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Green Wall Plants and Structures Studied for Energy Savings

Ivy-strewn brick walls are a familiar sight on many college campuses. But four plant-covered facades growing in College Park’s Research Greenhouse Complex this spring stood out for the mix of vegetation and the potential impact of these particular leaves on the “green” building movement.

Dr. David Tilley, associate professor of environmental sci­ence and technology, is studying green walls and how they might reduce energy consumption and, in turn, provide cost-savings in the real world. His is the only such research in the U.S. supported by Green Roofs for Healthy Cities. The not-for-profit industry association issued a joint $50,000 grant to Tilley and colleagues at the British Columbia Institute of Technology to study the thermodynamics of green walls and their possible effect on storm water runoff.

Tilley’s three-year project compares green wall prototypes to see which structures and plants yield the best energy-con­serving results.“I’ve always been interested in ecosystems and how they can be used to address human problems,” says Tilley, who heads the college’s ecosystem engineering design lab. “When you don’t know what the answer is, that’s the most fun.” Commercial green wall products aren’t novel, but much of the sci­ence behind them has been done in Europe. Tilley’s research will quan­tify their effects for North American consumers, and it may help the industry promote the walls’ money-saving qualities as well as their aesthetic appeal. Eventually, his findings could be used to determine credits toward environmental building certifications such as LEED or Sustainable Sites.

“A green facade’s utility to builders has up to now been based largely on empirical and aesthetic values from traditional, vernacular applications,” says Reu­ben Freed, chair of the green walls group for Green Roofs for Healthy Cities and director of research and project manager for greenscreen®, the predominant supplier of green façades in North America. “In the last decade, parallel with the growth in importance of the sustainable building and landscape movement and evaluation systems, metrics of the benefits of vegetated green walls and systems have become mandatory for the growth of our industry.”

Tilley comes to the green building movement as an ecological engineer who previously spent years constructing wetlands and studying them using satellite imaging. In 2004, he began working with graduate student Laura Schumann on a concept Til­ley called a green cloak.

While green roofs were gaining popularity for their ability to curb storm water run off, Tilley found that they were heavy and usually only appropriate for new construction specifically designed to bear their weight. He wanted to explore what could be done differently to create similar environmental ben­efits in smaller or existing buildings.

Instead of growing plants directly on the roof, they sought out climbing vegetation that could be suspended over a building’s top without weighing it down. The cloak experiment, completed in 2007, found that a building with a full overhead canopy was 11 degrees cooler inside during peak summer temperatures. Scaling the findings to a one-story, 2,000 square-foot home in the Mid-Atlantic region, they found that energy consumption would drop by 18 percent in the summer, with a cash savings of $100 to $200.

Tilley’s current research will determine what hap­pens when the vines grow up a vertical wall, instead of across a flat roof. The plants don’t attach directly to a building, but to a self-supporting structure placed less than a foot away from an exterior wall.

In January, environmental science and technology masters student Jeff Price ’10 planted eight varieties of grapevines and native plants at the base of 12, 4-by-8-foot panels. They include three types of commercial trellis products already on the market: a rigid, recycled steel panel; a stainless steel cable lattice sys­tem; and a stainless steel flexible net. A fourth type, Tilley’s cre­ation, uses thick, vertical manila rope.

Over the coming months, the team will determine how well the plants attach to the various panels to see if there is a clear difference between species that use tendrils, like grape, and ones that twine, like honeysuckle.“The question is, ‘How do they compete?’ ” Til­ley says. “The trick is getting the diversity right.”

In the meantime, Price is already collecting data from four larger prototypes with more selec­tive vegetation. Two of the 8-by-8-foot experimental buildings at the Central Maryland Research and Education Center in Clarks­ville are controls and have no cover. The other two each have their southern walls draped in grapevines.

Because vegetation can theo­retically cool the buildings by reflecting solar radiance or turning it into water vapor, each building is covered with dozens of sensors. A pyranometer measures the solar radiation. Two dozen thermis­tors measure temperature from inside to outside across a wall of each building. And because even a slight breeze may cool the building, two anemometers measure wind speed.

Data are collected every 10 minutes using a computerized system run by car battery, but Price says the most vital infor­mation will come “in the dead of summer” and will be used to measure peak benefits. “We’re trying to take the numbers we have from our buildings and scale them up to a full-size house using math­ematical modeling,” says Price, whose thesis will be based on the current research. “I’m really focused on tangible numbers for everyday people.”

Though the findings are too premature to share, Price says that even without complete coverage on the walls last year, there was “definitely an effect.” Tilley is excited about the heat-reducing possibilities, and he’s also seeking further fund­ing to measure storm water benefits. He has applied for a Pioneer Grant from the Chesa­peake Bay Trust that would support new testing equipment and on-going research.

“We’d need to measure the rate of runoff and the total quantity of rainfall to under­stand how much water the plant canopy can hold and for how long,” says Tilley, adding that 10 to 15 percent of rain evaporates from a forest before it hits the ground. “I’d hope we’d see something in that range.”

Freed, whose company’s product is one of the three commercial trellis systems being tested, says if proven, heat reduction and storm water control benefits could make green walls a key tool in climate change mitigation.

“Green roofs, vegetated walls and the integration of sustainable landscape practices remain our best tools in affecting the rate of climate change,” he says. “The maximum, public benefits are gained in large scale applications, but local benefits, as part of an overall energy and water conservation ethos, are certainly possible right now.”

By Kimberly Marselas

Photos by Edwin Ramsberg

 

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Last updated: 07/27/2010

What Grows Well on Walls?

Species being used in the green wall research include:

Vine
Commercial grapes