The Era of Thin-Layer Rooftop and Wall Greening, circa 2000

The Era of Thin-Layer Rooftop and Wall Greening, circa 2000
From the late 1990s through the 2000s, Japan's major cities were enveloped in a serious heat wave. Urban structures, with asphalt and concrete covering most of the ground surface, absorb sunlight and continue to radiate heat even at night. The heat island effect has become more pronounced over the years, and nighttime temperatures in midsummer are difficult to lower, leading to an increased risk of heat stroke and higher demand for electricity. A study published by the Ministry of the Environment at the time revealed that rising temperatures are closely linked to a decrease in green space in cities, and that the exhaust heat from buildings is pushing up the air temperature over cities. In response to this urban environmental crisis, rooftop greening and wall greening began to attract expectations as technologies that could fundamentally change the thermal structure of cities.

In the early days, rooftop greening was mainly based on a method that required a thick layer of soil, but its weight placed a heavy burden on the structure of existing buildings, hindering its widespread use. In the 2000s, thin-layer greening systems, which require only a thin soil layer of 5 to 15 cm, rapidly evolved and became widely applicable to urban buildings. Lightweight artificial soils with water retention properties were developed, as were mats and other porous, lightweight materials that provided both drainage and aeration, and vegetation centered on sedums, which are resistant to drought, came to be widely adopted. The thinning of layers was not only a technological innovation, but also created the social conditions for expanding greenery while making the most of existing urban spaces.

The nature of wall greening also changed dramatically during this period. In the past, it was often treated as an architectural design feature, but in the 2000s, experiments proved that it was effective as a heat control measure by suppressing the rise in surface temperature. In some cases, the transpiration effect of the plants lowered the air temperature near the wall surface and blocked direct sunlight, resulting in a 10 to 15 degree Celsius drop in the wall surface temperature. Patrick Blanc's vertical garden, which attracted attention in Europe for its urban renewal, was introduced in Japan, spreading a new perspective on the fusion of architecture and ecology. In Japan, a variety of construction methods have been developed, including the panel-type wire-attracting pocket type, allowing for flexible selection according to the type of building and its location.

On the policy front, a major turning point came in 2001 when the Tokyo Metropolitan Government announced its policy on heat island countermeasures, making rooftop greening mandatory for new buildings over a certain size. In 2003, the Ministry of Land, Infrastructure, Transport and Tourism issued an outline for the promotion of rooftop and wall greening, which, together with the 2004 Heat Island Action Plan of the Ministry of the Environment, established a trend for the entire city to accept greenery as part of its social infrastructure. Studies conducted at the time reported a reduction in rooftop surface temperatures of around 30 degrees Celsius, as well as a suppression of temperature increases inside buildings, demonstrating that greening contributes to the improvement of the urban heat balance. Various effects of wall greening were also confirmed one after another, such as reduction of noise and improvement of moisture circulation by adsorption of fine particles.

In recent years, thin-layer greening and wall greening have been increasingly recognized as climate change countermeasures, as they restore the ecological functions of urban ecosystems and contribute to CO2 absorption. From the perspective of ensuring biodiversity in urban areas, greening systems are beginning to provide an important foundation for encouraging the return of insects and birds. The maturation of technology is not limited to mere landscape improvement, but is changing a wide range of areas, including the regeneration of urban temperature structures, energy load ecosystems, and so on.