Review and Critical Analysis of International UHI Studies

4.5.3 Review for Green Roofs and Urban Green Areas:
4.5.3.1 Green Roof and Urban Green Areas Principles
Plants, trees, and vegetation increase the shading of the surface they occupy, thus less sunlight
that falls on the surface gets converted into heat. The heat that does reach the either the greenery
or the ground beneath it can often be dissipated by the process of evapotranspiration – heat in the
surrounding air/surfaces is used to evaporate water. Often such greenery and its cooling effects
are exploited on building roof tops as “green roofs” or strategically designed/naturally occurring
“trees and vegetation” around buildings/urban space. Through the processes of shading and
evapotranspiration the surfaces stay cooler than conventional rooftops under summertime
conditions. In principle, the use of green roofs will tend to lower the amount of solar energy
entering the conditioned space from the roof and thus reduce cooling loads. Whereas,
strategically positioning trees and vegetation can also provide shading to both pavements and
buildings alike resulting in a less of the sun’s energy being absorbed by the shaded surface
resulting in less solar energy entering into the conditioned space (thus a reduction in cooling
loads) and keeping the pavements cooler thus lowering the ambient temperature (indirectly
reducing A/C). For the same reasons green roofs tends to increase heating loads during the
heating season in non‐equatorial climates because less of the sun’s energy is converted to heat in
the roof and conducted into the conditioned space – to some extent this also applies to trees and
vegetation however, during the winter season evergreen and deciduous trees have no leaves
means that they provide less shading. Furthermore, they can often offer other benefits during the
winter such as shielding from cold winter winds for urban buildings/structures. Nonetheless, as
there is less solar energy in the winter than the summer the quantity of solar energy converted to
heat within the roof is greater in the summer than the winter, thus providing shading and
evapotranspiration displaces more summer heat gain than winter heat gain.
As discussed, the positive attributes of greenery in an urban environment not only has the
potential to reduce the air conditioning loads, and thereby the energy use and associated CO2
emissions, but also to reduce the amount of solar energy trapped in the troposphere by increasing
the reflection of solar radiation back into space (to a lesser extent than artificially designed
reflective surfaces). Thus in theory there is three mechanisms in which greenery either on
buildings or strategically positions around the urban space may mitigate global warming:
a) By lowering the space conditioning loads and associated power‐related emissions;
b) By reducing the amount of solar radiation converted into tropospheric heat;
c) By directly storing or sequestering carbon dioxide.
4.5.3.2 Green Roof and Urban Green Areas Summary of Literature and Findings
Studies from North America, Europe and Asia were examined covering a range of climatic
conditions investigating the use of green roofs and urban green areas as UHI countermeasures –
of these studies over a quarter examined green roofs and the rest were focused on the effects of
trees and vegetation. The studies reviewed for green roofs either simulated or conducted field
experimentation gathering data demonstrating the temperature reduction capabilities of green
roofs over conventional roofs due to evapotranspiration and shading. It was found that green
roofs offered greater cooling per unit area than light surfaces but less cooling per unit area than
Review and Critical Analysis of International UHI Studies
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