Nature has been prototyping designs far longer than humans have. And as architects strive to keep up with the rapidly evolving world of green-building standards, some of them are looking to exploit that experience by bridging the gap between nature and the built environment. The end goal: creating a functional interface between the two that improves building performance.

In the second half of the 20th century, buildings and landscape became disconnected. Many architects saw nature as an unruly force to be excluded at all costs. Nonetheless, a small but vocal group maintained interest in the interplay of the built and natural environments. And today, architects increasingly see biomimetic and biophilic approaches as practical strategies.

Contemporary systems that exemplify this interplay include green and blue roofs, green façades, living greenwalls, porous pavements, and associated systems for managing water and soil. But putting these green machines to work isn’t plug-and-play; it calls for patient cost-benefit assessment. Well-deployed natural features can improve water management and thermal control and reduce operating costs, but they are not a panacea.

Sponsored by: greenscreen
Sponsored by: greenscreen

“We have to break it down three ways,” says Signe Nielsen, a principal at the New York landscape architecture firm Mathews Nielsen. “We’ve got the up-front capital costs, the long-term maintenance cost, and then the long-term benefit to society.” In communications with clients, she recommends, “you ought to be prepared for developing an opinion on all three and backing it up with facts and dollars.” Specific metrics exist for irrigation, stormwater control, energy modeling, and benefits produced by trees, among others. Architects, Nielsen notes, can employ resources such as the National Tree Benefit Calculator, which takes location, species, tree size, and nearby land-use categories as inputs, and returns estimates of cost savings for stormwater control, electricity and natural-gas savings, air quality, property value, and carbon reduction as outputs. Related instruments exist for irrigation calculations in certain regions, but shading, thermal, and cost data require site-specific calculations.

These measurements can also be a reality check. Nielsen recalls writing a manual for green roofs in New York and noting that a 4,000-square-foot green roof with 6-inch-tall foliage does wonders for stormwater retention, but, because oxygen production is a function of leaf mass, the roof’s potential by that metric was equivalent to that of a single tree. “I remember trying to make my case to the city, and they said, ‘You know, if we just planted four trees, it would cost us a tenth the cost of a green roof,’” she says. And while the argument over including such features rarely rests on a single variable, it is important to know which will resonate with decision-makers.

Systematizing Interface Standards

The LEED system, says Frederick Steiner, Assoc. AIA, dean of the School of Architecture at the University of Texas at Austin, “did a pretty good job with buildings, but once you got outside the building envelope, there wasn’t much there. Basically it was ‘use native plantings; conserve water,’ both of which are worthwhile goals, but it doesn’t go into very much depth.” New site-scale standards are evolving. The American Society of Landscape Architects (ASLA), the University of Texas’s Lady Bird Johnson Wildflower Center, and the United States Botanic Garden have formed an interdisciplinary partnership, called the Sustainable Sites Initiative (known as SITES), with a complementary voluntary rating system for sustainable landscapes, with or without buildings.

“The USGBC, a stakeholder in the initiative, anticipates incorporating the SITES guidelines and performance benchmarks into future iterations of the LEED Green Building Rating System,” reports Mark Simmons, director of the Wildflower Center’s Ecosystem Design Group and a member of the SITES Technical Core Committee. SITES, Simmon’s colleague Steiner says, is organized around the idea of ecosystem services, the accounting of processes that nature provides gratis: clean water and air, oxygen, climatic mitigation, plant pollination. And there are other groups exploring these ideas as well. Jeffrey L. Bruce, the chair of Toronto-based Green Roofs for Healthy Cities—a group that increases awareness of the economic, social and environmental benefits of green roofs and green walls—also recommends the Cascadia Green Building Council’s Living Building Challenge, which is “projecting a standard that may take us decades to reach. They’re looking at net-zero energy, net-zero carbon, net-zero water,” he reports. “Totally off the grid.”

The trick is to determine which interfaces are appropriate. “Why do you want a green roof?” Simmons says. “What do you want your roof to do?” Beyond aesthetic appeal, choices involve thermal control, stormwater management, externality mitigation, and biodiversity. Extensive green roofs, with a light vegetative layer, differ from intensive roofs, with thicker soil, sturdier structures, and more ecological complexity. David R. Tilley, associate professor at the University of Maryland’s Department of Environmental Science and Technology, estimates that green roofs are “about five to eight years ahead of the greenwall industry in terms of market penetration, popularity, standards, and size.”

“Designers should ask clients, ‘Which of these do you want: just aesthetics, stormwater, biodiversity?’ ” Simmons says, then tailor designs to performance. “Then the onus is on the industry to say, ‘OK, you live in Atlanta, you’re limited to 100 pounds per square foot, you want to absorb a half-inch of rainwater, and you want to attract butterflies. OK, those are the specifications; thank you, we’ll go back and design it and give you a roof that can do that.’ Now, that implies a lot of accountability.”

Light, Shade, and Energy

Shade is vegetated surfaces’ primary service to the ecosystem. “Once you have a full canopy developed that’s three to four years old, and it’s matured,” Tilley says, “you’re looking at probably a 95 percent reduction in the solar load.” Canopy is measured according to leaf-area-index (LAI) relative to wall area; for each unit of LAI, sunlight decreases by about half. Effects on interior temperatures depend heavily on insulation: If walls already have a high R-value, even dramatic reductions in LAI will cut temperature only slightly, but at low R-values, a dense canopy reduces cooling costs appreciably. Replacing black asphalt with vegetation raises rooftop albedo, and evapotranspiration can add humidity to an urban atmosphere; both help mitigate heat-island effect.

The converse benefit—reducing heating loads with passive solar energy through the use of green façade systems—calls for deciduous species, which lose their leaves and thus allow light to penetrate into the building during winter. Native plants known to thrive under local conditions (climate zones, pest resistance, and soil compatibility, for instance) are preferable; consulting with local botanists is advisable.

Every region has its success stories and its problem children with regard to the plant varieties installed in a project. Maryland-based Tilley warns against using English ivy (Hedera helix), which adheres tenaciously and is aggressive enough to move beyond its support structure and enter a building through windows. Nielsen, based in New York, identifies wisteria as another potential monster: attractive and fragrant, but capable of growing 70 feet tall and forming a woody trunk powerful enough to crush metal and tear roof leaders off a building.