Hybrid in Chicago

Increasingly, green roofs are defying clear-cut categories of extensive and intensive. "Semi-intensive" refers to roofs with a medium-depth growing medium that supports a plant diversity in between those of extensive and intensive roofs. Also, some projects include both intensive and extensive plantings side by side.

An example of this is the 11-story Chicago City Hall, greened in 2000 as a demonstration project for the city's Urban Heat Island Initiative. Engineers assessed the loading capacity of the existing structure and discovered three conditions, suggesting three types of plantings. The team was able to plant trees and shrubs in relatively deep soil over the support columns. Over the frames for the former skylights, they installed semi-intensive plantings. The rest of the 20,300-square-foot roof was planted in ground cover in shallow soil. In all, there are 20,000 plants of more than 150 varieties.

The growing medium consists of customized soil mixes designed to provide firm anchorage for the plant roots and retain moisture and nutrients (nitrogen, phosphorous, calcium, magnesium, oxygen) but to be porous enough to avoid excess water accumulation. The mix of mineral and organic soil components weighs about 65 lb./cubic foot, slightly over half the weight of regular soil.

As part of the demonstration project, city employees conducted considerable testing after the planting was established. One remarkable result showed a comparison between the City Hall roof and a nearby black-tar roof. On a hot, sunny August day, the surface temperature at City Hall ranged from 91 to 119 degrees Fahrenheit, while the black-tar surface reached a scalding 169 degrees. In the spaces below the conventional roof, cooling systems were working harder to maintain comfort.

The Olu Bar & Eco-Lounge is home to Brooklyn's first living wall installation. The building was designed by Evangeline Dennie and is seeking LEED Gold certification. Photo Mister Gates

Green Walls

Vegetated walls have not achieved as much popularity in North America as green roofs. This may be partly because ivy and other climbing vines are notorious for causing damage to structures. They can grow under and pull away shingles or dig into and destroy mortar or stucco. Unchecked over time, they can cover up windows. So, half the challenge of greening a wall is to design an appropriate wall surface and support structure. Choosing the correct plant species can avoid future destructiveness or even prolong the life of the wall by shielding it from ultraviolet light. Thermally, urban green walls can be more effective than green roofs because there is more surface area available on high-rise walls than roofs. In contemporary practice, the greenery is usually held away from the walls, and its effectiveness in cooling by shading can exceed the effectiveness of roof vegetation in insulating. Walls are usually more visible from street level, improving the landscaped environment for passersby.

 

The Anne Demeulemeester Shop in Seoul features a vertical garden designed by Mass Studies. The material is geo-textile planted with herbaceous per-ennials to form a living facade. Photo Yong Kwan Kim

 

 

Green walls are generally categorized in one of two ways. "Facade greening" refers to walls that are covered by climbers-plants that are rooted in the ground and can theoretically grow to a height of 100 feet. On high-rises, they are usually planted on successive balconies along the length of the building. The support structure they require varies based on the plant species and their method of climbing. Ivy, for instance, is a self-clinging climber that extends small roots into any rough surface. As long as there are no cracks or other vulnerabilities in the material for the roots to penetrate and damage, this is an inexpensive facade-greening method. Twining climbers, such as clematis and wisteria, require additional support. Steel-anchor and cable systems are replacing traditional wood trellises for this purpose.

 

The foundation on the south facade of the Finnish Embassy in Washington, D.C., is almost completely hidden behind a metal screen covered in lush flowering vines. Photo Roger Foley