Designing buildings that are resilient to heatwaves and urban heat islands
Mariette Guermonprez pour Construction21
After record temperatures in September 2023 in countries across Europe, the month of October also begins with unusual rises in mercury. With these thermometers not going down, the problem of building and renovating to compensate for heat waves is also becoming a hot topic.
While heat waves will be more and more recurrent and intense in the coming years, what are the consequences for our cities and our buildings? How can we answer it? Deciphering this climatic phenomenon and the solutions available.
Heatwaves and urban heat islands: two sides of a burning problem
A heat wave is a period of several consecutive days during which temperatures are abnormally high, day and night, compared to seasonal averages. Thresholds at different geographical scales are defined according to the climate and the specificities of the territory.
This phenomenon, even if it increases in scale each year, is not new: from 2010 to 2019, average temperatures in France had already increased by 1.8°C compared to the beginning of the 20th century. In the IPCC's most pessimistic scenario, which corresponds to our current trajectory, by 2100 heat waves could last up to ten weeks without interruption.
The risks caused by heat waves are mainly health-related (heat stroke, cardiovascular and respiratory pathologies, etc.), but the consequences on the built heritage are not negligible.
The Sustainable Real Estate Observatory (OID) has identified thirteen categories of impacts in the real estate sector. Firstly, a rise in temperatures, particularly during the day, over several consecutive days increases the need for air conditioning and ventilation equipment. As a result, buildings are more energy-intensive and dependent on electricity. The lack of cooling at night causes poor natural ventilation, while indoor air quality is affected by pollution peaks or the release of substances from certain materials in the face of heat. Heat waves are also responsible for the deterioration of buildings: cracking of concrete, cracking of coatings, corrosion of plumbing networks, etc.
Beyond the building itself, plants and soils are dried out, which degrades biodiversity, reduces the associated benefits (shading, cooling, etc.) and increases vulnerability to other climatic hazards.
Faced with heat waves, not all territories are equal. In cities, urban heat islands aggravate the intensity of heat waves and their effects. These are caused by the greater presence of asphalt surfaces and construction materials which do not reflect heat but absorb it before releasing it slowly during the night. The morphology of cities also comes into play: tall buildings prevent wind circulation. The difference in temperatures between rural and urban areas can reach up to 10°C!
When the building takes the chill
Between prevention and adaptation, a vast range of tools is available to city and building stakeholders. These solutions are more or less effective, popular, expensive; and must often be applied in combination. Before implementing them, it is important to study in depth the vulnerabilities of the building depending on its location.
Designing buildings resistant to heat waves
This meteorological hazard must be taken into account from the start of a renovation or construction project. The materials chosen for the envelope and structure have an impact on the building's ability to limit overheating by storing heat. We must therefore favor materials with high inertia, hygrothermal, low emissions and reflective; such as earth, stone or concrete. Effective exterior insulation also makes it possible to maintain the thermal inertia of the wall and to treat thermal bridges.
The building's exposure to solar radiation is also a parameter that should not be neglected. Considering the orientation of the construction according to its environment, for example in relation to the shading of surrounding structures, limiting the glazed surfaces to those necessary for natural lighting and installing solar protection (shutters, sunshades, etc.) are strategies for controlling indoor temperatures in the event of a heat wave.
Frugality and performance of energy systems highly recommended
Installing appropriate ventilation systems is essential to limit the use of harmful air conditioners and peaks in energy consumption during heat waves. This can range from the traditional "Canadian well", a technique for refreshing the air by passing it underground, to controlled natural and mechanical ventilation systems, or even double-flow ventilation systems. The choice of ventilation system is made according to the orientation of the building; well-placed openings can, for example, allow winds to infiltrate at night and thus cool the interior for the following day.
Cooling systems can be installed in addition, such as mechanical compression systems on air or water, which can be coupled with renewable energies to reduce their carbon footprint. Geocooling, evaporative cooling and misting are other sustainable and effective alternatives.
Raising occupants’ awareness of good building management
Occupants can help reduce indoor temperatures by adopting certain practices. For example, ventilating the space at night when temperatures are cooler reduces the need for air conditioning during the day. Likewise, sun protection must be used correctly to protect against solar radiation. Technical Building Management systems, which optimize the operation of services such as heating, ventilation and air conditioning,can be useful in this regard.
Limiting the use of computer and household appliances during heat peaks and choosing energy-efficient equipment also makes it possible to control the indoor temperature while reducing energy demand during heat waves.
Raising occupant awareness of these practices plays a key role in managing heat inside buildings. Building constructors can produce guides detailing the measures to adopt for effective management of the equipment provided.
Refresh beyond the building
A building's resilience to heat waves also plays a role outside its walls. Vegetation and nearby water points absorb heat and reduce the outside temperature around the building. This reduces the pressure on the materials and provides occupants with pleasant spaces to cope with extreme temperatures.
Expand the scope of solutions at the local level
Finally, several of the solutions presented above can be encouraged by local urban planning policies. From controlling the artificialization of land to adapting architectural standards to allow more installations of solar protections, numerous possibilities are available to public stakeholders.
Encouraging the use of public transport and greener modes of travel to reduce pollution and urban warming is also essential. In addition, providing shaded and cooling public spaces protects populations during heat waves. The City of Paris, for example, lists the “islands of freshness” located within its perimeter.
By integrating these solutions into urban planning, communities can make cities more resilient to heat waves and create a more pleasant environment for residents.
Sources:
- Hazard sheet – HEAT WAVE; Sustainable Real Estate Observatory (OID)
- Melt the heat islands: Cerema presents the action levers for city planning; Cerema
- Urban heat islands – Directory of knowledge sheets; Institute of Planning and Urban Planning (IAU) Ile-de-France