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Green Roofs?

David Klein

Moderator
Green roofs to reduce the effects of climate change

Researchers from the Higher Technical School of Agricultural Engineering of the University of Seville have published a study in which they indicate that it would be necessary to have between 207 and 740 hectares of green roofs, depending on the scenario that is contemplated, to reduce the effects of climate change in relation to the maximum temperature rises of between 1.5 and 6 ºC that are estimated by the end of the century. This would require between 11 and 40% of the buildings in the city.

Read more here: Green roofs to reduce the effects of climate change

Here is the title and introduction of the research article on which the above is based:

The role of green roofs in climate change mitigation. A case study in Seville (Spain) - ScienceDirect

The role of green roofs in climate change mitigation. A case study in Seville (Spain)
Building and Environment
Volume 123, October 2017, Pages 575-584


1. Introduction

Numerous studies on climate change predict a global rise in temperatures. The consequences of this increase will be more troublesome in urban areas, where the temperatures are already higher than in surrounding rural areas. This heating phenomenon is mostly due to anthropogenic development in the urban area [1] and the increase of building covered areas [2]. The construction materials commonly used absorb most of the radiation and release it as heat. This generates the urban heat island phenomenon, which has direct and indirect impacts on the health and life quality of the citizens [3]. Urban heat islands vary in magnitude and structure according to two main groups of factors: climatological factors (such as climatic region, season, time of day, synoptic conditions and wind regime) and those related to the physical and human nature of the built environment, such as geographic location, topography, urban landscape geometry, type of building materials and intensity of human activities [4]. In fact, a study aiming to identity heat islands at different height levels conducted in Tel-Aviv (Israel) showed that parks and open areas were the coldest elements within the city during day and night [5]. There is a clear correlation between plant cover and land surface temperature [6,7], and consequently, an urban increase in green areas would contribute to mitigate the Heat Island [8]. Nevertheless, in many modern cities, there is a high density of building covered areas which does not allow raising the number of green areas. Thus, in order to increase the presence of urban vegetation, it is necessary to draw on systems implemented on existing buildings. Currently, the sum of all the building roofs represents a high percentage of exposition in urban areas. Estimations for dense cities prove that the fraction of roof area varies between 20 and 25% of the total area [9]. Because of this, the use of these surfaces to increase urban vegetation is an interesting option.

Green roofs are urban greening systems that precisely allow installing plant life in the roofs of buildings through more or less complex elements. They can be extensive, lighter, and with less substrate when establishing smaller species, or more intensive and heavier with greater amount of substrate where small trees and shrubs can be included [2]. Green roofs have existed for more than a thousand years, although their use has become more relevant in modern times and new technical solutions that favor their implementation have appeared. This development has come about since not only do they provide a nice relaxing space or scenery, but also ecosystems services such as microclimate regulation, rainwater management, improved building insulation (with an influence on inner temperature), noise absorption, decrease of air pollution, and biodiversity enhancing [3,10]. Moreover, they contribute to increasing the albedo of urban areas [11].

Many studies on green roofs are oriented to their capability to regulate temperature. However, depending on the climate and the type of green roof (different plant material, substrate, and construction features), their efficiency can vary [12]. The thermal efficacy of a green roof is closely related with the climate, and it becomes more significant when the environmental temperature rises [3]. This efficacy is measured from the point of view of energy savings in warm areas for their capacity to lower temperatures [2] of both the roof surface and the air above it [13]. For example, an analysis of the surface temperature before and after the placement of a green roof in Singapore showed a significant decrease once the green roof was installed, especially for high plant cover, making the maximum temperature difference approximately 18 °C [14]. Another study in Hong Kong proved that the heat stored in a bare roof was 75% higher than that of a green roof [15]. In the city of Chicago, the temperatures in summer of the surface of a green roof and a neighboring building were compared. The temperature of the green roof varied from 33 to 48 °C, while in the conventional dark roof of the adjacent building the temperature was 76 °C. The air temperature near the surface of the green roof was 4 °C lower than near the conventional roof [16]. This decrease in temperature happens because, in a green roof, the flux of sensible heat is low due to the high latent heat flux from evaporation, even if the net radiation is high. This works to lower the temperature in a specific area [17]. Also, some simulation studies indicate that green roofs can decrease the mean environmental temperature from 0.3 to 3 °C at a city scale, and drastically decrease the heat island effect [2].

Nowadays, in many cities of several countries, such as Germany, the U.S.A., Denmark, and Canada, their governments have developed a variety of norms, incentives, and technical services to promote the naturalizing of roofs [18]. These measures will foster the increase of the area covered by green roofs, which will have favorable consequences on the specific climatic conditions in the urban areas where they are installed. In fact, the mass installment of green roofs might work as a mechanism to decrease the Heat Island effect and counteract the temperature increase due to climate change. In order to do so, the remaining question would be how much surface would be needed to mitigate the effect of climate change. This is, precisely, the main objective of this study. This is why the effect on the temperature when increasing the vegetation areas by means of green roofs in the city of Seville (Spain) is assessed.
 

David Klein

Moderator
Here's a study that compared green roofs to cool roofs in central Illinois.

DK

An environmental cost-benefit analysis of alternative green roofing strategies - ScienceDirect
An environmental cost-benefit analysis of alternative green roofing strategies

Abstract.

Green roofs and cool roofs are alternative roofing strategies that mitigate urban heat island effects and improve building energy performance. Green roofs consist of soil and vegetation layers that provide runoff reduction, thermal insulation, and potential natural habitat, but can require regular maintenance. Cool roofs involve a reflective layer that reflects more sunlight than traditional roofing materials, but require additional insulation during winter months. This study evaluates several roofing strategies in terms of energy performance, urban heat island mitigation, water consumption, and economic cost. We use MLCan, a multi-layer canopymodel, to simulate irrigated and non-irrigated green roof cases with shallow and deep soil depths during the spring and early summer of 2012, a drought period in central Illinois. Due to the dry conditions studied, periodic irrigation is implemented in the model to evaluate its effect on evapotranspiration. Traditional and cool roof scenarios are also simulated by altering surface albedo and omitting vegetation and soil layers. We find that both green roofs and cool roofs significantly reduce surface temperature compared to the traditional roof simulation. Cool roof temperatures always remain below air temperature and, similar to traditional roofs, require low maintenance. Green roofs remain close to air temperature and also provide thermal insulation, runoff reduction, and carbon uptake, but might require irrigation during dry periods. Due to the longer lifetime of a green roof compared to cool and traditional roofs, we find that green roofs realize the highest long term cost savings under simulated conditions. However, using longer-life traditional roof materials (which have a higher upfront cost) can help decrease this price differential, making cool roofs the most affordable option due to the higher maintenance costs associated with green roofs.
 

David Klein

Moderator
Thinking about what kind of roof would be best? Here's some more info.

DK

From Berkeley Lab Heat Island Group

Economic Comparison of White, Green, and Black Flat Roofs in the United States | HEATISLAND

Economic Comparison of White, Green, and Black Flat Roofs in the United States

Abstract
White and “green” (vegetated) roofs have begun replacing conventional black (dark-colored) roofs to mitigate the adverse effects of dark impervious urban surfaces. This paper presents an economic perspective on roof color choice using a 50-year life-cycle cost analysis (LCCA). We find that relative to black roofs, white roofs provide a 50-year net savings (NS) of $25/m2 ($2.40/ft2) and green roofs have a negative NS of $71/m2 ($6.60/ft2). Despite lasting at least twice as long as white or black roofs, green roofs cannot compensate for their installation cost premium. However, while the 50-year NS of white roofs compared to green roofs is $96/m2 ($8.90/ft2), the annualized cost premium is just $3.20/m2 -year ($0.30/ft2 -year). This annual difference is sufficiently small that the choice between a white and green roof should be based on preferences of the building owner. Owners concerned with global warming should choose white roofs, which are three times more effective than green roofs at cooling the globe. Owners concerned with local environmental benefits should choose green roofs, which offer built-in stormwater management and a “natural” urban landscape esthetic. We strongly recommend building code policies that phase out dark-colored roofs in warm climates to protect against their adverse public health externalities.

 
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