2.2. Choosing the right type of materials
Great care should be taken with the materials that are used to build urban infrastructure. To remedy UHI it is also necessary to reduce as much as possible the amount of heat that is stored in buildings or in the ground during the days when solar radiation is intense. In fact, the greater the quantity of radiation stored during the day, the greater the nocturnal UHI. There are several possibilities to limit this absorption mechanism, but none is sufficient on its own.
It is possible to increase the albedo of the city by choosing light-colored materials that reflect solar radiation. This solution could be applied to buildings (facades and roofs), but also to artificial pavements.
The urban environment manages to trap the solar radiation reflected in the streets, where it is absorbed at the end. The streets are a trap for solar radiation (Figure 19). Studies have shown that the city absorbs solar radiation 15 to 20% more efficiently. At equivalent reflectivity, the albedo of cities is lower than that of the countryside. It depends on the narrowness of the streets (the narrower they are, the more effective the trap). To avoid this entrapment, the distance between the buildings must be increased.
Since it is necessary to prevent the heat which has been absorbed during the day to remain stored in the structure, it is necessary to use materials which have the smallest possible thermal diffusivity. In this type of material, heat will take a long time to pass through the thickness of the material and is not stored. As soon as night falls, this allows the structure to quickly remove the small amount of heat that it has absorbed during the day.
Insulating buildings is a key to limiting nocturnal UHI. The better it is done, the more it limits the loss of heat at night by emission of infrared radiation (Figure 21) and by sensible heat flux.
