By Alex Grandgirard, Urbanomy

It's supposed to be a solution to the ills of weather that makes you want to draw the curtains.

After two years of large-scale experimentation, the Greater Lyon area in France announced last summer that it was putting an end to its initiative to cover the city's streets with white heat-reflecting paint, to minimise heat absorption - also known as "cool pavement". 

For what reasons? Moderate effectiveness, considering the high cost of the coating. This finding, which was considerably less positive than expected, comes after only one year of test results. And yet the idea had only been applied to an area of around 100 square meters in Lyon's 7th arrondissement. The good news is that a massive tree-planting programme was favoured in order to face heat waves by 2050. And these are likely to be extreme. 

In this way, authorities intend to make the urban landscape greener, create more green corridors and integrate land-use planning into a strategy of resilience to climatic hazards. A parallel with the city of Bordeaux should not escape the informed professional: there, the city council has launched a greening plan. It has also committed to planting more than 30,000 trees within the next two years. 

In fact, all over France, towns and cities are planting acacias, almond trees, birches, holm oaks, gean trees and even Japanese medlar trees. A good omen, when you know that a single mature tree can evaporate about 450 litres of water a day; according to former London Councillor Jon Burke, that's the equivalent of five air conditioners working twenty hours a day. And as far as the Greater Lyon area is concerned, the 30,000 or so different species of flora and fauna and the 4,000 protected areas or natural zones of ecological interest in the Auvergne-Rhône-Alpes region certainly won't complain. So is this change of direction a wise move?
Photo de deux ouvriers peignant une toiture en blanc
roofimages / Pixabay

An easy-peasy principle implemented from San Francisco to Melbourne

The use of heat-reflective paint is becoming increasingly popular outside of France. On the other side of the Atlantic, major cities such as Los Angeles and New York, and even further out Melbourne in Australia, have been testing this approach for several years. San Francisco wins the prize: to combat extreme heat, the city recently launched a massive plan to cover the streets in several districts. A remedy that may seem extremely basic (or even "low tech"?) in a region that is synonymous of internet giants and artificial intelligence. This innovation seems to have been adopted in urban environments that are among the most exposed to climate risks. So how does one explain the U-turn taken by the Greater Lyon metropolitan area?

Let's take a closer look at the features of this wonder coating. This innovative process uses no solvents - the main component being ceramic beads made up of voids that create surface insulation. A revolutionary idea, yet as simple as clay components used in potting soil to block out humidity. The idea has been around for years, and is used in particular to insulate roofs and terraces. It reflects more than 90% of solar radiation, helping reduce indoor temperatures by up to 6°C when combined with insulation materials. A second benefit is that it reduces dampness issues by limiting cold-wall effects that may occur on roofs with little exposure. So why, you might ask, haven't we covered our roofs and roads sooner?

White light/White heat

The main problem with such a scheme, beyond financial concerns, is the light reflection it causes. The materials that make up most roofs and roads absorb more solar radiation. They release more of this energy in the form of heat into the surrounding air than the vegetation they replaced. In this way, the use of heat-reflecting paint, or "cool pavement", replaces the issue of sunlight absorption with that of reflection.  

On the downside, the white colour chosen for these paints lets the dirt show and would generate significant cleaning costs. All of these consequences suggest that this tool is theoretically effective, but simply not a lasting one for local authorities. In short, the thermal insulation performance is not good enough when compared with the financial investment. Resistance to urban stimuli is not future-proof and the inconvenience caused by dazzling is a major drawback. 

If we wished to replicate this innovation to meet the challenges of a city like Paris (among the most densely populated capitals in the world), we would quickly encounter a number of hurdles. Most Haussmann-style roofs are subject to strict urban planning regulations. It would be extremely difficult to get the Architectes des Bâtiments de France to approve the use of a whitish, highly reflective paint to replace the fifty shades of grey of the French capital's distinctive zinc.
Ouvriers à Los Angeles recouvrant une rue de peinture blanche
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Let me tell ya 'bout the birds and the bees and the flowers and the trees

What can we learn from all this? First, the ever-increasing night-time temperatures in urban areas are a key topic. During heatwaves, cities become pressure cookers. Cities warm up during the day and require a cooling period that far exceeds the length of a single night. The result: temperatures over 20°C at night, an inability to tackle heat islands in the long term, and the health of the most at-risk populations put to the test.

The decision to massively revegetate cities in an increasingly dense urban landscape seems to be the most appropriate response. It is vital to make urban areas more resilient to high temperatures, which become more extreme every year over ever longer periods. Provided we do it intelligently (choosing the right varieties, planting young trees that are better able to adapt, enlarging open grounds so that they are able to grow), planting trees appears as the most suitable way of responding to this serious issue.

Not only do trees sequester carbon and encourage biodiversity in our urban environments, but the shade they provide depending on the path of the sun is one of the most effective strategies to reduce heat in highly built environments - so long as we're not talking about cities that are forests... of skyscrapers, that is.
Alex Grandgirard
About the author

Alex Grandgirard

Alex is Consultant Manager for Urbanomy, overseeing projects and team coordination.

A graduate of the École Spéciale des Travaux Publics and holding a Master of Science in Bioclimatic Architecture from the Royal Polytechnic School in Stockholm, he has expertise in bioclimatic design, sustainable construction and urban development. He has worked with architects for a number of years as project manager.

In his spare time, Alex enjoys film photography, ceramics, cycling, surfing as well as long rail journeys.