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Over Our Heads: What Will Design Students Need To Know About The Revolution In Sustainable Roof Design?

By: Lynn Phillips

August 01, 2011

Designing a roof in New York City isn't what it used to be. In days soon to be bygone, the New York City roof was merely a hard-working servant of the edifice it covered. Anything beyond its parapets was none of its business. But now, city planners and politicians, environmentalists and urban designers are asking the roof to solve environmental problems well beyond its individual footprint.

Today's technologies give a designer color-coded options that do more than keep the weather out:

· "Green" roofs, structured to sustain plantings, stabilize urban climates
· "Blue" roofs, equipped to retain and control the release of excessive storm water,
· "White" roofs, coated with reflective materials, slow solar warming by deflecting solar radiation from the earth, and
· Solar roofs outfitted with cellular arrays to convert direct sunlight into electricity.

The selected option, or options, of course, will have to satisfy a building's owners while completing their noble assignments on behalf of the rest of us.
Heat Island Effect Source: EPA 2008

The one environmental villain that all sustainable roofs challenge is what's called the Urban Heat Island (UHI) Effect. City pavement, cement, and roofing tar soak up heat on hot days and release it at night, making overall temperatures in urban areas, according to the EPA, from two to over ten degrees Fahrenheit higher than rural surroundings. [1] The side effects of this added urban warming can be dire.

As climates trend warmer, it is alarming to tally up the health and mortality consequences of city heat waves. In 2006, the Centers for Disease Control and Prevention estimated that from 1979-2003, excessive heat exposure contributed to more than 8,000 premature deaths in the United States — more than caused by hurricanes, lightning, tornadoes, floods, and earthquakes combined. Clearly, the cooling effects of eco-roofing techniques give them impressive lifesaving potential.

Another UHI danger sustainable roofs mitigate is the spike in summer air conditioner use that overloads urban energy systems. Peaks in energy demand threaten the electrical grid with brown outs. This risk forces cities to site new, usually diesel-fired, generators to produce yet more energy (and create yet more heat globally).

Unlike the other UHI fixes on the architectural menu, white roofs, regardless of size or age, rarely need costly structural supports. A "white roof," by the way, need not be literally white. As long as it is properly installed with one of a large variety of highly durable, highly reflective, specialized shingles or paints, it should both insulate its building and repel the sun's heat. Because white roofs are cheap, easy and give a lot of bang for the buck, it's not surprising that ex-president Bill Clinton has taken a personal shine to them, or that populist ecology initiatives cluster around them.

The US EPA's Energy Star program, estimates that white roofs can lower rooftop temperatures up to 100 degrees F, and claims that maximum use of white roofing would reduce peak electrical demand by 10-15%. So Energy Star now requires low-slope roofs to have a Solar Reflectance Index (SRI) no lower than 50 after five years aging. (For LEED certification the SRI minimum is 78.) A recently released study by Lawrence Berkeley Lab physicist Hashem Akbari, projects that if all pavement and roofing in America's 11 largest cities were reflective, it would offset 44 metric gigatons of carbon dioxide worth over a trillion dollars. A man can dream.

Although blue and green roofs also help lessen the UHI effect, the real reason that they are Mayor Bloomberg's darlings is because he is rightly concerned with the city's aging infrastructure for handling storm water runoff. Currently, sewer water and storm water flow together after downpours, overwhelming treatment facilities and polluting the ocean. When warmed by UHI effects, storm runoff also overheats aquatic ecosystems. Both blue and green roofs, by releasing water more slowly than flat or shingle roofing of any color, can keep storms from crashing the system, saving taxpayers billions in clean ups and water treatment plan upgrades [2] — something neither white nor solar roofs can do. [3]

The industrial-strength supports, membranes and cachement pools of all-blue roofs are expensive to install, and many architects and builders are habitually wary of standing water on a roof no matter how thick the protective membrane containing it, so blue roofs tend to be used for larger, flat-topped institutional or industrial structures where drainage is a relatively serious problem (and key decision-makers are seldom either architects nor builders).

Green roofs

Green roofs can be extensive (shallow and low maintenance) or intensive (deeper and more labor-intensive, like the High Line). Whether designed as artworks or bought like carpeting in modules, packs or rolls, green roofs bless the city with increased bird and insect habitat, CO2 scrubbing and a higher prettiness profile — the latter a seemingly twee consideration but one which, for real estate developers, can have impressive payoffs.

Designers curious about how green roof techniques are applied can start at Greenroofs.com. The self-styled industry portal offers a free, searchable online catalogue of over a million square feet of green roofing already installed within NYC's five boroughs.

While crowning a building with a sustainable roof is a righteous and inspiring project, the chief constraint on the designer is — spoiler alert — cash. For the public good, the U.S., New York State and the City offer owners a variety of financial incentives to soften the sticker shock of ecologically responsible roofs. Energy companies will also pony up in some cases for projects that help blunt spikes in energy use. None of them, however, completely compensate builders for the cost of doing right.

Fortunately, because white, green and, to a lesser extent, blue and solar roofs better insulate the buildings they protect, sustainables can produce significant savings for whomever pays a building's electrical bill. Since environmental altruism is not every builder's mission, these operational cost savings are often a client's primary interest when shelling out for environmentally friendly roofing, and designers do well to provide them. New York City, keen on solar, recently published a solar map allowing a designer or building owner to type in an address, outline a potential installation and find out exactly how much sense it might make economically and environmentally to take this option. The EPA is so psyched on white that it offers reflective roof designers a free online savings calculator.

Each kind of environmentally sustainable roof has advocates, suppliers, and impressive statistics to recommend it. The job of the designer is to take all those recommendations to heart, but with a grain of skepticism, a genius for contingency and a gimlet eye for context.

Will a particular white roof reflect too much light at night from a neighboring apartment, streetlight or sign? Will it scald surrounding building materials by day? Will a green roof's bees fly into the windows of an allergic neighbor, causing death and lawsuits? Will rats who live in the first-floor restaurant make their way up to your edible plantings and chew their way through thousands of dollars worth of membrane, causing hundreds of thousands of dollars' worth of water damage? Would a blue roof as planned be too vulnerable to tornado attacks? Is it more urgent to move the continually flooding oil furnace out of the basement and onto the roof than to green that quadrant of it? Are you sure that your client's roof gets enough sun?

Ask any hands-on professional which sort of roof is best and the professional will answer sagely, "It depends." And of course it does. It depends on a client's wealth, interest and commitment. It depends on available tax abatements, offsets and rebates. It depends on a project's scope, on the state of the art in your chosen technology and on whether you are renovating or starting new. In many cases the best solution will be a clever intercutting of roofing types — water collection pools around the margins with tenant use in cordoned off areas and green swaths in others, or black passive solar water piping running graphically and usefully around a white roof. Because all of these arrangements, financial, technical and social are all relatively new, expecting the unexpected and being on the lookout for what Donald Rumsfeld famously called "the unknown unknowns" will be a key strategy in arriving at a design that survives the drawing board to see the sky.


[1] For the formula by which solar reflectance and infrared emittance are combined to form SRI, see ASTM E-1980.

[2] Curbing water runoff with blue and green roofs.

[3] Heat Island Impacts and Heat Island Effect Video Segments.



Lynn Phillips is a polymath media tramp who has worked in film, TV and many other media. She is a frequent contributor to the New York Times' T-Magazine, blogs regularly on addiction at Psychology Today and when she tweets, which is rare, it is at @highonnothing. Her humor book, "Self-Loathing for Beginners," has aided and abetted self-loathers of all degrees and levels. If, for example, you are hating your carbon footprint, SL4B will help you to spend the entire flight to your next eco-conference detesting yourself with enthusiasm.

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