Despite appearances, recent research has pointed towards cool roofing actually being more effective when it comes to keeping cities cool than vegetation covered green roofs. Although seemingly a much more minor change, a reflective coating or even just painting roofs white may actually do better for the environment than vibrant rooftop gardens. The research, published in Geophysical Research Letters mapped Greater London using a three-dimensional climate model to test the thermal effects of different passive and active urban heat management systems.
Although often considered the go-to solution for environmentally friendly roofing, green roofs were found to be the lesser effective option when it came to reducing temperatures compared to cool roofs. Of course this isn’t to say that Cool roofing is the better option for the environment overall, as there are many factors to consider including green roofs contribution to nature diversity. However, when it comes to exclusively lowering temperatures and counteracting the Urban Heat Island effect (UHI effect), cool roofs were the clear winner.
The research found that if adopted widely throughout London, cool roofs could reduce outdoor temperatures across the city on average about 1.2 degrees C, and up to 2 degrees C in some locations. In comparison, green roofs and street level vegetation were observed to only reduce temperatures by 0.3 degrees.
The research also analysed the effect of air conditioning units on the surrounding temperature. Air conditioning was found to increase the temperature of the outdoor urban environment by about 0.15 degrees C for the city overall, but by as much as 1 degree C in dense central London. The researchers also found that the increase in the distribution of air con units in their model could be entirely powered by photovoltaic solar panels if they were similarly installed to their fullest extent.
The team of researchers conducted their research by calculating the possible effects if each method was widely adopted across the greater London area.
Lead author Dr Oscar Brousse (UCL Bartlett School Environment, Energy & Resources) said: “We comprehensively tested multiple methods that cities like London could use to adapt to and mitigate warming temperatures, and found that cool roofs were the best way to keep temperatures down during extremely hot summer days. Other methods had various important side benefits, but none were able to reduce outdoor urban heat to nearly the same level.”
As the climate changes, residents within cities are particularly vulnerable to warming temperatures. Cities generally trap heat, inducing the Urban Heat Island effect that can lead to greater discomfort and mortality for residents during hot spells. Finding ways to adapt or mitigate these warming temperatures has become a high priority for city planners and designers, who have been exploring passive cooling methods like cool roofs, green roofs and expanded urban vegetation as well as active methods for building interiors like air con.
The UHI effect is a climate phenomenon that takes effect in dense urban areas, where man-made buildings and infrastructure causes heightened temperatures when compared to surrounding areas. This can have several negative effects including reducing the air and water quality and increased rainfall when compared with rural areas. This heat island effect is no surprise when taken into account that urban areas take up just 0.5% of the earth’s land surface, and contain more than half of the world’s population.
