Does densification lead to more heat stress in cities?
High temperatures and more frequent heatwaves are causing many people to doubt whether high-density urban planning is still sustainable. However, building physicist Jan Carmeliet argues that even dense cities can be cool if they are planned correctly.?
Switzerland is sweating – especially in the cities. Anyone travelling between the countryside and the city centre these days will feel the difference: cities are real heat islands where temperatures are even higher. Concrete and asphalt heat up mercilessly during the day and release heat deep into the night. This is why urban areas are particularly vulnerable to heat stress.
More vegetation and green spaces could help mitigate urban heat – but spatial planning demands that our cities develop inwards and become denser to stop urban sprawl. The dilemma is raising growing concern over whether current urban planning principles are still fit for the future.
And not without reason: in the past, densification was all too often at the expense of green spaces, reducing thermal comfort and increasing the risk of heat stress. There is therefore a widespread belief among urban planners, authorities and researchers that urban densification inevitably leads to higher temperatures.
As a researcher focused on reducing urban heating in cities, I believe this assumption to be wrong. The conclusion that we should not densify further is also too short-sighted in my view. The problem is not density per se, but how we design dense urban environments.
Dense structures are not necessarily hotter
As part of the SWICE project1 of the Swiss Federal Office of Energy, we examined the current debate on the role of urban densification (see box). To this end, we analysed the heat exposure of neighbourhoods in Geneva and Fribourg and used simulations to investigate how different scenarios for greening and densification are affecting the thermal comfort of residents in Sch?nberg in Fribourg.
The results are striking: Densely built-up neighbourhoods with well-placed greenery and many trees – as in some parts of Geneva – can offer more thermal comfort than open, yet less shaded and poorly ventilated areas such as the Sch?nberg neighbourhood in Fribourg.
In a recent position paper we also show that smart densification when thoughtfully integrated with green spaces can even enhance the local microclimate.2
Densification and greening must go hand in hand
The example of Sch?nberg is particularly compelling. We simulated three scenarios: pure densification, pure greening and a combination of both – in other words, increasing the residential capacity and integrating green spaces wherever possible, including green roofs and fa?ades, trees along the streets and in public squares.
The outcome: only the combined approach delivers meaningful progress. The key improvements stem from the increased shading provided by trees and buildings. This together with the cooling effect by transpiration from added vegetation reduces perceived temperature by up to 5 degrees Celsius.
“Local thermal comfort arises from the delicate balance between shading by buildings and trees, sufficient ventilation and the cooling effect of vegetation.”Jan Carmeliet
More shade from trees and taller buildings is one of the most important factors for enhancing thermal comfort. However, if planted incorrectly or positioned unfavourably, they can obstruct wind or block entire cool air corridors. Preserving urban ventilation corridors as far as possible is the key to efficiently dissipating heat.
While thermal comfort may still decline in some areas due to insufficient shade or blocked airflow from new buildings and trees, this impact can be minimized with careful planning.
Precise planning instead of one-size-fits all recipes
Thermal comfort results from the delicate interplay of shading from buildings and trees, unobstructed ventilation and cooling vegetation. Because this interplay is highly localised, an integrated approach is needed that accounts for the specific needs of each neighbourhood, the local climate and the available space and resources.
In contrast, general guidelines and generic solutions, are rarely effective. If we densify merely for the sake of density, there is a risk of the local deterioration of quality of life. We therefore need to consistently integrate ‘denser’ and ‘greener’ planning and carefully design wind flows, shadow patterns and heat distribution.
This requires precise tools such as microclimate simulations to accurately understand – and improve – the effects on people's well-being. I am confident that with thoughtfully positioned buildings of appropriate height and strategically placed vegetation, thermal hotspots can be largely avoided. The tools to make this happen are already at our disposal.
The city of the future is dense and green
My conclusion: densification is necessary to save scarce land. But it must never come at the expense of quality of life. If we build cities that not only offer more living space, but also more shade, cool air, green space and access to nature, densification is not a step backwards – but a smart and vital strategy to a climate-resilient future.
From my experience, urban planning can gain tremendous value from participatory living labs. These labs bring together all stakeholders– including residents – in order to co-create solutions that are both effective and broadly accepted by the community.
SWEET and SWICE
SWEET (SWiss Energy research for the Energy Transition) is a external page funding programme of the Swiss Federal Office of Energy (SFOE) with the aim of driving innovation for the Swiss Energy Strategy 2050. external page SWICE is a consortium funded by SWEET that aims to improve the quality of life of Swiss cities in the energy transition. The project comprises six participatory external page living labsin Switzerland, in which researchers work together with residents, authorities, property managers and urban planners to develop and test strategies to combat urban heat.
1 The external page SWICE project
2 Project website SWICE at the Chair of Building Physics