Green roofs can be a solution for cooling hot cities

Studies conducted in Athens over the last thirty years have confirmed that the city is getting hotter over time and that air conditioning use is a major contributor to this heating. Other studies have shown that urban greening mitigates the urban heat island.

Cities in hot countries tend to have flat roofs and Athens is no exception. Currently, the rooftops typical of Athens, and of many other cities around the world, are a barren concrete wasteland of unused urban space. They represent a huge potential for greening.

A green roof is a perfect way to cool a building and also the surrounding space.

A green roof cools a building in two ways. First of all, it shades the roof surface, protecting it from heat load and thereby reducing the temperature of the building. The second way a green roof cools a building is even more interesting in terms of city cooling.

The plants on a green roof evapotranspire water through their leaves. This uses heat energy in the building. So the plants effectively remove heat from the building and, by extension, the city. Consequently, less air conditioning is required and a “virtuous” cycle is established.

The urban heat island is a well-known phenomenon that affects all cities around the world. It is the difference in temperature between a city and the surrounding suburban area.

In countries such as Greece, the peak summertime temperature difference between a city such as Athens and its periphery can be as much as 10 degrees Centigrade. 

Air conditioning effectively heats the city while cooling the buildings internally. As the city gets hotter, more air conditioning is used to cool the buildings, making the city even hotter and increasing the requirement for cooling. Hotter cities mean more air conditioning, which means even hotter cities.

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Photo: Justin Williams

August 4, 2016