Cool paving materials

Trees in Prague - Source: atelier GROENBLAUW

Studies have been conducted in the United States [EPA, 2008] into the effects of paving materials on surface and air temperatures in towns and cities. As paving materials cover 30-40% of the urban surface, they play an important part in the heat in town and cities. In the summer months, tarmac and concrete can reach temperatures of between 45°C and even 70°C. The heat is trapped in the material and in fact passes on to the ground below; heavy summer rainfall can then pass on the heat to the rainwater running off the paved areas and into the surface water.

Paving material with light colours generally has a favourable influence on keeping the surface temperature down. However, the material’s permeability for water and air is of greater importance.

Important factors in the effect that paving materials have on surface temperatures and on the air temperature above them are reflection, thermal absorption characteristics, roughness of the surface, heat transport characteristics, emissivity and permeability for air and water.

In practice, this means that semi-paving materials should be used wherever possible, as they let through water and air and trap less heat, but primarily because they remain cooler as a result of evaporation. Light-coloured paving materials, such as light clinkers and concrete stone are better than black tarmac. However, their appearance suffers badly from pollution. This effect is even worse with light roads than with white roofs.

Asphalt collectors offer a way to use the heat trapped in the material and so to cool the tarmac.