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High-performance urban forestry for green infrastructure

The urban forest is perhaps the most conspicuous example of contemporary urbanism’s heartthrob, green infrastructure; witness the preponderance of “Million Tree” campaigns such as MillionTreesNYC, Million Trees LA, the Chicago Trees Initiative and Grow Boston Greener. After all, trees are good, especially a million trees, right? It depends. The planting techniques for most urban trees lag far behind what is necessary to steward them to old age and impressive size. 

The urban forest as green infrastructure, a primer
A robust urban forest produces myriad environmental, economic, social and cultural benefits, all while bolstering urban identity in ways no other infrastructure type can. As a biogenic public utility, it is unique in that it gains value over time rather than depreciating like most infrastructural assets.

The benefits of urban trees are well documented. Methods for assigning value to these benefits range from simple calculations, such as money saved on heating and cooling costs or increased property values to deeply contested and complex analyses. How does society determine the precise value, for instance, of the cleaner air and water or the increased biodiversity urban trees provide? The fields of environmental and ecological economics are hard at work creating metrics for such quandaries. At a minimum, it is widely accepted that the most important benefits the urban forest provides include a reduction of the heat island effect, improved stormwater management (which means higher water quality and reduced flooding), reduced air pollution, reduced energy use, increased property values, increased wildlife-habitat potential and a range of heightened aesthetic and spatial qualities.

Trees are remarkable biological machines that perform astounding feats of structural assembly and adjustment, self-defense and maintenance, organic chemistry, water pumping, atmospheric manipulation and power generation. The productive capacity of a tree increases in magnitude as it gets larger, and this is the key to understanding another maxim of urban trees: In terms of environmental and economic performance, bigger is better, by a long shot. 

The Center for Urban Forest Research calculates that large-canopy trees (greater than 50 feet in height and canopy spread) outperform small trees (less than 25 feet) by a factor of 15, and they do not start adding significant environmental performance until they reach 30 feet. Trees that do not reach this minimum will never be more than an aesthetic amenity. In the quest to make the urban forest into a high-performance green infrastructure, lots of big trees are required, especially in the most environmentally compromised zones: streets, plazas, parking lots, and commercial strips. These places have two things in common that make them hostile to trees: more than 90% impervious surface cover—what I like to call “extreme pavement”—and highly compacted soils.

An underground movement: soil resources
To grow large trees in the harshest urban conditions, a literal underground movement is necessary, and it’s all about the soil. Tree roots cannot penetrate the compacted soils of the urban environment; they need soil with lots of pore space and, ideally, organic matter. When a tree’s roots run out of space prematurely, one or more of the following happens: 1) The tree stops growing, 2) space-starved roots shove pavement and curbs aside in desperation and 3) the tree dies or is removed. Tree roots can and will break pavement, yet they cannot penetrate compacted soil. Because typical tree-root growth is relatively shallow and spreads laterally, roots need room to spread out. The eventual size and longevity of a tree depends on its ability to do that. 

A graph developed by landscape architect James Urban plots tree size, measured in crown spread and trunk diameter at breast height (DBH), as a function of available soil volume, measured in cubic feet. To grow a tree that achieves the minimum size for significant environmental performance, 800 cf of soil is required. A typical planting detail specifies a 4 feet by 10 feet tree pit (that is only 120 cf of soil!) carved out of highly compacted soil and surrounded by pavement. Trees planted in these conditions will be lucky to reach 15 feet in height, and the stormwater processing capabilities of that soil are negligible. High-compaction soil and extreme pavement are the dual enemies of urban tree growth and on-site stormwater management. Retooling underground infrastructure to accommodate high-quality, uncompacted soil enables the growth of large-canopy trees and the absorption of large volumes of stormwater.

Techniques to maximize soil
In pursuit of soil volumes required to grow environmentally productive urban trees (various experts recommend between 700 - and 1,000 cf), several techniques can be used that rely on the inventive use of space and augmented construction methods. Space permitting, the easiest way to achieve target volumes is an open planting area (1,000 cf = 10 feet by 34 feet by 3 feet tree pit, for example.) Since this is usually not possible in heavily paved areas, augmented techniques using covered soil in combination with open soil can be used. These include the use of root paths (narrow trenches of uncompacted soil under pavement to connect the planting area to nearby volumes of soil), structural soils and suspended-pavement systems. 

CU-Structural Soil™, developed by Professor Nina Bassuk and colleagues at Cornell University, solves the compaction problem by mixing angular one-inch crushed stone with planting soil, at a stone-to-soil ratio of 4:1. The stone pieces form a load-bearing “rigid lattice” that leaves space for uncompacted soil. CU-Structural Soil outperforms compacted soil, but the large proportion of stone in the mix means that 80% of the space is unavailable for root growth and water storage. Claiming a higher percentage of that space would boost performance.

Suspended-pavement systems offer the best combination of structural strength and large volumes of quality soil. A suspended-pavement system consists of an underground post-and-beam structure and a deck with pavement on top. The structure supports the weight of the pavement and additional loading by pedestrians and vehicles, leaving the space for large volumes of uncompacted soil for root growth and storm-water treatment. This approach also protects pavement and curbs from the rogue roots of cornered trees; they will always prefer expanding into tasty soil over pavement demolition. Storm water can be allowed to infiltrate the soil in several ways, such as via permeable pavements, drainage slots, curb-cut inlets and sheet flow to open planting areas. 

Providing that much high-quality soil opens the door to a much larger selection of species beyond the 10 or so that can survive in hostile urban soils. A greater mix of species helps fight pests and disease, and increases urban-habitat potential. This expanded planting palette may be one of the most significant ways to ensure the viability of the future urban forest.

San Francisco–based Deep Root Partners introduced its suspended-pavement system Silva Cell in 2007, and there are already hundreds of installations internationally, but performance data for mature trees is at least a decade away. Improvements in stormwater treatment can be measured immediately, however, and the benefits are clear. An installation in downtown Minneapolis intercepts stormwater for 6.6 acres with 179 trees planted in Silva Cells. Suspended-pavement systems are not new, and one of the best case studies is in Charlotte, North Carolina, where, in 1985, 170 trees were planted using custom suspended pavement along a 10-block stretch in the downtown business district. Each tree received 700 cf of soil, and the results have proved stunning. A 2009 survey found that the trees (167 of the 170 survived) have thrived, reaching an average height of 44 feet and 16 inches DBH.

The Achilles heel of suspended pavement is its high initial cost, adding as much as $10,000 per tree to install compared with conventional methods, a figure that will give pause to many! A life-cycle cost calculation goes a long way toward justifying this investment, and, in some cases, savings in up-front costs for traditional infrastructure can pay for it many times over. For instance, the City of Minneapolis chose a $1.5 million Silva Cell installation over a $7.5 million storm-sewer upgrade to meet the city’s storm-water goals. Still, the cost threatens to put suspended pavement out of reach for many smaller projects and out of the question for some private developers. 

Although many cities are writing minimum soil-volume standards into their zoning codes, cost is still a barrier. In Brookline, Massachusetts, the urban forestry budget is now part of the Capital Improvements Program, typically the domain of infrastructure construction and maintenance. By elevating the urban forest to public-infrastructure status, new funding possibilities emerge to offset the costs for suspended-pavement and other soil-intensive planting techniques. As a hypothetical, a city’s Department of Public Works could purchase materials and supplies such as Silva Cell in bulk and provide them at little or no cost to new projects that achieve stated environmental goals. As in the Minneapolis experience, the cost savings to the city could easily pay for such a program.

What about those million trees? Although a million is a great public-relations sound bite, the absolute number is not nearly as important as the quality of the plantings. Many trees will undoubtedly be planted in parks and open spaces where soil conditions are favorable, but thousands of trees are bound for those tough streets and parking lots. In that case, 100,000 healthy, large trees trumps 500,000 struggling or dead trees in every way. I, for one, hope that the program managers direct their resources to make this happen.

Case studies
Waterfront Toronto 
Waterfront Toronto is investing in a suspended-pavement system at an infrastructural scale, specifying SilvaCell for hundreds of its 2,000-acre waterfront revitalization project. (video)
Queens Quay (Toronto ON, Canada)
Waters Edge Promenade and Boardwalk  (Toronto ON, Canada)
Portland Slip
Bloor Street
DeepRoot case studies


Matthew Gordy is principal of On Land, a landscape and urban design practice based in Boston and in Memphis, Tennessee. He has been a design critic for studios at the Harvard Graduate School of Design and led graduate seminars at Northeastern University’s School of Architecture. He received the MLA from Harvard in 2005. Gordy previously practiced with Landworks Studio in Boston and Michael Van Valkenburgh Associates in Cambridge, Massachusetts.

A Passive (aggressive!) approach to energy use

In November 2010, Build Boston featured a comprehensive exploration of the Passive House standard by those who have helped it succeed in Europe and those on the forefront of its emergence in North America.

To whet the industry’s appetite for learning more about this cutting-edge approach to design and construction, the BSA recently sat down with Dr. Harald Rohracher, who was visiting from Austria’s Alpen-Adria-University, and Paul Eldrenkamp, of Byggmeister and the DEAP Energy Group, to discuss Austria’s great success in adopting the Passive House standard and how Massachusetts can emulate it.

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What is Passive House?

Eldrenkamp: Passive House is a very simple, clear building standard, which has just three criteria: the amount of total energy a house or a building uses; the amount of total energy that can go to heating and cooling; and an air-tightness standard that’s tested with a blower door. It doesn’t include any criteria for what’s traditionally been viewed as green building in the United States—for example, there’s no water-usage or material-sourcing category.

How does Passive House compare with LEED?

Eldrenkamp: The biggest difference is that LEED covers eight different categories, whereas Passive House covers just one directly. (Indirectly, it covers a range.) If you compare Passive House to the energy-usage component of LEED, you’ll find that LEED doesn’t really set a specific target: It gives a maximum that is really a pretty modest baseline. One of the backlashes against LEED has been that LEED buildings really have not demonstrated significantly less energy usage than buildings built to code. For example, with LEED residential construction, you only need to hit the Energy Star standard, which is only 15 percent better than code.

With Passive House, it’s all about the energy use. It dictates a very specific energy budget that you cannot exceed. By mainly focusing on energy efficiency, at the residential level, Passive Houses use only about one-fifth of the energy that a building constructed to code would use—for an 80 percent reduction in energy consumption.

How does Passive Haus achieve that level of efficiency?

Eldrenkamp: Passive House calls for super-insulation (which means triple-pane windows and thick walls), a ventilation system with very efficient heat recovery and, at least in heating climates, passive solar gains.

Rohracher: Thanks to this efficiency, the energy needs of Passive Houses are so low that internal heat production by their inhabitants—including people, computers and other systems—can meet a significant part of their energy-supply needs.

Is there a way to predict internal gains?

Eldrenkamp: The Passive House Planning Package (PHPP) is an Excel spreadsheet that has about 35 different tabs. It’s a very precise German tool with detailed data input to calculate the internal gains.

How successful have Passive Houses been in Europe?

Rohracher: The first Passive Houses were built in Germany in the early 1990s by Wolfgang Feist from Germany in cooperation with Bo Adamson from Sweden. They developed both the concept for the Passive Houses standard and the framework for its dissemination: the Passivhaus Institut in Darmstadt, Germany. The institute has developed an international network of professionals and coordinates the software package supporting the energy-efficient design of Passive Houses (the PHPP) and certifications for the different components, among other activities.

The Passive House concept spread to Austria a few years later, and now Austria has five times as many Passive Houses per capita than Germany. By the end of 2009, there were already close to 7,000 Passive Houses in Austria, and we are estimating that, by the end of this year, 25 percent of all new building construction in Austria will be to the Passive House standard. Around Europe, 22,500 Passive Houses have been built so far.

So the standard really has taken off, and it’s becoming more and more popular. Regulations are starting to adapt, and there are subsidies encouraging its more widespread adoption.

What is the state of Passive Houses in the United States?

Eldrenkamp: In 2002, the first Passive House was built in the United States by a German architect named Katrin Klingenberg. She had just moved here and was pretty discouraged with the energy-efficiency standards of the American housing market. She built a Passive House in Urbana, Illinois, and then another, before founding the Passive House Institute U.S. in 2005. The institute held its first round of Passive House consultant training in 2008, with about 20 people passing the exam and becoming certified Passive House consultants.

We’re not two years away from the completion of that first training session, and the Passive House standard has really taken on a life here. Many architects, builders and engineers are growing disillusioned by the green building movement and its overall lack of rigor, accountability and impact. It seems to a lot of us that the movement has been more about putting a greenish tint on business as usual than really moving the design and building industry toward making significant reductions in greenhouse-gas emissions. Passive House is an attractive standard because it’s rigorous and quantifiable.

Currently, there are maybe only 10 certified Passive House buildings in the U.S., but there are many more under construction, in design and in the pipeline.

Have any local projects met Passive House standards?

Eldrenkamp: There’s one on Martha’s Vineyard that I believe may have hit the final standard. I know it was getting very close. The project hadn’t achieved the air-tightness standard last I heard, and I don’t know if they’ve been able to make the necessary tweaks yet. But that’s been up and running for more than a year now.

There’s a project nearing completion in Shrewsbury. I’m involved in planning a Passive House project in Falmouth that should break ground in about six weeks. There is also an architecture firm in Maine that’s developing stock Passive House designs that could be adapted to local climates.

How does Austria’s climate compare with New England’s?

Rohracher: I believe it’s rather similar, though Austria’s mountains are sometimes a bit colder.

Eldrenkamp: I would agree. The Department of Energy has defined seven climate zones in the U.S. New England is climate zones 5 and 6, with our most- northern states being zone 6 and the rest zone 5. Austria is right around climate zones 4, 5 and 6, depending on the elevation.

However, Austria doesn’t have the same de-humidification and cooling needs as New England. We’re still trying to address those issues here and still fall within the Passive House energy-usage parameters.

What led to the speedy adoption of Passive Houses seen in Austria?

Rohracher: There were many different aspects, all of which were important. Austria focused strongly on architectural quality; in Germany, the movement was perhaps more engineering-driven. Passive Houses started off in a region in Austria where there was a strong emphasis on high-quality buildings, both in terms of energy efficiency and great design.

That region managed to set up a network for both architects interested in Passive Houses and the businesses that supplied and adapted components for them, such as window manufacturers and producers of ventilation systems. The area also was home to a strong intermediary organization, the Energy Institute, so there was a coordinated effort in setting up a structure for Passive Houses.

Nationwide programs, such as the Building Of Tomorrow program, focused on the research and development side—creating better building components, but also providing sociological research on perception and use.

There was also a strong focus on vocational training for diverse building-industry professionals and an emphasis on public awareness programs, including media campaigns and excursions to Passive Houses—with the ability to rent Passive Houses for a few days or weeks to “test them out.”

Can you only meet Passive House standards through new construction?

Rohracher: The original concept was focused on new construction, but there is huge potential to use Passive House knowledge and technology for retrofits. The latest available figures for Austria showed only 50 retrofits in 2008 meeting the Passive House standard, but that number is growing quickly. It’s certainly more difficult to meet Passive House standards via retrofitting, because you can’t change buildings’ orientation and factors like that. But you can apply different Passive House components, such as super-insulation and highly insulated windows, and get very close to the standard, if not all the way there.

Can you really apply Passive House standard to commercial buildings?

Eldrenkamp: “Passive House” is a direct translation from the German, where “haus” means “building.” So the concept can apply to diverse building types, from single-family homes to large office buildings.

Rohracher: Yes. The concept was originally used in Germany for modest residential buildings. But more recently in Europe, we have seen the Passive House standard applied in non-residential buildings, including schools, factories, supermarkets and office buildings.

How much does it cost to build to the Passive House standard?

Eldrenkamp: In the U.S., it may cost only 5 to 10 percent more to design and build a new home to the Passive House standard. But we have found that while it’s not impossible to retrofit an existing home to the Passive House standard, it’s prohibitively expensive in most cases: sometimes equaling 80 percent of the cost of new construction. So that’s problematic.

Rohracher: It depends on what the building looks like and how easy it is to retrofit. I’ve just seen recent figures from Austria, where retrofitting to the Passive House standard cost on average only 10 to 15 percent more than standard renovations. Our experience in Austria is that, with new buildings, Passive House construction only costs 3 to 5 percent more than regular construction. The economics are getting better all the time, as we become more experienced and Passive House materials become cheaper and more widely available.

What is the return on investment for Passive House construction?

Rohracher: In Europe, if you sell a building, you have to provide some certification of how much energy it uses. So an investment in Passive House construction increases the value of the property. If there is this kind of incentive around new construction, you don’t need to have long-term plans for a property to realize a return on your investment.

Eldrenkamp: In the U.S., however, people often have a very short-term view on construction costs. On average, people move about every seven years and tenant leases are typically only a few years long, too.

In addition, I don’t really like the idea of using simple payback formula for energy-efficiency improvements to homes and offices because the service life of a building is so long. In Boston, for example, we have buildings with a service life measured in centuries, whereas our ability to predict energy prices is measured in days.

That said, it is so much cheaper to build for maximum energy efficiency at a building’s outset, that it seems nuts to make value-engineering decisions that will last 100 years. One of things I found most compelling about the Passive House standard is that it says this is a useful energy budget, not only in terms of building economics but also in terms of sustainable use of the world’s energy resources.

And when you’re designing to a specific budget, the question becomes how do we most cost-effectively build this structure to get to this specific goal? That is a very different conversation from what’s cost-effective in terms of our ability to anticipate where energy costs are going to go over the next few years or even decades.

What are the barriers to moving toward adopting Passive House standards?

Eldrenkamp: One barrier is the general availability of components—primarily that really good windows are still inordinately expensive. We have not traditionally asked much of windows in the U.S., and the difference between the window you can use in a Passive House and the window that meets code is huge. You’re doubling or tripling the cost of your windows right there.

The HVAC and mechanical systems here are also lacking. In Europe, they have these elegant all-in-one units that do heating, cooling, ventilation and domestic hot water—all in a component that fits into a small linen closet.

Europe also has a wider range of insulation, more widely available. We can find all the same insulation here, but some of it is just very difficult to get in quantity or cost effectively.

So there’s the availability of building components part, which is solvable long-term, but there’s also consumer resistance to certain components. In a Passive House, you probably want to go with casement windows rather than double-hung windows. You likely want to have an electric range top rather than a gas cook top. You definitely don’t want an open fireplace. And ideally your dryer is a condensing dryer, so it’s not vented to the outside.

From the industry’s perspective, the biggest barrier is the initial cost: the investment in basically learning a new language. Austria is “exhibit A” that building to the Passive House level of performance gets easier with time. But it definitely costs a lot more to do your first Passive House, and not all those costs can be passed on to the client or the homeowner.

And then there are the perceived design limitations. Independent of those features I talked about, with regard to the consumer, the architects will find that they need to have simpler geometries. They have to do their floor plan layout so that most of the glass can face south. It’s also harder to do a Passive House with a basement than on a slab, so that’s a perceived design limitation. The cheapest way to insulate the attic is to put a couple feet of cellulose up in there, which means there’s no pull-down stair. So you start eliminating the storage in the basement and the storage in the attic, and there’s resistance both from architects and homeowners.

Rohracher: Many of these issues were also barriers to adoption in Europe. Many of them still are. And many of them you can work around.

For example, more than half of the Passive Houses in Europe have basements—you just have to change the design to decouple them firmly from the rest of the building. You need more collaboration between the different building professionals, because a Passive House is a much more tightly coupled technical system than a normal house. You have to integrate all building professionals into the planning process, and there needs to be a specific level of competence among all the different participants in construction.

Europe also has the same problems and issues in terms of uses and usage perception. Aesthetic quality is still an issue in Austria. It was so very important to show that you can make very good architecture and still meet the energy-efficiency standards. The proponents of Passive Houses would say that great architecture always comes from working with or against certain resistances and limitations.

Do we have to wait for U.S. manufacturers and suppliers to catch up to have access to Passive House building components and materials? Or can we work with what’s already available in Europe?

Rohracher: As Paul said before, it’s not just a matter of availability in the U.S. You have the technology, but it’s not widely available. And you do have not the same range of options seen in Europe. It’s something that will develop along with demand, I think.

Eldrenkamp: For some projects, people were actually shipping windows over from Germany because they were able to do that more cost-effectively than to buy North American triple-glazed windows.

But North American manufacturers are starting to respond. There’s a handful of window manufacturers now making windows efficient enough to meet the Passive House standard. A company in New Hampshire is starting to import Swiss mechanical systems and switching over the controls, so that you can use this Swiss equipment in the United States. You could normally do that with HVAC equipment because Europe is on 50 cycles a second, whereas the U.S. is on 60 cycles a second.

It's only a matter of time: The U.S. can build anything it puts its mind to. It just needs to put its mind to it.


Top photo: Mountain Retreat Schiestlhaus, Hochschwab. Robert Freund, ÖGUT.

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