The heat island is a real challenge for metropolises facing increasingly high temperatures, particularly in summer. Not only does this phenomenon have an impact on the health and well-being of city dwellers, but the risks are increasing as climate change accelerates.
The urban heat island reflects the inability of highly urbanized areas to dissipate accumulated heat throughout the day, once night falls. This can result in a temperature difference of several degrees with surrounding rural areas. The fight against the heat island is not only an urban planning issue, but also an eminently social and political one, and must be made a priority if we want our cities to remain livable as heatwaves are set to increase in number, duration and intensity.
The stakes are first and foremost health-related but also encompass energy and economic considerations.
What are the solutions? How can we rethink the city to confront this new climatic reality?
UNDERSTANDING HEAT BUILD-UP FACTORS
Some of cities features foster heat build-up. The materials used in construction play a key role, and some highly contribute to overheating. This is particularly true for dark materials, whereas light materials reflect more solar energy. Albedo measures this reflective capacity. In concrete terms, the higher the albedo, the greater the reflection and the less heat the material stores.
The growing demand for housing in cities has also prompted builders and communities to erect tall buildings that trap heat and and hinder air circulation, which is an aggravating factor. These are known as urban canyons, in reference to areas that are steep and, by their very nature, difficult to cool.
Heat islands also result from anthropogenic phenomena, steeming from human activities generating additional heat in urban areas 24/7. In addition to the pollution and greenhouse gases they emit, combustion-powered vehicles are a major example.
Another aggravating factor is the use of air conditioning. The efficiency of air conditioners designed to cool indoor air belies the amount of heat they release outside, raising the ambient air temperature in urbanized areas. All this comes at a very high energy cost.
If the outside air cannot cool down sufficiently during the night, then nighttime ventilation of buildings will be insufficient to provide effective natural cooling before another day of heatwave.
COMBATING THE HEAT ISLAND: A CHALLENGE FOR CIVIL ENGINEERING
There are two main points to consider. The first is to reduce the amount of heat absorbed by outdoor spaces and buildings in the urban fabric, by choosing the right materials and shading, greenery and by actively addressing unnecessary heat loss.
The second is to refresh the atmosphere, both indoors and outdoors, in the various living and working places.
Reflecting and shading
There are many solutions available to fight against heat islands, starting with those designed to reduce heat accumulation in urban spaces. One of them is to work with reflective materials, which have a high albedo. For instance, a white roof ends to reflect heat, while a black roof tends to absorb it. However, caution is needed regarding the impact of solar energy reflected by white surfaces on individuals nearby, as they may absorb and accumulate this heat, with potential risks to their well-being or health.
To provide effective protection against solar radiations, shading devices – whether fixed or removable, textile or integrated into the architecture – are being increasingly employed. They help protect outdoor surfaces that are too mineral and dark, as well as walls and terraces of buildings. Sunshades or movable shutters are preferred solutions, whereas interior blinds should be avoided as they trap heat behind glass surfaces. The new types of active glazing available to architects – thermochromic, electrochromic, photochromic (and even photovoltaic) – provide insulation from summer heat, while allowing beneficial solar gains in winter.
Greening and managing water resources
Greening is also a relevant solution, generating two beneficial effects. Firstly, whether climbing or tall-stemmed, vegetation provides natural shade. Secondly, it refreshes the atmosphere through evapotranspiration, the ability of its leaves to evaporate the water drawn by its roots, thus cooling the surrounding air.
However, this vegetation still needs to hold sufficient water throughout the droughts and heatwaves periods expected in the coming decades.
This brings us to a crucial concept: the management of water resources. A real civil engineering issue, the question of urban heat islands is inevitably linked to the management of rainwater and precipitation in urban areas. Ensuring proper management of local water reserves, whether under normal or exceptional conditions, calls for the development of reserves such as rain gardens, rain trees, Stockholm pits and reservoir pavements, as well as ditches and submersible gardens, which will also help to limit flooding. The restoration of mineral soils permeability will allow rainfall infiltration. Options include draining asphalt, concrete or paving stones.
As a corollary to these bio-inspired cooling mechanisms, the issue of the aeraulic design of neighborhoods is crucial to enable the circulation of fresh air according to prevailing wind patterns. Coupled with water features such as ponds, basins, or even water jets and misting devices, the circulation of cooled air will help lower temperatures in the vicinity.
Maximizing thermal inertia and phase shifting, dissipating heat
To improve summer comfort, we look for two qualities in our buildings: firstly, they must be able to accumulate and retain interior coolness over several days, known as thermal inertia. Secondly, they should delay the intrusion of external heat for at least the duration of the day.
Insulating materials with a high thermal phase shift, such as wood fiber panels, will then be used externally, while floor slabs and masonry walls will reinforce thermal inertia indoors. In addition, if we use geosourced materials such as raw earth, for example, for interior partitions, then passive regulation of indoor humidity will also be improved, enhancing perceived indoor comfort.
For optimum comfort during long periods of summer heat, it is necessary to «recharge» this internal thermal inertia during the cooler hours of the night by overventilating the building. Additionally, allowing the exterior insulating materials to «discharge» the amount of heat accumulated during the day is curcial, provided the outside temperature can drop sufficiently.
Reducing anthropogenic inputs
Another significant measure in the fight against urban heat islands involves reducing all anthropogenic sources of heat emitted outdoors. This notably means reducing, and if possible eliminating, the use of air conditioning, and switching from thermal mobility to electric or sustainable mobility.
Indoors, the focus should be on limiting heat sources such as lighting, by prioritizing natural lighting, skylights and LED lighting. As well as various electrical and electronic appliances, especially office or audiovisual equipment, particularly when not in use or being watched.
Cooling buildings
Once these previous measures have been taken, or if they prove insufficient or inapplicable, the challenge is to move away from the use of air conditioning and prioritize alternative cooling methods. Some, based on relatively ancient principles such as traditional Persian badguirs (wind catchers), Canadian wells, or Provencal wells, remain effective and inspiring today. Additionally, geothermal systems can be used, drawing coolness from the ground or water resources.
To cool indoor air, ventilation systems can be deployed, preferably double-flow systems, allowing for adiabatic cooling based on the cold generated by water evaporation. An interesting principle involves absorption heat pumps coupled with thermal solar panels, generating cold from the sun’s heat. When waste heat is produced, it can be captured, for example to produce domestic hot water or to be re-injected into a district heating network.
A COMMITMENT TO RESEARCH AND INNOVATION
Developing a transdisciplinary approach
ESTP is the benchmark engineering school for sustainable construction. In addition to our engineering training in thermal engineering and micro-climate control, we offer Specialized Master’s programmes in sustainable construction and housing expertise, as well as project management for ecological engineering works. In collaboration with the ESTP Foundation, we have also created a new chair this year focusing on ecological civil engineering, in partnership with AgroParisTech and the AgroParisTech Foundation, Art et Métiers, the Professional Union for Ecological Engineering (UPGE), VINCI Construction and Eiffage Group.
Our Research Institute is involved in various largescale projects to address the issue of urban heat islands, such as transforming the ESTP campus in Cachan into a living laboratory, and an open and collaborative innovation hub for studying solutions to combat urban heat islands.
Originally limited to a few thousand square meters, the project aims to provide guidelines for the development of nearly the entire 7-hectare campus, integrating into the green, blue, brown, black, or aerial networks of the town of Cachan and surrounding municipalities. This multi-scale aspect, from the local place to the metropolis is one of the most structuring elements of the systemic approach to be developed.
Together with other academic, entrepreneurial and territorial partners, we participate in collaborative projects aimed at building the knowledge, tools and techniques needed to advance the state of the art in this field.
The key to success of such project lies in working in a transdisciplinary way, integrating a wide range of expertise: agronomy, biology, climatology, geography, technology, urban planning, humanities and social sciences, economics, law, political science, functional and artistic design. Bringing together experts accustomed to working in their own specific fields raises major challenges in terms of team governance and hybridization.
Addressing issues and challenges
Working with existing infrastructures
By their very nature, urban renewal and climate change adaptation projects are carried out on existing urban fabric: buildings, roads and underground networks. Integrating planting pits, reservoirs and infiltration devices when the underground space is already occupied by electricity, gas, telecoms, water and sanitation networks, requires a major overhaul. That is generally only justified in the context of a major requalification of a district, introducing new uses, enhancing quality of life, creating socio-economic value and, of course, limiting the city’s vulnerability to the hazards of climate change and global warming.
When it comes to thermal renovation of buildings and improving their summer comfort, there are many technical and regulatory challenges, especially for heritage buildings. In such cases, a systemic approach will enable us to design innovative solutions, mainly active ones. For example, emphasis will be placed on adiabatic cooling and over-ventilation when it is not possible to modify the thermal phase shift of the envelope of a historic building, or add the necessary shading.
Making choices, thinking transitional and long-term
Urban renaturation also means rethinking the city with a long-term vision. While a shade sail immediately fulfils its function upon installation, this is not the case for a tree, which needs a few years or decades to cast shade on a building or cool several square meters.
Choices are also necessary to maximize the impact of the transformation, balancing technical possibilities, vulnerability reduction, created use value and economic constraints. Several scenarios need to be evaluated, in order to select the one that best satisfies different selection criteria.
Developing the digital design and measurement environment
The digital tools we have and need to continue developing, generally integrated under the name of digital twin, will enable these prospective analyses, as well as the observation and analysis of behavior during the entire life cycle of the system observed.
In the design phase, different scenarios representing global warming hypotheses will help to choose initial design options, and predict their evolution over the coming years and decades. These scenarios are based on the constitution and federation of physical, thermal, hydraulic, aeraulic, socio-economic, plant growth, biodiversity development and dissemination models. Consideration shoud be given to the impact of vegetation growth, or to projects of public space and building modification or densification, that can be anticipated.
We are engaged in a race against time to keep our cities livable. The time needed to think, design and implement this adaptation is met with the observed and undeniable speed of the rise in average global temperatures.
More than just a focus for research and innovation, it is ESTP’s mission to contribute to the emergence of the necessary solutions for preserving the urban living environment.
gbetis@estp.fr