Stuttgart, Germany, July 12, 2024. Current weather forecasts predict new record temperatures.Next week the maximum temperatures in south and south-east Europe are expected to reach 46 degrees, and in Italy the authorities have declared a ‘red alert’ in seven cities because of dangerous weather conditions – including the most populous city, the capital city of Rome. The effects of this heat wave are also expected to be felt in Germany. The heat will particularly affect cities, where the high proportion of land which is sealed with heat-retentive materials will lead to temperatures which are up to 15 degrees hotter than in the surrounding areas. “This so-called ‘urban heat island effect’ has an extreme impact on human health. Dehydration, cardiovascular problems and breathing difficulties especially affect the elderly and people with health problems,“ says Gregor Grassl. As Associate Partner and Head of Green Urban Development at the consulting firm Drees & Sommer, he knows what measures cities can take in response to heat.
The good news is that towns, cities and villages are not helpless in the face of the climate crisis. Examples of particularly effective measures can be seen in the projects monitored by Drees & Sommer: the city of Rastatt (federal state of Baden-Wuerttemberg); the Berlin TXL district development; the municipality of Dormagen (federal state of North Rhine-Westphalia).
Five Recommendations to Prevent Urban Heat Islands
1) Make sure there is enough shadow
The easiest and cheapest way to protect roads and open spaces from the heat is to use trees or other elements which offer shade. Simple features such as roofed bus stops or shaded benches offer a place for people to rest after great exertion. Gregor Grassl comments: “Trees in cities are extremely important. Trees not only give shade, but they also cool the air by evaporation.” In Rastatt, about 1,000 newly planted trees in the urban area ensure cooler temperatures in the summer. At the same time, they also absorb carbon dioxide and pollutants and produce oxygen, thus improving the air quality.
2) Use green façades for cooling, open up sealed ground
Opening up sealed ground areas also plays an important role in improving the micro-climate. “Open spaces and paths do not need to be covered in asphalt. Gravel surfaces like in beer gardens, or turf brick stones for car parking, reduce the heating effect and are actually cheaper than asphalt. Unfortunately, numerous areas such as traffic islands are still sealed, instead of making them into flower meadows which can contribute to biodiversity,” says Gregor Grassl.
The use of vegetation in cities includes not only trees and green strips of land, but also vertical green façades such as that of Drees & Sommer’s office building OWP12 in Stuttgart. There, the green façade covers an area of more than 100 square meters over three stories and a height of 12 meters. As a result, the building absorbs less heat, the plant-covered façade filters pollutants from the air and insulates the building from noise. An extra bonus is that the green area offers a habitat for plants and animals and thus ensures greater biodiversity.
3) Use light-colored surfaces with a high Albedo value
In addition to using more green spaces, light-colored reflective materials can also offer an effective remedy to the heat in cities. On hot days, these surfaces can reduce any excessive heat radiation. In urban planning this is known as ‘the Albedo effect’. Short-wave radiation is reflected and the material does not become hot. The Albedo effect has particularly positive results in densely developed areas with large roof surfaces. Light-colored concrete surfaces, concrete or natural stone paving and gravel-based surfaces are most suitable. A combination of coarse surfaces, porous materials and brighter colors in the covering materials provide a lower surface temperature and a higher thermal storage capacity. Where necessary, surfaces can be treated retrospectively by the application of a lighter color.
4) Use low-energy solutions for night-time cooling
Many people are not aware that air-conditioning systems actually reinforce the urban heat island effect. “Air-conditioning systems such as split design coolers are twice as bad for the outdoor air because they run at the times when the air is hottest. Although they cool the interior of the buildings, the exhaust heat emitted at the same time heats up the outdoor air. This means that even more cooling is needed. This is a vicious circle,“ Gregor Gassl points out.
He suggests that it is better to use low-tech systems in buildings. This involves installing a large heat storage mass in the building which can be cooled by the outdoor air at night. The doors and windows are kept closed by day. Gregor Gassl explains: “But this principle does not work when it is too warm outside at night. So in the future development, even buildings which are actually energy-saving and sustainable might need to be refurbished because of climate change.” As an alternative for natural cooling, underfloor heating can relatively easily be used as underfloor cooling in summer. One option is to cool the water circulation system at night and transport the heat of the interior rooms to the outside. It is also possible to use ceilings as cooling surfaces.
At the district level, low-energy networks can be used to provide both heating and cooling. In an ideal scenario, water is used for cooling in the summer and heated in this way. The heated water is then stored. In the winter, the hot water is used for heating and cooled again. In the summer, this process can even have a positive effect on the overall energy balance.
5. Build higher to counter overheating
As Gregor Gassl underlines: “Tall buildings provide mutual shade and protect apartments from overheating. If this is to work, the window surfaces must not represent more than 40 percent of the outer façade. Glass palaces are expensive in their consumption of energy, both in summer and in winter, because glass has a poor insulation value.“
Another plus point: skyscrapers create turbulence and updrafts. This improves the overall air flow in the whole district. If used systematically, tower buildings can improve cooling and have a comparable effect to natural landscape features such as rivers which offer cooling with the water, act as a conduit for fresh air and provide a natural ventilation zone because of the movement of the water.
Two Sides of the Same Coin: Climate Protection and Climate Change Mitigation
The protection of the climate and adaptation to climate change are two inseparable elements, the Associate Partner explains, which must not be played off against each other: “We often discuss whether we should install green roofs, or whether it is better to use them for photovoltaic systems. The first is a climate change mitigation measure, the second aims to protect the climate.” But numerous studies show that these two measures can easily be combined, and that they can even provide synergies. Photovoltaic modules on green roofs actually achieve a higher yield because they operate more efficiently because of the cooling effect of the green roof.
The impact of heat in urban areas is a major challenge for both the present and the future. But it also offers us an opportunity to make our cities more pleasant to live in, more sustainable and more resilient. We can take systematic action not only to reduce the heat load in cities, but also to gain further benefits such as an improvement in the air quality, more efficient use of energy and greater biodiversity.
Examples of Drees & Sommer’s participation in climate-related projects:
Climate adaptation plan of Rastatt: the city has prepared a plan comprising ten measures to be implemented over the coming five years. More green, more shadow, less concrete shall cool down the city.
Climate adaptation plan of Dormagen: an impact analysis constitutes the basis for the design of a climate adaptation plan.
Berlin TXL district development: comprehensive greening and unsealing of surfaces have been key factors of a district development that focuses on climate protection, climate change mitigation and biodiversity planning.
Neckarbogen district (heilbronn.de): roof greening, green courtyards and rain water harvesting for irrigation during periods of water scarcity ensure a cooler environment during the warmer periods.