Hong Kong’s public housing estates are going green. In recent years, the Housing Authority has been using its estates as laboratories for the latest green technologies, a move that could help reduce Hong Kong’s air pollution and encourage more sustainable building practices.
Some of the authority’s latest efforts can be seen in Yau Lai Estate, a newly-built housing estate in Yau Tong that opened last year. Standing near the estate’s main entrance are three green walls covered in a mix of grass and climbing plants. While the walls also serve a decorative purpose — the arrangement of red and green plants on one is based on a drawing of a fish made by Yau Tong schoolchildren — a study completed last November found that the greenery cooled temperatures on the walls’ exterior surface by up to 16 degrees. Temperatures on their interior surface dropped by 1.5 to 3.5 degrees.
If the green walls are adopted on a widespread basis, they could significantly reduce housing estates’ energy consumption by cutting air-conditioning costs, said the Housing Authority’s chief architect, Clifford Cheng Chiu-yeung. They would also help cool the outside ambient temperatures. That in turn would reduce Hong Kong’s urban heat island effect, which has been making summer weather even hotter and more unstable than normal.
More than two million people, or 29 percent of Hong Kong’s population, live in the Housing Authority’s 195 public rental estates. 15,000 new housing units are built every year.
In addition to the green walls, 10 green roofs have been built over the past few years, mostly on low-rise structures within the housing estates, such as wet markets and rubbish depots. 20 percent of each new housing estate’s land area is now required to be devoted to greenery and the authority plants one tree for every 15 residents.
“We’ve begun to rethink how we should build,” said Ada Fung Yin-suen, the authority’s deputy director of development and construction. “Slowly and gradually over the last 10 years, we’ve increased the greening ratio of our properties by planting more trees and gardens but also by looking into things like green roofs.”
Each green wall and green roof is a testing ground for different types of plants and maintenance techniques. Before planting, a root barrier and several drainage layers are built on the roof, followed by a lightweight soil mix. At Choi Ying Estate in Kowloon Bay, a layer of grass was laid on top of a shopping centre. In nearby Diamond Hill, Fu Shan Estate’s wet market was covered by different-coloured species of sedums — a small, water-retaining shrub — arranged in the pattern of a fish, a design meant to symbolize prosperity.
Hong Kong’s long, hot summers limit the number of plants that can be used. According to Evans Iu, the Housing Authority’s chief landscape architect, only three of 50 species of sedums can survive here. In one experiment at Ching Ho Estate in Sheung Shui, a green roof covered in sedums was not watered for three months in an experiment meant to test the plants’ durability. More than 70 percent died.
“Originally, we used some decorative plants in our greening projects, but we were criticized by ecologists, so we’re exploring more native species,” said Iu. One local species of wedelia, a creeping plant, was rejected for use on roofs and the ground because it attracted rats, but it has proven effective for vertical greening. After all said, Iu, “rats can’t live on walls.”
The Housing Authority’s willingness to experiment has been a boon for local developers of green building technology. The walls at Yau Lai Estate make use of a newly-patented green panel technology developed by Strongly International. Having a high-profile public body like the Housing Authority as a client has helped the company demonstrate that its technology is feasible, said its technical director, Jaime Yeung.
Strongly’s products use a soil substitute made from a mixture of organic materials such as bark and recycled paper. It weighs 90 percent less than natural soil and does not settle and harden when wet. For a green wall, the soil is placed in small plastic pots that are plugged into brackets on a steel frame. About thirty different plants can be used, depending on wind conditions, the local micro-climate and how much maintenance a client is willing to perform, said Yeung.
He said that Strongly’s own studies have shown that a green wall can reduce interior temperatures by as much as 8 degrees. But it’s harder to estimate how much of an impact a green roof or green wall has on the city’s wider environment.
“The heat island effect in Hong Kong is more severe than in other parts of the world because it’s very congested and the ventilation is poor because of all the highrises,” he said. “But the impact depends on the density of installations. If you only have a 10 square metre wall in an area like Mong Kok, it won’t make much difference. It really depends on government policy to make it more commonplace.”
The problem, however, is that green panel technology is expensive to implement and maintain. A green wall that Strongly developed for Sau Mau Ping Estate in Kwun Tong cost $6,000 per square metre to build. Depending on the type of wall, annual maintenance can cost three to eight percent of the wall’s initial construction cost. For the Housing Authority, which draws most of its revenue from rental income, the expense of building green walls and green roofs means that only a handful of projects can be completed each year.
“We want to green everything possible, but we can’t do it extensively unless costs come down,” said Fung. “We’re a developer with modest or even poor means.”
Jim Chi-yung, a professor of geography at the University of Hong Kong, is currently performing a vertical greening experiment on the roof of the university’s main library. “Very few people are willing to invest millions of dollars to green a wall,” he said. “Some developers are trying to put flower pots on walls, basically. We should make use of nature’s climbers without having to put soil on the walls. Otherwise the methods used are so expensive.”
Another problem is public awareness. Hong Kong has been slow to embrace green technologies, said Iu, because many people in Hong Kong still are not aware of the city’s environmental challenges. In one estate, he said, residents that were allergic to pollen from native cottonwood trees asked the Housing Authority to chop the trees down. The authority refused.
“Most people brought up in such an urban environment have no knowledge about nature,” he said. “Many people are scared of caterpillars without realizing they are harmless, for example. We need to change the mentality of people towards greening if we want to go forward. The most critical thing is to get our residents to accept and agree with what we’re trying to do.”
For that reason, the Housing Authority is investing in so-called environmental “software” to complement hardware like green roofs. Last year, it invited tenants to grow their own plants in estate gardens. Ho Kei-yim, 66, a resident of Un Chau Estate in Cheung Sha Wan who works as a gardener in the Botanical Gardens, jumped at the chance to participate.
“When I was small, I lived in the New Territories and had a big garden. Planting things was my only hobby,” he said. “I don’t understand much about Hong Kong’s environmental policy but the air quality here is very bad. It’s more acceptable in the estates. My friends told me they really loved my plants.”
Back at Yau Lai Estate, the green walls have been well-received. Oscar Wong, 19, was walking to the MTR when he stopped to gaze at one of them. “I think it’s good for the environment,” he said. “In Hong Kong, the air is very polluted, and maybe this can help make it cleaner.”
This story was originally published in the January 24, 2010 edition of the Sunday Morning Post.
Tags: Green Roofs, Hong Kong, Housing, Kowloon, Vertical Greening