Against the heat How effectively green roofs and facades cool in Bonn

Bonn · Green roofs and facades are still the exception rather than the rule in Bonn. But a lot can be achieved with small means. Two women from Bonn show how effective plants on buildings can be against heat.

 A green facade is more a matter of conviction than effort, find Andrea Muno-Lindenau (left) and Hildegard Boisserée-Frühbuss (right).

A green facade is more a matter of conviction than effort, find Andrea Muno-Lindenau (left) and Hildegard Boisserée-Frühbuss (right).

Foto: Martin Wein

At best, planted roofs and facades don't just look pretty. Especially in densely built-up urban heat islands such as downtown Bonn, the center of Bad Godesberg, or the commercial areas of Beuel, they can also significantly improve the microclimate. On the one hand, the shading of the building envelope provides cooling. On the other hand, the plants retain rainwater and later cool the surroundings by evaporating it. In addition, there are other advantages because, for example, bitumen membranes or plastic films on flat roofs are no longer directly exposed to the sun and therefore last longer.

So much for the theory. But if you walk through Bonn in summer temperatures, the heat continues to build up in concrete and asphalt canyons as far as the eye can see. Green spaces are the exception rather than the rule in many neighborhoods. Green facades are almost non-existent. Even the City of Bonn's emergency program from last year, in which homeowners are reimbursed up to 50 percent of the costs by the municipality, has not substantially changed this situation.

At the Wissenschaftsladen Bonn, they are trying to reverse the trend toward more urban greenery with various projects. On a muggy afternoon, Hildegard Boisserée-Frühbuss, an agricultural economist, seeks shade under a carport at Godesberger Straße 55 in Friesdorf. Josef Küpper has installed a photovoltaic system on the roof of the shelter. It supplies electricity for electric vehicles. Solar thermal energy heats the water. Contributions also to climate protection. The microclimate in the company courtyard is nevertheless hot and stuffy. Apart from a tiny strip of greenery with five pruned trees between the parking spaces, there is no greenery. Everything is paved and without shade.

Project aims to promote greening of buildings

That's about to change in the "Factor Green" project - sponsored by the Federal Environment Agency. The grassy area could be planted, and the bare wall of the neighboring property could be greened over, Boisserée-Frühbuss hopes. "That would have a noticeable effect because the heat wouldn't bounce back as much," she says.

A planned addition to the hall-like company building would have a green roof terrace and a green facade facing southwest. Employees are expected to help shape the design. "It's also about a mental shift in the mind," Boisserée-Frühbuss says. The idea that plants and animals have a place in the city, at best in ornamental gardens, must be overcome, she says. Instead of plucking every blade of "weed," parking lots, for example, could simply be unsealed with so-called grass pavers. When it rains, moisture can then seep away instead of flooding the overloaded sewage system.

At Andrea Muno-Lindenau's home in Duisdorf, you can see what the result can look like. On the way to her house on a side street, a lone rose bush in a gravel area graces a front yard. The path leads over a driveway with said grass pavers.

Behind the house, a vine has climbed up the south façade for two decades. "The only thing we do is cut the gutters clear once a year in the fall," Muno-Lindenau says. How much heat that keeps out, she can't say. After all, the house meets low-energy standards anyway and is insulated accordingly.

Gardens as cool oases in the city

While the heat builds up outside on the residential street with its asphalt lanes, sidewalks and a spacious turning ramp, her garden is a cool, green oasis. There are no stones here. Clematis and other climbing plants have conquered the wooden fences to the neighbors. In the early evening sun, the insect hotel is as busy as Cologne-Bonn Airport these days. The terrace is made of native Douglas fir wood. The rainwater from the roof collects in a wooden barrel and thus remains on the property. The water is sufficient to occasionally fill the small pond in a corner of the garden, which serves as a breeding ground for insects and a watering place for birds.

The additional costs for greenery are kept within narrow limits, says Muno-Lindenau, who in return can offer guests fresh tomatoes to try. The cost of greening and shading measures is also often low, adds Boisserée-Frühbuss. Entire industrial estates can be greened in cooperation between local authorities and companies. However, those wishing to convert should seek the advice of experts to ensure that trees or climbing plants do not attack the fabric of the building years later.

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