How Green Roofs Mitigate Urban Heat Island Effects
by Dr. Anna Zakrisson on Wednesday, October 23, 2019
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Mitigating urban heat islands with green roofs
Urban heat islands (UHI) particularly develop in urban areas and describe locally increased temperatures due to changes in the landscape going from green-to-gray. What was once green, moist, and cool is now asphalt, stone, and concrete and thus quick to heat.
Most of us are well-aware of how hot stones and asphalt can get when walking barefoot in summer while grass provides a pleasant surface to walk on. At least, most of us have some distant childhood memories of this.
Urban heat islands can negatively affect urban populations by directly increasing heat-induced illness and mortality . From an economic perspective, heat islands increase summertime peak energy demands such as air conditioning costs . Also, urban heat islands result in increased greenhouse gas emissions and air pollution .
Adding back some of that greenery helps to keep temperatures down and ensure that our cities become pleasant and healthy environments to live in . Green roofs and living walls provide are excellent ways to achieve this in crowded urban areas.
In this article, we investigate the published scientific literature on green roofs as heat island mitigation tools. All articles are referenced in the text, and we hope that we have given a balanced view of the topic.
Urban heat island effects - what are we up against?
Heat island effects on surfaces
Heat islands can have a severe negative effect on the urban environment. Bergdahl et al. 1997  showed that a sunny day could heat streets and pavers to 27-50°C (50-90°F) hotter than the ambient air temperatures. However, surfaces that remain shaded or wet did not deviate far from the air temperatures.
We must take this matter seriously.
Atmospheric heat island effects
Not only surfaces can heat disproportionately, but the heat island effect phenomenon can also apply to the atmosphere. Oke et al. 1997  could show that on a sunny day for a city with about 1 million inhabitants, the annual mean temperature could increase by 1-3°C (1.8-5.4°F).
It’s always better to take one concrete example to illustrate an issue, so let’s take a look at New York City: Susca et al.  could show a mean temperature difference of 2°C between the most and least vegetated areas of the city.
This might not sound a lot, but a 2°C increase from 30-32°C significantly increases the heat mortality risk of the population [1, 8]. People actually die.
Most hot cities rely on air-conditioning. Europe just experienced two extremely hot summers and an increasing number of people are purchasing these units to cool their homes.
As it gets hotter, the harder the air-conditioning has to work, and the more electricity they require. In a way, this is wasted energy as we could use nature-based solutions that cool our cities passively with no extra energy added. Also, we should not forget that air conditioning units simply are transferring heat from the inside-to-the-outside actually exacerbating the whole heat island problem. Air-conditioners are frankly a terrible and unsustainable way to cool our indoor urban environment.
So, how can we deal with this serious matter and prevent it from occurring?
Urban heat island effect mitigation - what options are there?
Generally, in order to reduce heat islands, one strategy is to ensure that our outer building materials have a high so-called albedo.
Albedo is a measure of how well a surface reflects energy. If a surface reflects a lot of light, it has a high albedo and looks white, whereas if the surface absorbs most of the light, it will look dark and have a low albedo. Black asphalt has a low albedo and is thus hot to walk across on a warm summer day.
There are a few options available to mitigate heat island effects. One example is to install so-called reflective or cool roofs. These white roofs have a high thermal emittance and will not heat as effectively as a traditional roof would do [9, 10]. Similarly, cool pavers can be used based on similar technology.
Nonetheless, it might be useful to ask the question of whereto the energy is reflected? Someone once compared this method of urban cooling to tossing your garbage over the fence to your neighbors. Your yard might be nice and cool, but what about your neighbor? Is this really a sustainable solution?
Also, these solutions come with few secondary benefits.
The most obvious solution to the heat island problem is to increase plant coverage. Simply because greenery is better as dispersing heat than pavers, stone, and asphalt . However, increasing the number of parks in a city can often be tricky due to the present crowding and colliding business interests. Just take a look at a city like NYC – where should the park be built? and what will the cost be to build one on such valuable land?
Green roofs are another possible solution that comes with a range of secondary benefits such as stormwater management, increased urban biodiversity, and simply bringing more nature into our cities, something that has been shown to improve mental health [12, 13]. Also, roofs are generally unutilized areas, so the colliding business interests are few.
How do green roofs mitigate heat island effects?
So how do green vegetated roofs manage to keep roofs cool?
Despite the comparatively dark color of vegetated roofs and living walls, their albedo is still relatively high . Berardi et al. report albedo values of 0.7-0.85 for living roofs, which is significantly higher than the 0.1-0.2 measured for gravel, tar, or bitumen roofs.
Many studies have been undertaken on how well green roofs can reduce heat island effects. One study from Hong Kong showed that gray roofs stored 75% more heat than green roofs ! But this is no high fluke number as many other studies conducted in other climates report similar numbers [16, 17, 18].
For cold climates, there have been reports showing that roof temperature fluctuations are significantly reduced for green roofs. One study in Canada showed that the daily roof temperature fluctuation was 45°C for a gray roof, but merely 6°C for a green roof . These reductions in temperature fluctuations not only act as a buffer for local climates but also protects the roof membrane from damage, ensuring a longer membrane life span.
In short: if you replace a traditional roof with a vegetated roof, you will gain thermal benefits, and heat island effects will be reduced.
Nonetheless, it should be noted that for very cold climates, there still is a debate running, mainly due to lack of data on how effective green roofs are at mitigating urban heat islands .
But, how do green roofs fight urban heat island effects?
What is behind green roof thermal regulation?
The magic trick of green roofs for dissipating heat is vegetation and the evaporative cooling process coupled with the energy consumed during photosynthesis .
Evapotranspiration - the driver of cooling!
Evapotranspiration is the combined processes of evaporation and transpiration. Both processes result in evaporative cooling, the energy-requiring process in which water is transformed into its gaseous phase. The energy required is taken from the air, and so the temperature goes down.
It’s essentially the same process when humans sweat to cool down.
If you want high evapotranspiration rates on a green roof, you need to have high retention capacity. Retention is the water that never leaves the roof as runoff, and which leaves as water vapor.
Evapotranspiration increases with increased roof capillary capacity. The often-used well-drained soils of green roofs generally have quite low water holding capacity and water travels down preferential flow-paths and macro-pore filling is low. However, materials like mineral wool (preferably phenol-formaldehyde free mineral wool) added underneath the soil layer improves macro pore space-filling and hence retention capacity significantly. This ultimately leads to better cooling due to increased evapotranspiration.
This means that you need to carefully design your green roof to fit your climate. A flawed design here can result in high and expensive irrigation needs, or dead plants and dead plants will not significantly help mitigate heat island effects...
In order to help you with your climate-adapted green roof design, we have developed a green roof modeler that calculates plant stress and based on your selected green roof setup and your local climate. It is free to use and built on several years of data from Green Roof Diagnostics’ rain laboratory. If your city has not yet been added, please let us know: CONTACT PURPLE-ROOF.
Green roof thermal insulation – temperature reduction inside buildings
Not only the city as a whole benefit from green roofs but also building owners as they are able to reduce heating/cooling costs significantly. Exactly how much this is, of course, depends on the building’s characteristics, for example, how well insulated it is beneath the green roof and the local climate.
If a building is poorly insulated, adding a green roof will have more effect than if a green roof is added to a well-insulated building. Also, very tall buildings will provide a completely different case than very low buildings due to the differences in volume vs. surface area. Also, shaded buildings such as in cities like Hong Kong may display lower cooling effects than those directly exposed to the sun .
One study conducted in Greece with a hot Mediterranean climate could show a 4°C reduction in indoor temperatures when the roof was vegetated compared with a non-vegetated gray roof . Many other studies in different global cities have shown similar results with several degrees difference in indoor temperature if a green roof was installed over the room compared with a conventional gray roof .
Hence, green roofs will improve the building’s energy consumption and save money, but the magnitude will depend on the factors mentioned above. Nonetheless, don’t forget that this is only one of the many benefits of vegetated roofs.
Living walls and urban heat island effects
A short final note on green walls since green roofs are not the only green building solution for our cities.
Green walls are excellent at mitigating heat island effects partly due to the direct effect of the vegetation, but also as they are able to break down certain types of wind patterns that especially form within so-called skyscraper canyons.
Different types of wall greening systems appear to have a different effect on the reduction of heat island effects. In a study by Perini et al. , it was shown that living walls outperformed direct and indirect facade greening systems but that all systems showed significant improvement compared with a bare wall. The temperature reductions of the three types of walls were 1.2, 2.7, and 5.0 °C, respectively.
Once again, remember that these are no small numbers. A mere difference of one degree can literally save lives in many climates!
Heat island effects and green roofs – where do we go from here?
So, where do we go from here? Well, we install green roofs! That’s what we do.
The evidence is there, and our cities are boiling. We need to cool our cities to reduce heat-related mortality, sink our summer cooling costs, and to reduce the exacerbated pollution issue that these heat islands are causing.
I work with purple roofs, in comparison to traditional green roofs or blue-green roofs, but the ecologist in me honestly doesn’t care what kind of roof your install. What matters is that we go from gray-to-green.
I want my daughter to grow up in a world where kids can be outside in summer and where pollution isn’t making us sick.
We have the technology to make this future possible, and it comes with a range of other benefits. Let’s change the world, one roof at the time!
- Dr. Anna Zakrisson, Purple-Roof
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