Innovative ground surfaces to improve hygrothermal comfort in cities
As part of the redevelopment of Lyon’s Part-Dieu district, Egis tested innovative ground surface solutions based on the reflective power (albedo) of certain coverings and materials. There are various new solutions to ensure a pleasant temperature and humidity level in cities, whatever the season. Among them, “cool ground” surfaces are one of the most promising.
As part of the redevelopment of Lyon’s Part-Dieu district, Egis is testing innovative “cool ground” surface solutions based on the reflective power (albedo) of certain coverings and materials. The principle is simple: to come up with new ground surface materials with specific thermal properties that help combat urban heat islands (temperature increases due to urbanisation) and improve thermal comfort in winter.
All current research suggests that ground surfaces not only have a significant effect on the urban climate, and therefore on the heat island effect, but also that they are one of the main contributors. The properties of different materials will also influence the local microclimate through the albedo effect, which measures the amount of sunlight reflected by a surface, as well as their insulation and thermal inertia properties. In order to fight against the heat island effect, it is essential to reduce the surface temperature of ground surfaces, especially in the summer. One of the most important aspects of these often highlighted phenomena is the albedo, that is to say the proportion of radiation reflected, of materials used in urban spaces. For example, the zinc rooftops in the historic centre of Lyon reflect about 60% of sunlight and heat less than classic roof tiles, which reflect about 20%.
Initial results (from March 2017 to mid-October 2017)
During the heat wave in June, dark concrete and asphalt surface temperatures rose to 65°C, where the variable surface albedo tests remained below 55°C. On the experiment table, initial results show significant differences in thermal behaviour depending on the covering used (up to 20° difference between the tested surfaces). They confirm, in particular, that the use of granite (planned for major future constructions in the district) is more virtuous (less than 50°C surface temperature) than the more traditional materials for public spaces such as asphalt and concrete (more than 65°C surface temperature).
Research into increasing albedo is important but only focuses only on the issue of reducing ground temperature. At Egis, our goal is to find a material that has a low surface temperature in summer, but also – as unnatural as it may seem – a higher temperature than normal in winter. This material would not only help prevent heat islands in summer, but would also mitigate the drawbacks of the cold winter weather. Independently, these two requirements are achievable, but the innovation here lies in combining the two properties into a single material.
A 50 sqm experiment table, made up of 12 ground cover samples, was installed on Rue Bouchut to carry out these tests. Six of the samples are existing materials used as a yardstick (concrete, asphalt, stabilised soil, etc.). Six others are sample prototypes of “variable albedo” surface coverings, with different characteristics. The aim is to compare the properties of the materials over several seasons and under different climate conditions.
Egis is conducting this experiment as part of Lyon’s Ecocité project, which focuses on the redevelopment of several districts in the city. The project, sponsored by SPL Lyon Part-Dieu, receives funding from the Programme of Investments for the Future (PIA), operated by the Caisse des Dépôts. For more information, see the Elioth Lab publication (french version).