Written by: Sara Dennis
Days over 100 degrees F, projected scenarios. (Source: US EPA, Reduce Urban Heat Island Effect)
In our last article we discussed what Urban Heat Island Effect is, how it’s created and what effects it has on its surroundings. This becomes scarier when considering the higher temperatures that climate change is likely to bring. Even under a low greenhouse gas (GHG) emissions scenario, the City of Chicago’s “new summer heat index could increase to around 93°F by the end of the century—similar to current summer conditions in Atlanta, Georgia” (Reduce Urban Heat Island Effect). More drastic is the high GHG emissions scenario, in which Chicago might have 30+ 100°F days per year. The increased heat from either scenario could lead to health issues for residents, as well as a potential decrease in tourism. Luckily there are a variety of ways to combat urban heat island effect and limit its impacts.
Shade & Vegetation
As explained in our last article, natural landscapes have the opposite effect of heat islands, and actually cool the air through transpiration (the process of releasing water from leaves) and evapotranspiration (when that transpired water evaporates into the surrounding air, which cools it, similar to how sweating cools us off). Trees and taller vegetation also provide valuable shade and can help improve air quality. Finally, trees and vegetation that directly shade buildings and homes can reduce the need for air conditioning, putting less strain on the electric grid and reducing GHG emissions.
Green roofs have a long history in Europe and have been popular for over 30 years.
Green Roofs
A building’s roof is another place where improvements can be made to mitigate heat island effect. When the sun is out, sunbeams hit the roof, heating it up. This thermal energy (heat) is then transferred into the building. On a warm day this extra heat means that the building will require even more energy to keep cool, creating an alarming feedback loop.
A green roof is a specially designed roof that allows plants and vegetation to grow on it. The vegetation removes heat from the air through transpiration and evapotranspiration, cooling both the roof and the surrounding air. And because the roof itself is cooled, less heat is transferred to the building, and less energy is needed to cool the building in the summer. Green roofs also provide energy benefits in the winter, as they help insulate the building and reduce heating costs. Finally, though these roofs require maintenance, they provide habitat and food for wildlife, help mitigate stormwater, and can provide enjoyable spaces for people to hang out, depending on the style of the roof and building.
“Depending on plant selection, some green roofs do not require watering and can absorb up to 70% of rainwater”
Green roofs on buildings in Sydney, Australia.
A “cool roof” using high SRI material and solar panels to reduce heat island effect and for energy savings & efficiency.
Cool Roofs and Reflective Pavement
Another strategy to mitigate heat island effect is to prevent the heat from being absorbed in roofing or pavement, by reflecting the solar heat from the surface instead. The Solar Reflectance Index (SRI) is a measure of the ability of a material to reflect or return solar energy back into the atmosphere. A material with a high SRI value will be cooler than a low SRI material while exposed to the same solar energy conditions. According to LEED, “a reflective roof will lower energy costs in all climates, but some building owners have seen energy use reductions of 40% during times of peak energy use” (LEED Green Associate Study Guide). A study cited by the EPA also found that in California “cool roofs can provide annual energy savings of almost 50 cents per square foot” (What You Can Do to Reduce Heat Islands).
Solar panels providing shade for a parking lot and renewable energy in our Rainbow Women and Children’s Center project in Cleveland.
Paved surfaces also absorb heat and release it back into the atmosphere later. With 43,500 square miles (~the size of Ohio) of paved surfaces, including roads, highways, parking lots and buildings, in the continental United States (LEED Green Associate Study Guide), this can add up. Fortunately, we can apply the same SRI properties that help cool roofs, to pavement. Thus, a high SRI paving material will reflect the solar energy rather than absorbing it.
Another pavement/parking lot heat island mitigation strategy is to place parking under the cover of shade or a roof. The shading device or roof may be a high SRI roof, a green roof or a roof that supports solar panels, wind turbines, or another method of energy generation.
In Conclusion
There are several ways to mitigate heat island effect, including planting trees and vegetation, installing green and/or cool roofs, using high SRI pavement, and covering/shading parking lots. Our next article will give a few examples of these success stories.
Sources
“Chapter 4 - Site Selection.” GBES BD+C v4 Study Guide (v2), Green Building Education Services, LLC, Atlanta, GA, 2018, pp. 57–60.
“Chapter 4 – Sustainable Sites.” LEED Green Associate Study Guide, LEED v4 Edition, 3rd ed., Green Building Education Services, LLC, pp. 98-105.
“Reduce Urban Heat Island Effect.” EPA, Environmental Protection Agency, 23 Mar. 2022.
“What Is the Solar Reflectance Index?” CoolRoofs.org, Cool Roof Rating Council, July 2022.
“What You Can Do to Reduce Heat Islands.” EPA, Environmental Protection Agency, 21 July 2022.