildings cooler in summer and warmer in winter. The energy savings vary depending on materials and location, but according to one study, by the National Research Council of Canada, a green roof can reduce the average daily energy demand of a building by more than 75%. A study by a Columbia University group, meanwhile, found that green roofs can reduce the average daily energy demand of a building by about 15%.

Urban Thatch

Such insulation prevents some of the temperature-driven expansion and contraction that takes place when a roof is completely exposed to temperature extremes. Experts say that while a typical asphalt roof can reach 160 degrees on a summer day, green roofs rarely exceed 80 degrees. A protective layer of greenery "doubles the life of the roof," says Leslie Hoffman, director of Earth Pledge, a New York nonprofit that acts as a consultant on sustainable environmental practices.

Vegetation also captures rainfall and can slow the rate at which runoff goes into storm-water systems, reducing the load on urban sewers -- and possibly the tax burdens of city residents. When runoff is produced by heavy rains, it can "overwhelm city water systems...becoming a very expensive problem for municipalities, as well as violating the Clean Water Act," says Ms. Hoffman.

"As individuals, we don't think about storm water as having an effect on our bottom line," she adds, but cities build elaborate storm-water-management systems, and "we pay those costs through taxes."

View Slideshow

Earth Pledge

TOP GROWTH Green roofs on residential commercial buildings in Kansas City, Mo.

On a larger scale, green roofs may help ease global warming by reducing what is known as the urban-heat-island effect. Heat islands are created when dark-colored, impermeable surfaces absorb heat energy and radiate it back into the air. The effect makes cities two to 10 degrees hotter than nearby suburbs, increasing energy use and causing health problems as well. "As the temperature goes up, the air quality goes down, affecting the overall health of the cities' residents," says Amy Norquist, founder of Greensulate Inc., a consulting firm based in New York and California that focuses on environmentally friendly building techniques.

According to a study by Canada's environmental agency, Environment Canada, planting vegetation over at least 6% of Toronto's total available roof space (70 million square feet) could reduce summer temperatures in the city by 1.8 degrees. Such a reduction, they predict, would subsequently lower cooling demands by 5% for all buildings in Toronto, and that decrease in energy use would reduce temperatures an additional degree.

Another boon: Lower roof temperatures prevent overheating in solar-energy panels placed there, and thus make them more efficient, Ms. Norquist says.

While most green roofs have been installed as part of new buildings, retrofits are increasing. However, these aren't "do-it-yourself projects," Ms. Hoffman warns. Just as you wouldn't attempt to build a traditional roof, installing a green roof may require input from an architect, landscape designer and green roof consultant, among others. Building owners must be careful to check out any insurance policies that apply. Weight and fire hazards may exist.

Going to Sedum

Maintenance costs depend on the complexity of the project. The simplest of green roofs are planted with a 3-inch to 4-inch layer of grass or ground cover, like sedum, a plant whose leaves store water. Such greenery requires minimal maintenance. "These could go months without any kind of watering," Ms. Norquist says.

More elaborate projects, including flowers, vegetable gardens and trees, require continuing attention. But the aesthetics can add to a property's real-estate value as well. "It's definitely an amenity," says Michael Gubbins, resident manager of the Solaire, an environmentally friendly residential building with two rooftop gardens in the Battery Park City area of New York.

While green-roof technologies have been around for years in Japan and in some European countries, like Germany and Switzerland, the idea started gaining ground in North America only in the past decade. In 2001, Chicago Mayor Richard Daley, inspired by a trip to Hamburg, Germany, decided to cover about 20,000 square feet of the roof atop Chicago's City Hall.

City officials in Chicago now estimate that their green roof saves the city about $3,600 a year in cooling and heating costs. If all of Chicago's roofs were greened, they add, the city's peak energy demand could be cut by 720 megawatts -- enough electricity for 750,000 people. The load on the city's storm-sewer system, meanwhile, would be slashed by about 70%.

Mayor Daley says that Chicago offers grants of as much as $5,000 toward the cost of installing a green roof, and that buildings with plans to install green roofs are pushed through the permit process quicker.

"We decided that the city had to lead by example," by installing green roofs on government buildings and then "offering economic assistance to get the private sector involved," the mayor says.

"Now everything from libraries, schools and apartment buildings have green roofs," he says. Mr. Daley estimates that Chicago now has about 400 green roofs in the downtown area.

Earth Pledge

Green roofs on commercial buildings in Kansas City, Mo.

The city of Chicago also has begun hosting conferences about green roofs for other U.S. city leaders.

Cities from Tokyo to Toronto, New York, Washington, Portland, Houston and Seattle have gotten involved, offering builders and companies tax incentives, rebates and higher-density building allowances for installing green roofs. That means builders can get permission to construct bigger buildings in highly coveted downtown spaces if they incorporate a green roof into the plan. Tokyo requires new buildings with total floor space of more than 10,000 square feet to make at least 20% of their usable roof area green.

New York's Battery Park City in 2002 started requiring new buildings to plant greenery covering 75% of the nonmechanical area of their roofs. That rule has resulted in about two acres of green roofs installed or planned in the neighborhood since then -- about a third of a typical New York City block. Then, in June of this year, the city of New York began offering a one-year property-tax credit of as much as $100,000 to building owners who install green roofs on at least 50% of available rooftop space. The credit would be equal to $4.50 per square foot of planted area, or about 25% of the typical costs associated with the materials, labor, installation and design.

The amount of green roofing in New York more than doubled to 123,074 square feet from 2004 to 2007, according to surveys by Green Roofs for Healthy Cities-North America Inc. That is an area roughly equal to about 10 Olympic-size swimming pools. Green Roofs, an industry association based in Toronto, says the study polled only members of its association. But the group estimates that its survey reflects about 60% of all green-roof activity in the U.S. and Canada.

Earth Pledge

The layers of one kind of green roof system.

http://online.wsj.com/article/SB122305471179402733.html?mod=googlenews_wsj

—Ms. Banjo is a staff reporter for The Wall Street Journal in South Brunswick, N.J.