New KU Leuven research shows that targeted changes to buildings and urban space can significantly reduce the impact of extreme heat. So-called 'cool roofs' – roofs that reflect solar radiation instead of absorbing it – turn out to be particularly effective. In Brussels, up to a quarter of the heat-related deaths during the July 2019 heat wave could have been prevented with the implementation of cool roofs.

Extreme heat constitutes a growing risk to public health, especially in highly urbanised regions like Belgium. During the unprecedented heat wave of July 2019, with the highest temperature ever measured in Belgium, urban temperatures rose sharply, with serious consequences for vulnerable groups. In a study recently published in Environment International, scientists from KU Leuven, Sciensano, VITO, and University College London investigated how Belgian cities can better adapt to this kind of extreme weather conditions.

One of the interventions investigated in the study is the implementation of cool roofs. These are roofs that are intentionally made more reflective, for example by installing white or light-coloured roof covering, by applying special coatings, or by using specific materials that reflect solar radiation. Traditional dark roofs absorb solar radiation, which warms up the roof significantly, whereas cool roofs reflect much of that radiation.

As a result, buildings with a cool roof store less heat, which means there is a lower increase in the temperature of the surrounding air. When cool roofs are implemented on a large scale – for example in entire neighbourhoods or cities – this can lead to a decrease in the outdoor temperature, especially during hot, cloudless summer days.

‘You can compare it to the difference between wearing a black T-shirt or a white T-shirt in the sun,’ says KU Leuven researcher Fien Serras. ‘You stay cooler in the white T-shirt. The same principle applies to roofs, but with an impact on the whole city.’

Using a high-resolution (1 km) climate model, the researchers simulated how Belgian cities would react to different adaptation strategies during the 2019 heat wave. In this study, they took into account differences in residential density, urban structure and population distribution.

‘The simulations show that cool roofs lower the maximum temperatures during the day,’ says Fien Serras. ‘On average, the air temperature in urban areas would decrease by just over 2 degrees Celsius, with even greater effects in densely populated city centres with plenty of roof surface area.’

In addition to cool roofs, the scientists also investigated the impact of more urban green spaces. This translates, for example, into more trees, parks, and unpaved areas. More green spaces in combination with cool roofs proved important to lower the temperatures at night, which is crucial for the human body to recover after hot days.

The researchers linked the results they obtained on temperatures to an existing heat-mortality correlation studied by Sciensano in the context of heat-related mortality in Brussels. During the five-day heat wave in July 2019, an estimated 47 deaths were linked to extreme heat. According to the study, a large-scale application of cool roofs could have prevented around 25% of those deaths.

Combined strategies – cool roofs and more green – bring significant health benefits by ensuring cooler temperatures both day and night.

Belgium is one of the most urbanised countries in Europe, and climate change will increase the frequency and intensity of heat waves in the future. This research provides policy-makers with scientifically based and readily applicable strategies to make cities more resilient against extreme heat.

‘The results show that relatively simple changes to existing buildings – such as increasing the reflective capacity of roofs – can have an important social impact, especially when strategically combined with policy on green spaces,’ concludes co-author Inne Vanderkelen.