[I wrote this for the "Sustainable Design as a Way of Thinking" class at the Boston Architectural College.]
When natural systems are destroyed in the process of creating a building, it is important to replace them by incorporating them, or a reasonable replacement, as much as possible into the building’s design. This is the only way to create a fully sustainable built environment. The majority of the world’s existing buildings do not even meet “net zero” or similar energy goals, so in order to slow – and hopefully, reverse – climate change and other detrimental impacts of the built environment on the natural, new construction must go beyond carbon neutrality, and give more back into the environment than it removes. We must replace the natural systems we have to destroy in the process of construction.
A great deal of attention is paid to how buildings can be designed to be highly energy efficient, using strategies and goals like zero carbon, Passive House, and net zero, in the interest of both reducing the carbon put into the atmosphere to keep the building running, and lowering the costs of operation for the building owner. Less attention is paid to how buildings can be designed to have a net positive impact and to perform more like an integrated part of the environment. There are a variety of ways that a building’s envelope can be handled to provide benefits to the environment, including creating habitat, using materials and coatings that absorb or break down CO2 and pollution, and managing stormwater run-off. Some of these treatments, like vegetated surfaces, have additional benefits, like reducing the urban heat island effect.
It is unquestionably necessary to reduce the amount of energy used to keep buildings running. According to the Environmental Protection Agency, buildings in the United States account for 30% of the country’s greenhouse gas emissions. Since the majority of this electricity is produced by burning fossil fuels, reducing a building’s energy use will help to lower the amount of carbon put into the atmosphere, which is required to reduce the amount of climate change we will see.
But being net zero, or even creating more energy than it consumes, is not enough to make a building an integrated part of its local environment, to make it function like a living part of the ecosystem that provides environmental services to its surroundings. That requires looking at how the building relates to the local ecosystem, including stormwater, amplification of local weather (does it contribute to the urban heat island?), air quality, and habitat for local wildlife. Incorporating these techniques provides a variety of benefits for the building, its inhabitants, and its energy needs, too.
One of the exciting recent inventions in building materials is the development of concretes and coatings that can actually break down airborne pollutants, including carbon monoxide, nitrogen oxide, and benzene (Giussani). The active ingredient is titanium dioxide, which interacts with sunlight in a process called photocatalysis that speeds the decomposition of pollutants that come in contact with the surface into their component elements, which then wash away in the rain. The decomposition reduces them to less harmful chemicals, like nitrates, carbon dioxide, and water (Giussani). One test site, 8,000 square meters of treated pavement in Bergamo, Italy, showed a reduction in nitric oxides of around 45% (Giussani).
Treating surfaces with these materials has an aesthetic benefit, too: by breaking down pollutants, they are self-cleaning. Richard Meier, famous for designing predominantly white buildings, wanted the “sails” of the Dives in Misericordia Church in Rome to remain white in Rome’s smoggy environment, so they were made with a concrete that includes a photocatalytic compound (O’Reilly).
The church has three large, curved walls, each a section of a sphere, made of Styrofoam-filled precast concrete (Richard Meier & Partners). It was constructed between 1996 and 2003. The concrete used, “TX Active,” was developed by Italcementi to provide the necessary pollution-destroying qualities in addition to strength and durability (Italcementi, “Dives in Misericordia”).
Italcementi’s photocatalytic concretes have been used in at least 30 additional projects in the United States and Canada. Many projects use permeable pavers incorporating titanium dioxide, but several of them have incorporated it into the exterior cladding of the structures (TX Active, “Project Profiles”). To learn more about how the material lasts, the Cité de la Musique et des Beaux-Arts in Chambéry, France, has colorimetric monitoring in place to see how its grey photocatalytic concrete holds color over time (Italcementi, “The active photocatalytic”).
Constructing buildings with pollution-destroying concrete or coatings gives them the ability to provide an air-cleaning service, like trees and shrubs, which can remove a variety of airborne pollutants through both the leaf stomata and surface (Nowak 115).
Vegetation incorporated into a building can also help remediate some kinds of pollution. While roofs planted with sedums have little ability to remove pollutants (compared to trees and shrubs), vegetated roofs can trap a significant quantity of heavy metals, like cadmium, lead, copper, and zinc (Dunnett and Kingsbury 50). Up to 16% of zinc, and up to 90% of the other metals, can be trapped (Dunnett and Kingsbury 50). Some of these metals are used in roof construction, so the presence of a green roof over the structure can reduce contamination of water by limiting the contact that rainwater has with the metals in the roof.
Green roofs are an important part of a site’s stormwater management system. They reduce the percentage of impermeable surface on a site that leads to runoff. Both the vegetation and substrate can absorb water directly, as well as slow the release of unabsorbed water, which reduces the strain on city sewage, preventing overflow and pollution problems in the sewer system. In cities with combined sewage and stormwater drainage, this is especially important. Planted roofs have become important parts of city stormwater management plans in places like Portland, Oregon, and Toronto, Canada (Earth Pledge 121 & 127).
In 1994, Portland became the first city in the United States to pass legislation encouraging the use of green roofs, as a result of a state order to comply with the Clean Water Act (Earth Pledge 121). That year, an employee of the Portland Bureau of Environmental Services, Tom Liptan, learned about vegetated roofs, and after a couple of years of further research, including experimenting with his own garage, the city began a more serious investigation into green roofs, including financial commitment (Earth Pledge 121).
Two residential projects in Portland, the Hamilton Apartments and Buckman Terrace, were the first municipally-funded green roof test sites in the United States. The first year of data collected showed that water retention is most effective during short, intense summer storms, with a 5-inch-deep roof able to absorb an average of 69% of annual rainfall. In the dry season, the roofs absorbed from 65-100% of rainfall, and 10-35% in the season (Earth Pledge 121). A report completed 5 years after the Hamilton project was completed shows that in an average year, the roof retains 53.5% of stormwater.
Green roof design can vary a great deal, from fairly shallow “extensive” roofs with shallow growth media and sedum mats to heavily planted “intensive” roofs holding shrubs and even trees. These factors, along with the number and type of layers making up the green roof, will affect the amount of water the roof is capable of holding. The water that is not immediately absorbed by the media or plants will be stored for some time in the roof system before being released, which will reduce the impact on the city drainage: instead of the roof runoff hitting the drains at the same rate as the rainfall, it will be delayed, so the system is less likely to overflow (Dunnett & Kingsbury 47). Most research shows that the yearly reduction in runoff will range between 60 and 80 percent (Dunnett & Kingsbury 48). The addition of water storage systems, like cisterns, or water features like ponds or rooftop wetlands, can simply retain what the roof doesn’t absorb, further reducing or even eliminating the runoff.
KPMG’s building in Düsseldorf, Germany, has a wetland on the roof of its parking garage that stores and uses rainwater. The wetland solved stagnation problems in an existing pond by adding a system to filter water by using wetland plants. The project includes a well that holds rainwater and then releases it into a creek, which flows into a swamp, and then into the pond. Volcanic rock lining the waterways further cleans the water and provides an anchor for the plants. This roof has improved the local microclimate, saved KPMG money through reducing stormwater fees, and provided habitat for birds and insects (Earth Pledge 28).
Stormwater management and removal of pollutants are not the only ecological services that green roofs provide. They also reduce the urban heat island effect and provide habitat that has been destroyed by the act of building.
Much like vegetation does at ground level, plants on rooftops can reduce the urban heat island effect. One of the major ways that a green roof functions in this regard is simply by not being a typical black asphalt roof. Two roofs in Chicago were compared on a day when temperatures were in the 90′s to show the difference: the surface temperature of the green roof ranged from 91-119° F, while the dark roof on the neighboring building was 169° F (EPA 3). The temperature of the air immediately above the planted roof was 7° F cooler than the air over the conventional dark roof (EPA 3). A model study in New York City, assuming 100% conversion of all roofs to green roofs, estimated that the city as a whole would be 0.4° F cooler over the day, and 0.8° F cooler at three in the afternoon (EPA 4).
In addition, since the green roof prevents heat from penetrating into the building interior, the building’s energy needs for cooling will be reduced. When wet, they can hold additional heat, and when dry, they act as an insulating layer (EPA 5). A study in Florida showed that the average rate of heat transfer into the interior was more than 40% less for the green roof than a nearby light-colored roof (EPA 6).
The term “green roof” can be misleading when talking about roofs that are designed with the intent of providing habitat for wildlife. In Portland, roofs that are designed to provide environmental services, rather than amenities for people, are referred to as “ecoroofs” (roofs for human use are more often considered rooftop gardens). Unlike a rooftop garden, which is designed to be used like a park, an ecoroof is likely to be designed in a way that isn’t necessarily aesthetically pleasing, or even literally green by being heavily planted.
The wild plants that are useful for wildlife are not always considered decorative, and if the roofscape is left to develop naturally, from seeds blown in on the wind or left by birds, the arrangement of plants on the roof will not be tidy. Further, roofs that are built to provide rocky or gravel habitat will not have the same density of vegetation that even a sedum mat roof will. However, if one of the design goals is to provide habitat for invertebrates, many of them prefer landscape without heavy vegetation (Gedge, “Zurich railway”).
The Laban Dance Center was one of the first buildings designed to provide a self-planting rubble rooftop environment, replacing the landscape that the construction of the building destroyed. On this “brown roof,” the primary material is rubble from the construction site, which has been allowed to develop naturally. This was done to maintain an important foraging ground for the black redstart, an endangered bird. Since then, additional buildings in London have constructed brown roofs for the birds.
Another striking example of a low-vegetation roof is the roof of the Zurich Railway, described by Dusty Gedge, which was designed to provide habitat for the local wall lizards, which prefer a dry, stony habitat, and a rare species of wasp that needs dry glacial riverbeds. This project uses the structure of the building to help provide varying levels of habitat – deeper areas of biomass have been arranged directly over structural members, which have the strength to hold it. This provides some heavier vegetation, which increases diversity of plant and thus habitat for invertebrates. Gabion pillars at the ends of the roof allow the lizards and crawling invertebrates to travel between the rooftop and the ground.
Roofs can be designed primarily for invertebrates, too. The roof of the Wat Tyler in Essex is in the middle of a region that is home to a number of rare invertebrates that live in brownfield landscapes (Gedge, “Bee hotel”). Wooden posts and walls with holes drilled in them were placed on the roof to provide habitat and nesting sites for mining and other solitary bees, and many bees took up residence there in the roof’s second year (Gedge, “Bee hotel”). Other green roofs in Switzerland and the United Kingdom have proven their value as habitat for bees – theexperimental field station at Barking Riverside in the East of London was visited by the rare Brown Carder Bee. Considering that the landscape around this building is devoid of nectar-bearing plants, designing the roof to include a variety of wildflowers is an important part of providing supportive habitat for this rare insect species (Gedge, “First record”).
Many people put honeybee hives on rooftops, but it is also crucial, especially in dense urban areas without a lot of flowering vegetation, to provide the bees with habitat. A project on the roof of a warehouse in Baltimore, Maryland, was built to be one of the first green roofs to provide both habitat and hives for honeybees (Gedge, “Hives and Habitats”).
The green roof on the Rossetti Bau, described in Earth Pledge’s Green Roofs: Ecological Design and Construction, was created not to replace habitat destroyed by its creation, but to lower the building’s overall environmental impact. Like the Zurich railway roof, this roof has multiple levels of soil. The roof was only minimally planted, so that native species could naturally take over. Some rare spiders, and over 50 beetle species, have been found on the roof. Some of these species have been found outside of the local riverback habitat only rarely, making the roof a model for providing habitat.
While many people may find birds to be the most appealing creatures to attract into our cities, it is extremely important for the ecosystem, and for human agriculture, to ensure that all of the insect species that act as pollinators do not lose too much of their habitat. Thoughtful design of urban roofs can provide the flowering plants, sand, and wood that provide food, habitat, and nesting space for bees, increasing their presence in cities (good for urban apiaries) and aiding agriculture. Providing habitat for a diverse population of invertebrates besides the major pollinators helps to support the local bird and bat life.
In addition to foraging resources, roofs can provide nesting places for a variety of birds. A number of species have already adapted to the built environment, and have found ways to use building structures for nesting: house sparrows and starlings will nest in crevices in the eaves of houses, holes in walls, or dryer vents. Pigeons, of course, will nest on anything that provides a ledge, matching their cliffs of origin. Peregrine falcons also make their nests on cliff faces, and will nest in boxes placed on building roofs and windowsills. However, the change in roof design over the years has lead to a decrease in some species, like nighthawks.
Nighthawks like to nest on gravel roofs, but as asphalt roofs became more popular, they stopped nesting in cities including Portland, Oregon. During a discussion on KBOO radio with ecoroof experts Dusty Gedge and Tom Liptan, they mentioned that people have been discussing the possibility of trying to encourage nighthawks to come back into Portland. Other species that are attracted to a gravel or rubble roof for nesting include killdeer (“Killdeer”) and skylarks (“Skylarks”). Pointed out in the discussion is the importance of designing such a roof so it doesn’t get too hot, which could kill the birds’ eggs. Rooftop nest sites can be especially important for birds because they are safer than being at ground level, where they are vulnerable to a variety of predators.
Wetland environments on roofs can not only provide valuable water services, like the KPMG roof, but attract birds. The Possman Cider Company, in Frankfurt, Germany, uses a similar kind of system as the KPMG roof, that collects rainwater that ends up on the roof after being circulated through the factory to cool holding tanks (Earth Pledge 88). This system also uses water collected from the parking area, which is filtered by the rooftop plants. The roof attracts local birds and has become a favorite bird-watching location.
Rooftops are not the only opportunity for adding vegetation to a building. Climbing plants provide a number of benefits for both the building and the local environment. They can be trained up the sides of the building, either allowed to cling directly to the cladding, or through the use of support structures specially designed for the species being used. The use of a separate structure for the climbing plants is more common these days than allowing the plants to adhere directly to the building. In addition, while ivy and Virginia creeper can attach directly to surfaces, a number of other climbing plants require a supporting trellis, or wires, to twine themselves around (Dunnett & Kingsbury 127).
Having a separate structure just for the plants also protects the building from any damage that might be caused by the plants, makes maintenance of the building surface easier (because the trellis with the attached plants can be pulled away or removed), and helps encourage desired patterns of growth (Dunnett & Kingsbury 127-128). Climbing plants can provide many benefits to the building itself, including providing shade in the summer, hiding boring or unattractive facades, and, if evergreen species are used, adding an insulation and wind chill blocking layer in the winter (Dunnett & Kingsbury 128 &131).
In the summer, shading the walls with climbing plants can reduce their temperature fluctuation significantly. The combination of shade and insulating air layer can change the fluctuation from ranging between 14° F and 140° F to a narrower range of 41° F to 86° F (Dunnett & Kingsbury 130). Shading is significant for the building’s interior, because it prevents direct solar radiation from reaching the building; a 10° F reduction in the temperature at the building surface can lower energy needed for cooling by 50-70% (Dunnett & Kingsbury 131).
By reducing the heat of the exterior of the building, climbing plants reduce the convention currents that would otherwise be created. This reduces the urban heat island effect and lowers dust generation (Dunnett & Kingsbury 131).
Consideration of local climate is important in choosing the plants because of their impact on the building’s interior temperatures. In places with cold winters, it might make sense to choose deciduous species, so the building can absorb solar radiation in the winter (Dunnett & Kingsbury 131). The intertwined stems of the plant will reduce some amount of wind chill, too. However, evergreen species will create a better insulating and wind chill blocking effect. Wind chill has a major impact on heating requirements – air infiltration and cooling of the walls by the wind is the cause of 30% of residential heating demand. Heating demand can be reduced by 25 percent if wind chill is reduced by 75 percent. Climbing plants also protect the building physically from hail, rain, and ultraviolet light (Dunnett & Kingsbury, 132).
Climbing plants provide other environment services. They can trap dust, absorb pollutants, and improve biodiversity (Dunnet & Kingsbury, 132-133). Vines like Parthenocissus tricuspidata can remove heavy metals like lead and cadmium from the air and rain by absorbing them into their tissue; proper disposal of the dead leaves and wood that contain the metals is required to minimize the environmental impact. The amount of dust that is trapped is proportional to the amount of leaf surface to wall area. Studies show that 0.012 ounces of dust per square foot can be captured by P. tricuspidata in one growing season, and 0.018 ounces per square foot by Hedera helix. A recently published study shows that if street canyons are planted properly with climbers and other plants, street-level concentrations of pollutants in those canyons can be reduced by up to 40% for nitrogen dioxide and 60% particulate matter (Pugh). This would significantly improve the air quality at street level and improve public health, by reducing people’s risk of asthma and other respiratory problems caused or exacerbated by these pollutants. Like all plants, vines also absorb carbon dioxide, which helps with climate change (“Green Walls”).
Vines increase biodiversity by providing habitat for a variety of invertebrates, which feed birds and bats. Foliage and flowers provide food sources and hibernation sites for insects (Dunnet & Kingsbury 133). Older vines, with denser branches and leaves, also provide safe nesting and roosting places for birds; some species can offer some protection from predators even in the winter, because the density of the branches keeps cats and predatory birds away. Evergreen species provide better winter shelter, particularly those that develop thicker growth (Dunnet & Kingsbury 163). Vines that bear fruit provide food for insects and birds, and can be an especially valuable food source for birds in the winter (Dunnet & Kingsbury 163). If vines are incorporated into a building that also has a green roof, and the vines reach the roof, they will provide a route for non-flying invertebrates to travel between ground level and the roof (“Green Walls”), which can increase biodiversity on the roof.
Climbing plants are not the only way to create a green facade that protects a building, hides an unattractive facade, and creates habitat. Certain species of shrubs and even fruit trees can be pruned and trained into growing flat against a wall, though they cannot reach the same heights as vines do (Dunnet & Kingsbury 145). Due to the need for pruning, creating a green facade with these plants requires more maintenance than using climbers (Dunnet & Kingsbury 146).
More intensive living walls, that hold non-climbing plants, can also be incorporated into building facades. Like green roofs and vine-covered facades, living walls can protect the building, lower energy costs, reduce pollutants, and create habitat and biodiversity. This type of facade treatment requires creation of some sort of support structure, containing growing medium, to hold the plants and distribute water. Hydroponic systems use a layer of propagation felt or capillary matting fixed in place over a waterproof layer of PVC that protects the building wall from the water (Dunnett & Kingsbury 182). Cuts are made in the felt or matting, creating pockets into which plants are inserted. Drip irrigation provides a steady stream of water and nutrients. This system was developed by Patrick Blanc, who has several walls installed in Paris, the most famous of which may be the living wall on Jean Nouvel’s Musee du quai Branly (Lee).
Another system for creating living walls uses a vegetation mat, similar to what might be used on a green roof. According to Dunnett and Kingsbury (183-184), most of these mats use sedums, due to their tolerance for drought conditions. However, their long-term success is not guaranteed unless they have irrigation to help them through especially dry times. In addition, sedums move around and get leggy as they mature, which can turn an initially lush and attractive wall into one that is less aesthetically pleasing (Philips).
Plants are a vital part of creating a healthy habitat for human beings, too, and not just for their ability to moderate local climate and clean the air. There are numerous psychological benefits for people who have plants around them, so in especially dense urban environments, where there is not much open ground for parks, it is especially important to consider incorporating vegetated roofs and walls into buildings.
According to research reported on by Jonathan S. Kaplan, on Psychology Today’s website, spending time around plants, like going for a walk in park, or having potted plants in an interior space, has numerous benefits, including reducing stress and increasing concentration. Seeing trees creates feelings of relaxation, and can shorten recovery time in hospitals (“Benefits of Urban”).
A growing number of hospitals are creating carefully designed healing gardens for their patients and families, for both the psychological and medical benefits offered by spending time in a peaceful, natural environment. The reduction of stress and anxiety from spending time in a garden improves healing (Kreitzer) and increases people’s feelings of general wellbeing and hopefullness. Even having a view of trees from a window can help patients recover faster (Thornburgh).
Outside of hospitals, studies have found that people who live in apartments with trees nearby report “greater effectiveness and less procrastination” than people in identical housing without trees (Thornburgh). That study reported, “it seems that trees help poor inner city residents cope better with the demands of living in poverty, feel more hopeful about the future, and manage their most important problems more effectively.” Thornburgh’s article also reports that people “felt safer in areas with trees, children were twice as likely to play in such an area, and children and adults were more likely to interact together in landscaped areas.”
Research also shows that spending time in cities can damage people, mentally: spending time on crowded, busy streets reduces memory and affects self-control (Lehrer). One of the reasons for this is the lack of nature, and the overwhelming amount of stimulus on a busy city street. As described in Lehrer’s article, the kind of stimulus you get from an urban environment requires a specific kind of attention that takes significant energy and effort. By contrast, while natural setting are also full of things that capture our attention, they do so without triggering negative emotional reactions. Seeing a squirrel running up a tree is not the same as a blaring bus horn or overloud conversation.
Adding natural features to a busy, dense urban environment, whether they are street trees, a green roof visible from a higher building, or a vine covered wall, can help balance out the overwhelming aspects of the built environment by providing the right kind of natural stimulus.
Living systems and technological systems can be integrated for the good of both. Adding solar panels, either PV, for electrical needs, or solar thermal, for domestic hot water or radiant heating systems onto a green roof is an excellent way to lower the building’s need to import energy from a power plant.
Solar panels and green roofs benefit each other. Incorporating panels thoughtfully onto a green roof, to shade areas of it, can provide a more diverse habitat and provide additional shelter for wildlife. The vegetation on the roof benefits the solar panels, too: the plants reduce pollutants and dust that would settle on the surface of the panels, and evapotranspiration cools the surface temperature of the roof, both of which improve peak efficiency of solar PV (Anacostia Watershed Society; “Special constructions”). Panels are reported to produce up to 16% more energy when placed as part of a green roof (Anacostia Watershed Society).
Incorporating either kind of solar panel into a green roof also reduces load on the roof. As described by the International Green Roof Association’s “Special constructions” webpage, “Earlier the solar units were mounted on concrete bases or slabs and partially filled with gravel; however, they are now mounted on framework which is fixed to plastic boards. The profiled plastic boards are covered with substrate and allow rain water to drain through; thus, allowing plants to grow underneath the solar panels. With the solar panels mounted on the plastic boards the load distribution is spread over a large area and prevents the roof construction from being damaged by point loads.”
All of the techniques mentioned so far are what are happening right now. There are at least a couple of different ways these treatments of building envelope can be developed in the future. One is to simply develop better technologies and greater understanding of the ecology and process of growing plants on the sides and roofs of buildings. Another possibility is to design buildings, especially high-rises, to incorporate opportunities for cliff-like habitat on the sides of the buildings (Bernstein). For example, Portland is near the Columbia River Gorge, the south side of which has fairly steep, basalt cliff sides. What would it look like, to develop buildings in Portland that imitated that kind of environment? What would a similar cliff ecosystem look like in other parts of the world, where cities are built near natural cliffs?
When designing rooftop habitat, it is important to look at local ecology, and the overall changes in ecology over an entire city, when designing green roofs or other living systems (Bernstein). If the new building is going up next to a dry, scrubby area, the local wildlife will be best served by creating a dry, scrubby roof, like the Laban Dance Center did for the redstarts. But a few miles away, buildings may be adjacent to wetter or woodier environments, and their vegetated systems should fit into that. This will also likely be better for the health of the roof systems, too, since the plants growing on it will be appropriate to the local – if slightly harsher, for being at roof level – conditions. Zoning and master planning of a city could take on an entirely different level of meaning when the roofs and walls are seen as a component of the “ecological landscape.” This would be an exciting way to have our density and our connection to nature, too.
We do not have to wait on new construction to increase the connections between the built and natural environment. Existing building stock offers an abundance of opportunities to install climbing plants and increase the integration of the built environment and the natural environment on a larger scale, increasing the amount of vegetation and the resulting benefits in cities. Warehouses, big box stores, and parking garages all provide windowless or otherwise blank surfaces that could be turned into a functional component of the environment. They should be easier to add to existing building stock by virtue of not requiring the same kind of structural support that a green roofs does. Many people who live in apartments with balconies already grow plants in pots.
Given the growing, and increasingly alarming, signs of climate and ecological change, it may be a matter of some urgency to put more effort into greening our existing buildings. The energy savings alone, merely by moderating the energy needs for heating and cooling, would be substantial. Increasing the natural environment in our urban environments, where over fifty percent of the human population now lives, will be vital in providing us with the connections to the natural world that we need to be healthy. Reminding people that nature is not “out there,” separate from us, is necessary to increase interest in preserving the natural world and restoring what we have damaged.
As Richard Fuller, an ecologist at the University of Queensland said, “We worry a lot about the effects of urbanization on other species . . . But we’re also affected by it. That’s why it’s so important to invest in the spaces that provide us with some relief.” (Lehrer) We are a part of the natural world, but we have increased our disconnection from it, with many bad outcomes. It is time to reverse the trend.
Works Cited
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