By Mary Kelley Hoppe

Stand on an asphalt road or rooftop in downtown Tampa on a sweltering summer’s day and you can feel the urban heat island effect. On hot summer days, cities can be 6 to 8 degrees hotter than their suburban counterparts.

That’s because paved and dark-colored surfaces absorb, rather than reflect, the sun’s heat, causing surface and ambient air temperatures to rise.

And heat isn’t the only problem. Hotter air in cities increases smog-producing ozone, prolongs and intensifies heat waves, and increases demand for electricity to cool buildings, which, in turn, amplifies climate change through the burning of fossil fuels.

Turning down the heat is relatively easy, with planning and changes that can be incorporated in new construction and redevelopment, and promoted through building incentives.

Think cool roofs, cool paving, trees, trees and more shade trees. Cities across the U.S. are getting the message.

In Houston, the city’s “Cool Houston” plan calls for the widespread use of cool-paving technologies for new and existing parking areas, new streets and road resurfacing; cool-roofing materials on all flat roofs; and 10 million new trees in 10 years through public-private partnerships, combined with vastly improved conservation efforts.

In Chicago, vegetated “green roofs” are sprouting atop inner-city buildings in impressive numbers, thanks to attractive incentives including fast-track permitting for developers who install them. While green roofs cost more than their conventional counterparts, their documented advantages – energy savings, cooler and cleaner surrounding air, and reduced runoff – benefit the public as much as building owners.

In Miami-Dade, a business that won a county-sponsored free roof-whitening contest saw a $100 drop in its energy bill the first month after its roof was lightened. An educational message applied to the new cool roof is now seen by 95,000 Metro-rail riders each month.

Cool roofs
So-called “cool roofs” have two important surface properties – a high solar reflectance, or albedo, and high thermal emittance, the ability to radiate energy away from the surface after it is absorbed. Most cool roof applications have a smooth, bright white surface to reflect heat, reduce heat transfer to the building interior, and save on air conditioning.

Conventional dark-colored tar-and-gravel roofs – which still account for over 90% of the roofs in the U.S. – have low reflectance and high thermal emittance, reaching mid-day summer temperatures of 150 to 190 degrees F.

By contrast, cool roofs with high reflectance and emittance remain up to 70 degrees F cooler than traditional roofs during peak summer months.

Associated energy and cost savings can be substantial. A Department of Energy analysis of estimated savings for 11 metropolitan areas suggests that Miami could save $20 million per year on energy costs alone by converting to light-colored rooftops.

From her 21st-story office at County Center in downtown Tampa, Hillsborough County Architectural Services Manager Swati Bose surveys a sea of mostly tar and gravel rooftops. But things are beginning to lighten up.

“We routinely do white roofs now,” says Bose, pointing to workers completing a new roof on the old county courthouse and the large white roof capping the Tampa convention center. At the courthouse, the county is replacing a black tar roof with a modified bitumen, or “built-up” roof, adding a white top layer.

Bose, whose office supervises the design of high-rise county buildings, watches the bottom line, mindful that every taxpayer penny counts. “Whatever we do has to be practical – there has to be a payback,” she says.

For Hillsborough County’s 22 downtown buildings, last year’s energy bill topped $3.4 million. Even a 10% reduction in energy use achieved through greater building efficiencies would produce significant savings.

Rooftops are important, says Bose, but their footprint in a downtown such as Tampa’s is relatively small against the larger hardscape of the buildings themselves and the pavement surrounding them.

“It’s important to consider the entire building envelope,” says Bose, ticking off a half-dozen or so energy-smart cool city strategies, including:

• lighter roofs and lighter building envelopes
• higher glass reflectiveness
• more ground cover
• and, most important of all, in her estimation, less impervious pavement.

Bose points to sidewalk width requirements as an example, an issue that generates heated debate. A quick Google search of sidewalk width requirements turned up 716,000 results, including a plethora of “Citizen Joe and Jane” blogs.

“Think about it, if we could reduce sidewalk size from 6 feet to 4, that could incrementally have a huge effect on the urban environment,” she notes. For overflow parking, she adds, cities could encourage parking on turf grass or other porous surfacing material.

“We’ve gotten into the habit of huge driveways, wide roads, big parking lots,” she says.

“It’s such a no-brainer,” says Josh Bomstein, business manager for Creative Contractors in Clearwater. “There are so many little things you can do to make buildings and sites more energy and cost-efficient.”

Creative Contractors builds a lot of white roofs nowadays, both single-ply and modified bitumen. The company’s latest crowning achievement is the white-capped Dunedin Community Center, which recently won U.S. Green Building Council LEED® certification in recognition of its comprehensive sustainable features. Designed by Collman&Karskey Architects, modeling analyses indicate that the building will be about 30% more energy-efficient than standard code, with associated savings of $28,000 per year.

And that’s just energy savings, notes Bomstein. The figure does not include cost savings associated with water-efficiency.

Paved paradise

Elie Araj has been preaching against the super-size mentality all too prevalent in some engineering quarters and local building codes since his days as manager of Hillsborough County’s stormwater engineering section.

“We overdesign buildings and parking lots,” says Araj, president of Applied Sciences, a civil and environmental engineering firm specializing in water management and sustainable site development.

“We need to change the mindset enshrined in our land development codes that everything has to be built to peak capacity. If we used more pervious paving material and built parking lots for average traffic flow – instead of designing for the holiday peak – we could dramatically improve our urban environments.”

Cool paving options abound. Concrete, for example, can be whitened by mixing it with recycled fly ash and slag at very low or no additional cost, says Bomstein, who used the material in sidewalks at the Dunedin Community Center.

Other cool-paving technologies include white cement; white topping – a relatively new cement product used to cover existing asphalt pavements; porous concrete and interlocking concrete pavers; and asphalt concrete, combining asphalt binder with rock aggregate.

Adding aggregate that is 30 to 40% more reflective to concrete increases its reflectively by 10 to 30%. A study in Japan found that an asphalt pavement with an albedo of 0.1 reached temperatures of 150 degrees, while concrete pavement with an albedo of 0.45 was 30 degrees cooler.

Porous pavement offers further advantages by allowing water to drain through, reducing and slowing runoff pollution, which remains the single greatest to the health of Tampa Bay.

Cool roofs on display: the Dunedin Community Center, which recently achieved U.S. Green Building Council LEED-Silver certification; a 1,200-square-foot green roof demonstration project at the University of Central Florida: and the new Hillsborough County Courthouse and re-roofing of the old county courthouse (foreground).

Budding green roof market

If the thought of tending plants on the ground overwhelms, the idea of a green roof might seem downright puzzling.

If so, it may be time for some chiropractic adjustment in thinking.

Vegetated green roofs offer numerous benefits for flat-top commercial buildings and residential high-rises, particularly in urban centers where green space is limited and dark rooftops collect heat that increases interior cooling costs.

Green roofs are not new, nor are they experimental. They have been used extensively and successfully in Europe for many decades. While they have only recently begun to emerge in the U.S. market, in places like Chicago, they are already becoming a fixture in the urban landscape and gaining critical mass.

While green roofs today can cost twice that of a conventional rooftop, their benefits can easily outweigh the costs, experts say. Research in Florida is helping to further quantify those advantages.

At the University of Central Florida, a 1,600-square-foot green-roof demonstration project is thriving atop the school’s student union building, alongside a conventional roof which also is monitored.

“This is the first university in the state to install a green roof,” says Dr. Marty Wanielista, director of UCF’s Stormwater Management Academy. Constructed with the help of a grant from the Department of Environmental Protection, the UCF green roof is projected to slash building energy costs by 50%. Additionally, says Wanielista, green roofs can double or triple a roof’s lifespan, providing significant cost savings for building owners.

Green roofs also do yeoman’s work in reducing stormwater runoff. The UCF green roof, which connects to a 1,500-gallon cistern, reduces stormwater runoff by 80-90%; the water is recycled for ground and occasional rooftop irrigation.

	4 ways to reduce urban heat islands
		trees and ground vegetation for cooling shade, improved air quality and reduced runoff
		cool paving, including use of materials such as porous concrete and white cement
		cool roofing by modifying existing dark-colored roofs with highly reflective white-topping or by installing single-ply white roofs
		impervious paved surfaces

For developers eager to maximize buildable space, that could mean the ability to reduce the size of ground-level stormwater treatment ponds required by state regulation.

Developer Brady Kelly is a believer. The Managing Director of Orlando-based Overture Development Group is set to break ground in January on a six-story office complex that will include a 25,000-square-foot accessible green roof.

Kelly’s decision to build a green roof for The Conservatory at Celebration was “just being intelligent as a businessperson,” he says. The Conservatory will be the first LEED-Platinum office building constructed in Florida.

Kelly calls UCF’s green-roof research “credible and compelling.” Given the projected extension of roof life, the company was able to calculate replacement reserves on a different schedule, he says.

That, coupled with the overall reduction in heat and energy use – estimated to be 30% for common areas, and much more for occupied offices – clinched the deal.

Kelly does not forsee cumbersome maintenance for the roof, given the green roof’s low-maintenance design.

The Conservatory’s green roof will cost about $25 per square foot and required special permission from the county.

While Kelly acknowledges some up-front reservations, he’s now convinced that the green roof was “not only the right thing to do, but also the smartest thing.”

Scheduled to open in 2009, The Conservatory will use an 80,000-gallon ground cistern to capture and recycle stormwater for landscape irrigation and for restrooms inside the building. “We won’t be flushing a single drop of potable water down the toilet.”

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