Why Urban Heat Island (UHI) Reduction is critically important and how Green Roofs can help

Posted by Jure Sumi on Oct 13, 2015 9:32:20 AM

The urban population of the world has grown rapidly from 746 million in 1950 to 3.9 billion in 2014. In 2014, the United Nations Department of Economic and Social Affairs (DESA) published a revised version of the World Urbanization Prospects. According to the report, 54 per cent of the world’s population today lives in urban areas, a figure that is expected to rise to 66 per cent by 2050.



Projections indicate that with the current rate of urbanization combined with overall global population growth, the world’s urban population could surpass 6 billion by 2045 and reach as much as 6.4 billion adding a total 2.5 billion more people to the world’s urban population inside the next 35 years.

“Managing urban areas has become one of the most important development challenges of the 21st century. Our success or failure in building sustainable cities will be a major factor in the success of the post-2015 UN development agenda,” says John Wilmoth, Director of the UN’s DESA Population Division.



Urbanization negatively impacts the environment, especially with the generation of big Urban Heat Islands. UHI is defined as the rise in temperature of any man-made area, in comparison to more rural or green areas or other natural habitats. According to many studies, heat island effect negatively impacts not only residents of urban environs, but also other people and their associated ecosystems located far from the world’s cities. In fact, UHIs have been indirectly connected to climate change because of their contribution to the greenhouse effect, and therefore, to global warming.

According to the report “Urban heat islands,” prepared by the Met Office, the UK’s National Weather Service, the 2003 heatwave across Europe is estimated to have resulted in additional 35,000 deaths, the majority of them in major towns and cities. It can’t be determined exactly how UHIs contributed to that, but one thing is clear – every summer cities are becoming increasingly hotter, regularly breaking heat records for “downtown” areas.



Cooling the outside surfaces of buildings should be a priority of all municipal areas around the world, and Green Roofs should become one of most common tools with which to fight UHI. Therefore, understanding how vegetated roofs can cool roofs down is of paramount importance.

The “Laboratory for Sustainable Technologies in Buildings, Faculty of Mechanical Engineering, University of Ljubljana, Slovenia, EU is currently embarking on the third year of their study of thermal responses of green roofs in different climatic conditions.

UHI is one of the major inputs being explored in detail. The results reveal that Living Roofs on buildings can dramatically change the temperature on roofs, by converting old “heat radiator roofs” into cool green areas. The findings also confirm that there are certain elements that can drastically improve the positive effects of Green Roofs on UHI.


The two most critical elements you need to study in-depth to understand how and why vegetative roofs are affecting real performance and improve positive effects on UHI:


  • Plants provide good shading effects as well as reflect radiation from the sun;
  • Without plants, this component of UHI reduction is eliminated – having healthy green roofs is key.

Sun shining directly on dark soil is bad for UHI. So if you want to see an immediate reduction in UHI, make sure you provide instant green cover already the first day after green roof installation.


  • Plants, together with growing media, capture a certain amount of water, which is the basis by which evapotranspiration takes place, reducing air temperatures and generating a net cooling effect on the surroundings.
  • Evapotranspiration positively effects UHI reduction, as long as the Green Roof and plants still contain some moisture.

Make sure to design a green roof that will have enough water dispersed in the growing media such that prolonged dry periods will not dry it out completely.


In order to see the effect of a Green Roof solution on the reduction of roof temperatures have a look at the comparison below, of two roofs / green roofs in Barcelona, Spain, but with one important difference:

  • The first is non-irrigated;
  • The second was designed with sub-ground drip irrigation, which was running when the moisture content dropped below a certain predefined level.



A simple glance at the chart shows that the average green roof performs far better than the traditional “Barcelona-type” flat roof; however, it loses its performance advantage when it dries out entirely. 

reference roof & Urbanscape non-irrigated green roof temperaturesChart 1: Reference roof & Urbanscape green roof temperatures


In Barcelona, this can actually happen, as there is simply not enough rain during the hottest and driest months, like the summer months of June, July and August. During those months the green roof still performs, on average, better, but the difference is smaller than one would expect. If the difference in temperatures in May between a regular flat roof (temperature on the roof surface up to 70ºC / 158F) and the green roof (temperature on the green roof surface up to 40ºC / 104F) was still 30ºC / 54F, this difference can be dramatically lower for the month of August.

In the worst case scenario, when the green roof has dried out entirely, and vegetation is largely dormant, leaving a lot of unshaded ground, the difference in roof temperatures has diminished dramatically, even entirely.



On the other hand a quick glance at the irrigated roof shows an entirely different picture.

reference roof and Urbanscape irrigated green roof temperaturesChart 2: Reference roof & Urbanscape irrigated green roof temperatures

It is clear that an irrigated roof performs far better in terms of temperature reduction on the roof. Even in the hottest, driest month – which in Barcelona is August – the temperatures on a regular “Barcelona-type” roof have easily exceeded 75 degrees C (167 F), whereas the green roof has barely reached 40 degrees C (104 F).



The University of Ljubljana, together with the Urbanscape team, have included the Urbanscape Green Roof UHI performance evaluation in the Urbanscape Performance Evaluation Tool (PET).

The Urbanscape PET helps us design Urbanscape green roofs and define irrigation protocols for them, with the aim of reaching maximum UHI reduction potential.

If you are interested in receiving a sample Performance Evaluation Report for a selected town or country, please click here:


And if you would like us to prepare a Project-Specific customized evaluation of your green roof performance, follow the link below:



To learn more, check our website www.green-urbanscape.com or download our Green Roof System Brochure.


Topics: Heat island effect (UHI), Measuring performance