Environmentally Sustainable Green Roof Design for Energy Demand Reduction

Citation: Azkorra-Larrinaga, Z.; Romero-Antón, N.; Martin-Escudero, K.; Lopez-Ruiz, G. Environmentally Sustainable Green Roof Design for Energy Demand Reduction. Buildings 2023,13, 1846. https://doi.org/ 10.3390/buildings13071846 Academic Editor: Antonio Caggiano Received: 20 June 2023 Revised: 18 July 2023 Accepted: 18 July 2023 Published: 21 July 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). buildings Article Environmentally Sustainable Green Roof Design for Energy Demand Reduction Zaloa Azkorra-Larrinaga * , Naiara Romero-Antón , Koldobika Martin-Escudero and Gontzal Lopez-Ruiz ENEDI Research Group, Department of Thermal Engineering, University of the Basque Country (UPV/EHU), Alda. Urquijo s/n, 48013 Bilbao, Spain *Correspondence: [email protected] Abstract: Green roofs are artificial ecosystems that provide a nature-based solution to environmental problems such as climate change and the urban heat island effect by absorbing solar radiation and helping to alleviate urban environmental, economic, and social problems. Green roofs offer many benefits in terms of heat and water conservation as well as in terms of energy costs. This work proposes the design of an extensive and environmentally sustainable green roof for the Faculty of Engineering building in Bilbao. The green roof will be made from the composting of food waste generated in the building’s own canteen. Therefore, the main objective of this study is to calculate the solar efficiency of a sustainable green roof, evaluate its thermal performance, and quantify the impact that its implementation would have on energy consumption and the thermal comfort of its users. The results obtained confirm that an environmentally sustainable green roof has a positive effect on summer energy consumption and that this effect is much greater when there is water on the roof, as shown by the difference in energy savings between the dry ( − 53.7%) and wet ( − 84.2%) scenarios. The data show that in winter the differences between a green roof and a non-vegetated roof are not significant. In this case, the estimated energy consumption penalty (0.015 kWh/m 2 ) would be 10% of the summer gain. Keywords: green roof; energy savings; solid waste management; sustainability 1. Introduction High urban population growth rates, driven by industrialization, have intensified the urban pollution and caused severe natural phenomena and presenting significant risks to human health and the environment [ 1 ]. The world’s urbanization ratio (urban population as a % of total population) has grown from 43.4% in 1991 to 55.7% in 2019; this requires an understanding of the multi-dimensional nature of urban development [ 2 ]. Urban sustainability is therefore becoming increasingly important due to climate change, human activities, and increasing urbanization [3]. The need to reduce urban air temperature increases and increase green spaces, especially in city centers, is serious. Added to this, there are growing concerns about the global energy crisis and waste management. As a result of the above, the 2030 Sustainable Development Strategy aims to make Europe a resource-efficient society and establishes the principle of a hierarchy of waste management options, according to which prevention is the best option, followed by preparation for reuse, recycling, other forms of recovery, and finally disposal. That is why green roofs are an environmentally sustainable option for cities to reduce the impact of urbanization. Green roofs thus provide nature-based strategies for promoting urban environmental sustainability which include a combination of mitigating and adaptive actions mainly focused on restoring and preserving the balance between the biotic and abiotic elements of ecosystems by increasing biodiversity, developing green spaces, and promoting the sustainable development of cities by building a habitable urban area [4–6]. Buildings 2023,13, 1846. https://doi.org/10.3390/buildings13071846 https://www.mdpi.com/journal/buildings Buildings 2023,13, 1846 2 of 22 The world’s cities are of great relevance in the global development scenario as they will become even more urbanized in the next decade [ 7 ]. They represent around 85% of total global GDP (Gross Domestic Product) while consuming approximately 70% of the world’s natural resources and 70% of all energy produced, producing around 50% of all waste, and (3) producing 70% of all greenhouse gases [8]. The use of waste in construction materials and the environmentally friendly development of buildings and infrastructure are some of the mechanisms put in place to conserve natural resources and contribute to environmental and socio-economic sustainability [ 9 ]. Designed and implemented as engineered ecosystems, green roofs (GR) enhance urban sustainability by providing multiple functions and benefits. A GR system consists of a root protection and storage layer, a draining layer, a root permeable filter layer, a substrate, and plants [ 10 ]. In recent years, the scientific literature has discovered a significant number of related benefits across a wide variety of sustainability zones, which makes GR a common engineering application universally applicable to the fight against climate change [ 11 ], namely the mitigation of the urban heat island (UHI) [ 12 , 13 ] and improvement of urban air and water quality [14,15]. The GR is one of the technologies that have demonstrated energy reduction for cooling or heating buildings [ 16 – 19 ], being considered as an additional thermal insulation compared to bare or graveled roofs. However, energy savings have been found to vary considerably depending on several factors, mainly related to the site weather, the thickness of the existing insulation layer, and the type of vegetation used for the roof [ 20 , 21 ]. In addition to reducing UHI intensity and minimizing the extent of urban overheating, green infrastructure helps manage urban wastewater [6]. Research has shown that increasing the R-value of a building envelope does not guarantee a reduction in energy demand and can result in insufficient performance in present or future climates [ 22 – 24 ]. The evidence shows that climate change will have a considerable effect on the energy efficiency of a building. Therefore, it is essential to design the thermal transmittance of the building envelope for the current and future climate [25]. From a thermal point of view, the first effect of green roofs is to reduce heat transfer to the building [ 26 – 28 ], thereby improving indoor comfort conditions, mainly by reducing ceiling temperatures [29,30]. In terms of energy, the impacts of GRs on heating and cooling loads have also been studied in terms of their potential to reduce the UHI, as they can reduce outdoor temperatures in their vicinity in different climatic contexts [31]. In addition to the research into the impact of the reduction in ambient temperature that can be achieved by GR, a number of studies were completed investigating the insulation benefits it can provide. He and Jim [ 32 ] assesses the thermodynamic transmission in a GR based on a simulation model of the traditional Bowen Ratio Energy Balance Model (BREBM). It found that the GR captures and retains great quantities of heat to form a good thermal protection from variations. Additional work by Pérez et al. [ 33 ], shows that passive systems, due to the shading provided by the plants and the insulation provided by the plants and growing media, can be used to reduce the energy consumption of the system. Morakinyo et al. [ 34 ] presented a parametric paper of the result of four types of green roofs on outdoor/indoor temperature and cooling demand. It was found that semi-intensive green roofs were more effective than their fully extensive counterparts in reducing both outdoor temperature and cooling demand. In a similar way, Ouldboukhitine et al. [ 26 ] evaluated the heat flux through the roof. They found that, in summer, the passive cooling effect of the rooftop was three times greater with the green roof, while in winter, the green roof decreased rooftop heat loss on cold days. Rakotondramiarana et al. [ 35 ] demonstrated that GR protects the roof structure in conditions of extreme temperatures and large temperature fluctuations. The plants reduce the maximum indoor air temperature and increase the thermal comfort of the building during summer days. Composting is considered a sustainable option for the treatment of urban organic waste and its reuse as a soil conditioner and fertilizer [ 36 ]. As well as being a rich nutrient supply Buildings 2023,13, 1846 3 of 22 for plants, including nitrogen, phosphorus, and potassium, compost is also considered to reduce the incidence of soil-borne plant diseases. These benefits make composting an excellent solution for dealing with the vast quantities of biodegradable solid waste produced in the world [ 37 ]. As such, composting can help achieve circular economy goals in both developed and developing countries [38,39]. Aerobic composting is an excellent way of transforming manure into a hygienic, humusand nutrient-rich, stabilized, and phytotoxic-free material that has attracted the attention of national and international bodies [40,41]. Various chemical properties of the organic waste, such as excessive moisture or low porosity, can limit the efficiency of composting [ 42 , 43 ]. The effective composting of organic waste therefore requires good control of a number of factors for the production of a superior agricultural product [ 44 ]. These include the composition of the compost raw materials (C/N ratio, pH value, particle size, and moisture content) and the control of the process (aeration rate and temperature). These factors may have an impact on the intensity of microbial metabolism and biochemical transformation and thus on the final products [ 37 ]. Various strategies, such as aeration, raw material mixes, blowing agents, process management methods, additives and microbial inoculants are used in composting to achieve shorter composting times, reduce costs, and thus improve the quality of the final compost product [45,46]. Municipal solid waste (MSW) treatment is one of the essential services provided by any city. Neglecting its needs can be problematic for the city’s inhabitants. The rapid growth of cities and the resulting growth in urban populations have caused a significant expansion in the generation of municipal solid waste (MSW) [ 47 ]. The trend towards sustainable waste management (WM) according to the Reduce, Reuse, Recycle, and Recover (4 R) principle is growing worldwide [ 48 , 49 ]. Universities are also becoming more concerned with initiatives to mitigate their environmental impacts [ 50 ]. University campuses can make a significant contribution to the general sustainability of their local communities. They reduce the environmental impact of the traditional model and encourage healthy and sustainable living within the campus community [51]. The organic component of mulch substrates is effective in maintaining vegetation and increasing water retention. These organic materials are not always available locally. It is therefore important to design and use locally produced growing media as this reduces the financial and energy costs of transport [ 52 ]. The incorporation of local waste materials is preferable as it transforms low-value materials into a valuable material, clearly lowering costs and helping to promote the implementation of green roofs. Today, there is a need for a holistic approach to waste management (WM) in university campuses. Composting food waste (HWW) to create a green roof on campus can reduce the adverse environmental effects and be part of a circular economy. The compost can serve as a substrate for the green roof and then be added as fertilizer, ending the waste cycle. Applying this type of waste strategy brings campuses closer to the goals of carbon neutrality and sustainability [53]. In this work, the design of an environmentally sustainable vegetation cover was proposed from the substrate originating from the composting of food waste generated in the canteen of the building itself. This project proposes an extensive green roof with low maintenance and no need for irrigation. This is a future construction on the nontrafficked roof which houses the building’s ventilation installations and, since it is an already constructed building, it is necessary for the vegetation cover to be light, shallow, and suitable for the previously constructed concrete support structure. The aim of this work is to calculate the solar cooling/heating efficiency of a sustainable green roof. The presented study uses the solar efficiency method, which simplifies the complex physical models of heat transfer and radiation on green roofs to calculate the marginal cooling or heating performance. This is an indicator of the performance of innovative passive solutions that can be installed on the last layer of the building surface with respect to solar radiation. The calculation of these solar cooling and heating performances requires the calculation of the conduction heat transfer, i.e., the energy balances of the Buildings 2023,13, 1846 4 of 22 outermost surface of the building, in this case, the roof. This allows the temperature of this surface to be estimated, which in turn allows greater accuracy in estimating the heating and cooling requirements. Thus, it is intended to meet the following objectives: 1. A more energy-efficient building; 2. An improved and landscaped building and an attraction for the university community and citizens with the following agenda: reflecting the commitment of the UPV/EHU to the needs of Basque society as well as those derived from its history and its socioeconomic, political, and cultural transformations; disseminating the knowledge of universal culture and science; and exercising its daily activity in an economically, socially, and environmentally sustainable manner (UPV/EHU Strategic Plan 2022–2025). The new plan defines the public university as “a decisive agent for the development of the Basque community” as well as “a space for the generation of new ideas”; 3. The valorization of waste generated in the canteen of the same building through composting in pursuit of the principles of openness and dissipation of ecosystems, hierarchy, self-sufficiency, and zero waste. This work would represent a step forward for the UPV/EHU in the 2030 strategy through the analysis of the possible implementation of composting as a waste management measure. 2. Materials and Methods 2.1. Experimental Site Description The experimental roof site for this study is located in Bilbao (4315.8522 ◦ N, 256.9635 ◦ E) in the north of Spain. Bilbao is the largest city in the north of Spain and the tenth largest city in Spain, with a population of about 347,000 inhabitants in 2023. The urban area of Bilbao has 1,037,847 inhabitants, making it the largest populated urban area in northern Spain. It has an urban area of 18.22 km 2 . The Bilbao School of Engineering is located in the San Mamés area next to the San Mamés football stadium, the EiTB headquarters, and the bus station (Termibús). The building that houses the Faculty of Engineering in Bilbao is located between the San Mamés stadium and the access to Bilbao (see Figure 1), which, being a busy road, has an acoustic barrier along the side of the building. The building is also clad with a ventilated façade along which the UPV/EHU logo is displayed. The part of the roof of the EUITI-EUITMOP building where the vegetation cover will be installed has a total surface area of 1106.55 m2. Buildings 2023, 13, x FOR PEER REVIEW 5 of 22 Figure 1. View of the study area from above. In an extensive vegetation cover, the substrate is particularly important as it provides nutrients, water and oxygen, filtration, and physical support for the vegetation. It is therefore the key element that determines the success of the implementation of a green roof. This substrate layer, together with the drainage and filtration layer, forms the so-called rooting layer of the green roof system. A meteorological station was placed at a height of 2.5 m above the roof, a. Air temperature and relative humidity (HMP155A-L, Campbell Scientific Ltd., Shepshed, UK) as well as rainfall (TE525L, Campbell Scientific Ltd., Shepshed, UK) were measured and recorded every 30 min on a data logger (CR850, Campbell Scientific Ltd., Shepshed, UK). Various studies [54] recommend that the percentage of inorganic components in the green roof substrate should be greater than 80% to reduce the overall weight of the roof [55]. The use of 100% compost should also be avoided in extensive roofs as this could lead to the smothering of the vegetation, encourage weed growth, and increase the load on the roof, compromising the long-term success of the project. The roof drainage quality is also influenced by the substrate, the vegetation, and the age of the green roofs. The growing substrate, which could be either inorganic, organic, or a specific mix of the two, is a critical component of the design of green roofs. Appropriate substrate materials need to have appropriate water and nutrient retention properties as well as adequate drainage and lightweight and chemical balance to provide support for the plants’ regulatory functions [56,57]. To meet these conditions, substrates are frequently evaluated according to key physical and chemical properties such as bulk density, weight, porosity, particle size, water retention capacity, pH, and organic matter. The desired ecosystem of the green roof is also important for selecting the appropriate amount and type of organic components for the growing media [58]. Green roof growing media are usually made up of the following elements: - Perlite is a material obtained as a result of the thermal treatment of volcanic siliceous rock of the rhyolite group at 1000–1200 °C. It is presented as white particles whose dimensions vary between 1.5 and 6 mm with a low density and are generally less than 100 kg/m3. It has a water retention capacity of up to five times its weight and a high porosity; its pH is close to neutral (7–7.5); - Vermiculite is obtained by exfoliating a type of mica at temperatures above 800 °C. It has an apparent density of 90 to 140 kg/m3 and is supplied in 5–10 mm flakes. It can hold 350 L of water per cubic meter and has a good aeration capacity, although it tends to compact over time. It can contain up to 8% assimilable potassium and up to 12% assimilable magnesium. Its pH is close to neutral (7–7.2); - Sand: The grain size of the sand ranges from 0.5 to 2 mm in diameter. Its bulk density is similar to that of gravel. Its water retention capacity is medium (20% by weight and Figure 1. View of the study area from above. The climate is atlantic; rainfall is very abundant and almost always exceeds 1000 mm. In addition to being abundant, rainfall is well distributed throughout the year, with a Buildings 2023,13, 1846 5 of 22 maximum in autumn–winter and a minimum in summer, although no month receives less than 30 mm. The 150–160 days of annual rainfall mean that, on average, it rains every other day. Rainfall intensity is low. Under these conditions, the relative humidity is high throughout the year (80–90%). Average temperatures are mild, ranging from 12 ◦ to 13 ◦ . The average temperature in January does not fall below 6 ◦ C and in July it does not exceed 20 ◦ , giving a temperature range of 9–11 ◦C. Summers are mild with very rare periods of extreme heat. All environmental conditions were measured throughout the year using the weather station located on the roof of the Faculty of Engineering building in Bilbao. On the other hand, the indoor conditions are kept constant at 20 ◦ C and 25 ◦ C for the summer and winter seasons, respectively. In an extensive vegetation cover, the substrate is particularly important as it provides nutrients, water and oxygen, filtration, and physical support for the vegetation. It is therefore the key element that determines the success of the implementation of a green roof. This substrate layer, together with the drainage and filtration layer, forms the so-called rooting layer of the green roof system. A meteorological station was placed at a height of 2.5 m above the roof, a. Air temperature and relative humidity (HMP155A-L, Campbell Scientific Ltd., Shepshed, UK) as well as rainfall (TE525L, Campbell Scientific Ltd., Shepshed, UK) were measured and recorded every 30 min on a data logger (CR850, Campbell Scientific Ltd., Shepshed, UK). Various studies [ 54 ] recommend that the percentage of inorganic components in the green roof substrate should be greater than 80% to reduce the overall weight of the roof [ 55 ]. The use of 100% compost should also be avoided in extensive roofs as this could lead to the smothering of the vegetation, encourage weed growth, and increase the load on the roof, compromising the long-term success of the project. The roof drainage quality is also influenced by the substrate, the vegetation, and the age of the green roofs. The growing substrate, which could be either inorganic, organic, or a specific mix of the two, is a critical component of the design of green roofs. Appropriate substrate materials need to have appropriate water and nutrient retention properties as well as adequate drainage and lightweight and chemical balance to provide support for the plants’ regulatory functions [ 56 , 57 ]. To meet these conditions, substrates are frequently evaluated according to key physical and chemical properties such as bulk density, weight, porosity, particle size, water retention capacity, pH, and organic matter. The desired ecosystem of the green roof is also important for selecting the appropriate amount and type of organic components for the growing media [58]. Green roof growing media are usually made up of the following elements: - Perlite is a material obtained as a result of the thermal treatment of volcanic siliceous rock of the rhyolite group at 1000–1200 ◦ C. It is presented as white particles whose dimensions vary between 1.5 and 6 mm with a low density and are generally less than 100 kg/m 3 . It has a water retention capacity of up to five times its weight and a high porosity; its pH is close to neutral (7–7.5); - Vermiculite is obtained by exfoliating a type of mica at temperatures above 800 ◦ C. It has an apparent density of 90 to 140 kg/m 3 and is supplied in 5–10 mm flakes. It can hold 350 L of water per cubic meter and has a good aeration capacity, although it tends to compact over time. It can contain up to 8% assimilable potassium and up to 12% assimilable magnesium. Its pH is close to neutral (7–7.2); - Sand: The grain size of the sand ranges from 0.5 to 2 mm in diameter. Its bulk density is similar to that of gravel. Its water retention capacity is medium (20% by weight and more than 35% by volume); its aeration capacity decreases over time due to compaction; - Gravel has a diameter of between 5 and 15 mm and an apparent density of between 1500 and 1800 kg/m 3 . It has good structural stability; its water retention capacity is low although its porosity is high (more than 40% of the volume). Buildings 2023,13, 1846 6 of 22 Taking all of the above into consideration [ 59 – 61 ], the substrate to be used will be a mixture of 20% compost from the Faculty of Engineering canteen waste, 30% perlite, 20% vermiculite, 10% sand, and 20% gravel from reclamation. This mixture will be spread around the selected perimeter of the roof to be landscaped by pouring, raking, and lightly compacting. The plants will be planted on this substrate. The roof will be provided with a water inlet so that, at the time of application, the substrate mix will have sufficient humidity to work on the roof and prevent it from drying out and avoiding wind erosion. The plants to be selected are weather resistant and will allow for minimum maintenance which will avoid installing an irrigation system and allow the reuse of rainwater, thereby providing an extra element to the environmental quality. Figure 2shows a schematic section of the green roof to be installed on the roof of the UPV-EHU building together with its components. The first layers are the vegetation and the substrate which is composed of 20% compost and can absorb excess moisture to prevent the roots from rotting. The drainage layer made of polyethylene absorbs excess water and the root barrier prevents plant roots from damaging the material. Part of the rainwater is recollected by the drainage layer and is available for use during dry periods. The insulation layer between the vegetation and the roof slab acts as a temperature barrier for the plant roots, in this case, 5 cm extruded polystyrene. Buildings 2023, 13, x FOR PEER REVIEW 6 of 22 more than 35% by volume); its aeration capacity decreases over time due to compaction; - Gravel has a diameter of between 5 and 15 mm and an apparent density of between 1500 and 1800 kg/m 3 . It has good structural stability; its water retention capacity is low although its porosity is high (more than 40% of the volume). Taking all of the above into consideration [59–61], the substrate to be used will be a mixture of 20% compost from the Faculty of Engineering canteen waste, 30% perlite, 20% vermiculite, 10% sand, and 20% gravel from reclamation. This mixture will be spread around the selected perimeter of the roof to be landscaped by pouring, raking, and lightly compacting. The plants will be planted on this substrate. The roof will be provided with a water inlet so that, at the time of application, the substrate mix will have sufficient humidity to work on the roof and prevent it from drying out and avoiding wind erosion. The plants to be selected are weather resistant and will allow for minimum maintenance which will avoid installing an irrigation system and allow the reuse of rainwater, thereby providing an extra element to the environmental quality. Figure 2 shows a schematic section of the green roof to be installed on the roof of the UPV-EHU building together with its components. The first layers are the vegetation and the substrate which is composed of 20% compost and can absorb excess moisture to prevent the roots from rotting. The drainage layer made of polyethylene absorbs excess water and the root barrier prevents plant roots from damaging the material. Part of the rainwater is recollected by the drainage layer and is available for use during dry periods. The insulation layer between the vegetation and the roof slab acts as a temperature barrier for the plant roots, in this case, 5 cm extruded polystyrene. Figure 2. Composition and layers of green roof. 2.2. Waste Measurements Local composting is seen as a sustainable option for bio-waste recovery and is increasingly demanded by society. University campuses are no exception. In Bilbao, as in many European cities, growth and development have led to a lack of green spaces. Consequently, one of the problems to be solved in the case of composting organic waste generated in the university canteen would be the distribution of the compost produced. For this purpose, the composting and its subsequent distribution will be carried out on the green roof of the building itself. Measurements [62] have shown that in the canteen of the university building about 8.1 tons of waste were generated per academic year and that the percentage of organic waste, excluding wooden boxes but counting bread, was equivalent on average to about 60% of the mass of waste generated. Waste from the building’s canteen is currently separated into the following fractions: waste, packaging, and glass with varying degrees of efficiency and, according to the latest Figure 2. Composition and layers of green roof. 2.2. Waste Measurements Local composting is seen as a sustainable option for bio-waste recovery and is increasingly demanded by society. University campuses are no exception. In Bilbao, as in many European cities, growth and development have led to a lack of green spaces. Consequently, one of the problems to be solved in the case of composting organic waste generated in the university canteen would be the distribution of the compost produced. For this purpose, the composting and its subsequent distribution will be carried out on the green roof of the building itself. Measurements [ 62 ] have shown that in the canteen of the university building about 8.1 tons of waste were generated per academic year and that the percentage of organic waste, excluding wooden boxes but counting bread, was equivalent on average to about 60% of the mass of waste generated. Waste from the building’s canteen is currently separated into the following fractions: waste, packaging, and glass with varying degrees of efficiency and, according to the latest count (Table 1), about 33 kg of organic waste per day or about 5 tons during the academic year. These data are consistent with other authors who have found that higher education institutions (HEIs) generate an average of 0.08 kg/day per capita. Between 22% and 55% of the total waste generated in HEIs is usually biodegradable organic material [ 63 ]. Therefore, the total production rate is calculated to be 89.50 g/user/working day [64]. Buildings 2023,13, 1846 7 of 22 Table 1. Waste accounted for in kilograms after the implementation of waste minimization and management measures per day. Sample Number Organic (No Bread) (kg) Bread (kg) Packages (kg) Total (kg) 1 31.8 3.7 10.0 45.5 2 27.2 1.5 5.6 34.3 3 35.7 4.2 10.9 50.8 4 27.4 2.8 10.0 40.2 Average 30.5 3.1 9.1 42.7 It must be taken into account that during composting, organic matter tends to decrease due to its mineralization and the consequent loss of carbon in the form of carbon dioxide; these losses can represent almost 20% by weight of the composted mass [65]. According to RD 506/2013 of 28 June on fertilizer [ 66 ], the organic matter content of compost must exceed 35%. In the study by Montejo et al. [ 67 ] in which 30 compost samples from 10 different composting plants were analyzed, it was found that the average organic matter content was close to 45% while the average density of the resulting compost was 1.06 g/cm3. Considering all these aspects, it was calculated that the volume of compost that can be produced during the academic year is 8.39 m3. In view of the above, this work proposes the design of a green roof on the building itself, using a substrate produced by composting the food waste generated in the building’s canteen. The aim is to make efficient use of the organic fraction and to obtain high-quality compost through selective separation at the source. 2.3. Calculation of Green Cover Efficiency This section begins by explaining the solar efficiency methodology used to calculate this indicator of the performance of a green roof in relation to heat gains due to solar radiation. This is conducted by defining the theoretical maximum and minimum temperatures of the external surface of the green roof. These two theoretical extreme temperatures, together with a calculated external surface temperature, are related by an efficiency parameter relative to solar radiation. The concept of plant cover efficiency is applied whereby the thermal performance of plant cover for specific environmental conditions can be quantified in a simple way. In this study [ 68 ], in the energy balance of the outermost layer of the building it was shown that, since the qcond value is negligible compared to the sum of the values of shortwave radiation, longwave radiation and convective heat exchange occur in the outermost layer of the roof or façade. In this way, it is possible to accurately estimate the value of the temperature in the outer layer (T g ) by simply applying the energy balance at the outermost surface of the building envelope, without taking into account the term qcond in the energy balance when solar radiation is present. Therefore, efficiency is defined as the radius between two temperature differences: ε=Tmax −Tg Tmax −Tmin (-)(1) where T g represents the temperature in the outer layer of the substrate and T max and T min represent the limits of this temperature. Depending on the period in which one wants to study the efficiency of the roof, T max and T min will represent the best or the worst possible scenario. In summer, the efficiency will be maximum when the temperature in the outer layer of the substrate (T g ) is equal to the minimum temperature (T min ). On the contrary, in winter the best scenario is found when T g is closest to T max , therefore the optimum value of the efficiency during winter will be equal to 0. Buildings 2023,13, 1846 8 of 22 • T g is the measured temperature of the exterior surface of a monitored building. It might be a vertical or a horizontal component [69]. In buildings with passive elements, such as green systems or ventilated facades, where complex heat transfer phenomena such as evapotranspiration and natural and/or forced ventilation can occur, the choice of the layer in which T g is to be estimated is crucial. Once T g is known, the solar efficiency equation can be used to accurately model the heat flow through the component with this complex behavior. In the absence of a green roof installed at the Bilbao School of Engineering, we have studied the thermal behavior of a green roof based on the work carried out by Erkoreka [ 70 ] which allows us, in a simple way, to quantify the thermal performance of a green roof for specific environmental conditions. The model presented by Sailor [ 71 ] was taken as a starting point and the following simplification was made in order to obtain this simplified model for the calculation of T g under specific working conditions; the water content in the substrate and in the drainage layer, the evapotranspiration rate, LAI, albedo, photosynthetic rate, convective heat transfer from the roof to the outside air, and heat flow through the substrate were considered constant or insignificant. The simplified model is reflected in the following equation: Tg=Tout + αe f f ective Gsolar hheat−mass,e f f ective (-)(2) where T out is the outdoor air temperature [ ◦ C], G solar is the global solar radiation incident on the surface [W/m 2 ], and h heat-mass is the effective transfer of the coefficient heat mass [W/(m2C)]. • The maximum temperature (T max ) can be calculated using the experimental method provided by ASHRAE [72] which calls it Tsol-air. Tmax =Tsol−air =Tout +α·Gsolar hcomb − ε·∆R hcomb (◦C)(3) where T out is the outdoor air temperature [ ◦ C], α is the absorptivity of the surface for solar radiation, G solar is the global solar radiation incident on the surface [W/m 2 ], h comb is the combined coefficient of heat transfer by long-wave radiation and convection at the outer surface [W/(m 2◦ C)], ε is the hemispherical emissivity of the surface (-), and ∆ R is the difference between the long-wave radiation incident on the surface from the sky and the surroundings and the radiation emitted by a hypothetical blackbody at outdoor air temperature [W/m2]. According to ASHRAE, the maximum possible T sol-air temperature is obtained when: • The term α /hcomb has a value of 0.052, which represents the usual maximum value for this term; • The term ( ε·∆ R)/hcomb has a value for horizontal surfaces facing the sky of 4 ◦ C and for vertical surfaces of 0 ◦C; • While the minimum temperature, considered as the ‘wet bulb temperature’, can be obtained using the outdoor temperature and relative humidity [ 73 ], T min is defined as the temperature of the air when it is adiabatically saturated with water, in which case its temperature will decrease to the temperature called the wet bulb temperature. This, in turn, can be calculated as a function of the outdoor air temperature and the relative humidity, using the following empirical algorithm [74]. Tmin =Twet−bulb =Tout atanh0.151977 (RH%+8.313659)1 2i+ atan(Tout +RH%)−atan(RH%−1.676331)+0.00391838 (RH%)3 2· atan(0.023101 RH%)−4.686035 (◦C) (4) Buildings 2023,13, 1846 9 of 22 2.4. Calculation of the Energy Demand after the Installation of a Vegetation Cover This section presents an estimate of the energy required by a cooling/heating system to maintain comfort conditions assuming the installation of a green roof. For this calculation, only the heat flow through the roof has been taken into account. For this analysis, the location of the building (Bilbao) was taken into account. In order to achieve greater accuracy in the calculations, the information collected by the meteorological station located on the roof of the UPV/EHU Faculty of Engineering building in Bilbao has been used; this information includes measurements of the external air temperature, the relative humidity of the external air, rainfall, and solar radiation. Once the temperatures of the outer layer of the vegetation cover of GR had been determined, we then estimated the solar efficiencies throughout the whole year using the meteorological data collected during 2021 at the weather station located on the roof of the building of the Faculty of Engineering in Bilbao. The methodology used was as follows. The energy required per square meter is equal to the heat flow through the roof. This can be expressed in the following equation: q A=UTin −Tg(W/m2)(5) where T in is the temperature inside the building. According to thermal comfort standards, this value is 25 ◦ C for summer and 20 ◦ C for winter. T g is the temperature in the external layer of the substrate. On this occasion, it was calculated based on the efficiency values developed in Equation (1) above. This would become: Tg=Tmax −ε(Tmax −Tmin)(◦C)(6) U (W/(m 2◦ C)) is the thermal transmittance including the thermal resistances of the insulation layers under the vegetation cover: U=1 Rsi +e1 k1+e2 k2+· · · +en kn+Rse (W/(m2◦C)(7) where R si and R se are the surface thermal resistances of the enclosures in contact with the outside air, e 1. . . e n are the thicknesses of the different layers, and k 1. . . k n are the thermal conductivities of the respective materials. According to the Spanish regulations CTE [ 75 ], Bilbao is located in climate zone C1. In the same basic HE energy saving document, the maximum limit of thermal transmittance of roofs in contact with the air is defined for the different climatic zones. The following assumptions were made in the model. The growing part of the substrate was considered a homogeneous layer of 5–6 cm. The substrate may be dry, partially saturated, or fully saturated with water. This degree of saturation determines the conductivity and thermal capacity of this layer. The reference value for the thermal resistance of the green roof is taken as 0.44 (m2◦C/W) [18]. The geotextile filter of less than 2 mm was considered to have negligible heat transfer. The drainage layer was the most complicated to model as it is the only non-homogeneous layer. The effective thermal conductivity concept described by Çengel [ 76 ] was used. This concept assigns an effective thermal conductivity to a non-homogeneous layer such that the actual thermal resistance is the same as that modeled as homogeneous with this effective thermal conductivity. The insulating layer is made of 5 cm extruded polystyrene (k = 0.03 m ◦ C/W) which provides a thermal resistance of 1.66 (m2◦C/W) to the green covering. The impermeable layer was assumed to be a homogeneous 5.85 mm thick layer, 2.85 mm ‘preflex’, and 3 mm ‘graviflex’ with negligible heat transfer. Buildings 2023,13, 1846 16 of 22 to an average of 15 W/m 2 which means that three times more energy is required to achieve the same indoor comfort conditions. The energy savings from installing a green roof can therefore be calculated. In this case study, the average daily energy saving for the dry vegetation scenario is estimated to be 0.092 kWh/m 2 which represents a reduction of 53.7% in the energy required to cool the building. On the other hand, the daily average energy consumption for the scenario in which the vegetation cover is saturated with water is 0.144 kWh/m 2 , which represents a significant saving in consumption related to the cooling system of 84.2% less than for a traditional roof. Green roofs reduce energy consumption in the warm season which is consistent with the findings of Kostadinov et al. [ 86 ] who found that green roofs in summertime increase the thermal efficiency of the roof by 57% and decrease the thermal transfer. Dwijendra et al. [ 87 ] showed that the presence of a green roof, as opposed to a normal roof, has the capacity to decrease a building’s energy consumption by 30.7%. The majority of the findings agree with the literature on green roof systems, justifying them as a good passive cooling technology. 3.4.2. Winter Period The winter calculations were carried out taking the period of the year when the heating system can normally be used to maintain the internal temperature at the fixed value of 20 ◦ C into account. The period considered is from 23 September to 19 March (plotted from 15 to 31 December). The temperatures of the layer below the vegetation (T g ) are calculated in a period in which solar radiation is significant; otherwise, the calculation of Tmax becomes meaningless. In winter, when radiation is slightly lower than in summer, a minimal radiation of 50 W/m 2 was selected. After calculating T min and T max and with consideration of the meteorological data recorded every hour of the period, we can calculate the external surface temperature of the ground. It is important to note that on a sunny winter day there is less incident solar radiation on horizontal surfaces than on a summer day. Figure 8shows the heating demand required to maintain thermal comfort inside the building once the green roof is installed. It can be seen that every day the set temperature is higher than the outdoor temperature, so heating is required to maintain indoor comfortOn some sunny days when the outdoor temperature is around 15 ◦ C, e.g., 15, 16, and 18 December, the heating demand is lower. On colder days, the heating demand peaks at up to 8 W/m2. In the same way, the Figure 8shows that the average heat flux thanks to the vegetation cover remains at an average value of 3.7 W/m 2 which means an energy consumption of 0.030 W/m 2 . The results indicate that the installation of a green roof would increase the energy consumption of the heating system by 0.015 kWh/m 2 per day. This is because during winter the waterlogged vegetated cover repels more solar energy than a nonvegetated cover. A comparison of the two periods, namely summer and winter, shows that the increase in energy consumption in winter is only 10% of the total savings that can be achieved in summer. This is consistent with the limited amount of research that has investigated the thermal performance of green roofs in cold winter conditions [ 88 – 91 ]; in general, they found a moderate thermal advantage for green roofs in colder climates but a much smaller advantage in warmer climates. Green roofs had only a small negative (i.e., cooling) effect on the urban heat island surface in winter, as reported by Teemusk and Mander [ 92 ], but in diurnal periods the existence of green roofs, caused surface cooling and low evapotranspiration even during winter weather periods, can cause this cooling in the green roof system [ 93 ] comparable for a green roof to [ 94 ]. However, the green roof had a higher average surface temperature of about 1 ◦ C. This meant that it had a beneficial (i.e., warming) role at night compared to the Buildings 2023,13, 1846 17 of 22 traditional roof. This is probably caused by its substrate, which has a high heat capacity and thermal inertia, allowing the surface to cool more gradually during the night. Buildings 2023, 13, x FOR PEER REVIEW 17 of 22 Figure 8. (A) Temperature of the internal air, the external surface during wet winter, and solar radiation. (B) Heating demand per square meter from the 15 to 31 of December. In the same way, the figure 8 shows that the average heat flux thanks to the vegetation cover remains at an average value of 3.7 W/m2 which means an energy consumption of 0.030 W/m2. The results indicate that the installation of a green roof would increase the energy consumption of the heating system by 0.015 kWh/m2 per day. This is because during winter the waterlogged vegetated cover repels more solar energy than a non-vegetated cover. A comparison of the two periods, namely summer and winter, shows that the increase in energy consumption in winter is only 10% of the total savings that can be achieved in summer. This is consistent with the limited amount of research that has investigated the thermal performance of green roofs in cold winter conditions [88–91]; in general, they found a moderate thermal advantage for green roofs in colder climates but a much smaller advantage in warmer climates. Green roofs had only a small negative (i.e., cooling) effect on the urban heat island surface in winter, as reported by Teemusk and Mander [92], but in diurnal periods the existence of green roofs, caused surface cooling and low evapotranspiration even during winter weather periods, can cause this cooling in the green roof system [93] comparable for a green roof to [94]. However, the green roof had a higher average surface temperature of about 1°C. This meant that it had a beneficial (i.e., warming) role at night compared to the traditional roof. This is probably caused by its substrate, which has a high heat capacity and thermal inertia, allowing the surface to cool more gradually during the night. 3.5. Limitations The efficiency of this sustainable green roof for the EHU building in Bilbao was calculated according to that of a green roof in Vitoria. As the water use of the green roof depends on the precipitation and the evaporation process of the soil, as well as on the 0 50 100 150 200 250 300 350 400 450 0 5 10 15 20 25 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 0 12 15/12 16/12 17/12 18/12 19/12 20/12 21/12 22/12 23/12 24/12 25/12 26/12 27/12 28/12 29/12 30/12 31/12 Solar radiation [W/m 2 ] Temperature [ºC] A T in Tg winter G solar -8 -6 -4 -2 0 2 4 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 01 2 15/12 16/12 17/12 18/12 19/12 20/12 21/12 22/12 23/12 24/12 25/12 26/12 27/12 28/12 29/12 30/12 31/12 Heating demand [W/m 2 ] B Q_wet winter [W/m^2] Figure 8. ( A ) Temperature of the internal air, the external surface during wet winter, and solar radiation. (B) Heating demand per square meter from the 15 to 31 of December. 3.5. Limitations The efficiency of this sustainable green roof for the EHU building in Bilbao was calculated according to that of a green roof in Vitoria. As the water use of the green roof depends on the precipitation and the evaporation process of the soil, as well as on the transpiration and photosynthesis of the vegetation to produce carbohydrates, the data from Bilbao were used to calculate the available water so the efficiency was estimated as being slightly above the real value. 4. Conclusions This study investigates food waste from students in university canteens and analyzes its use as compost to create green cover on campus to reduce energy demand. It was found that food waste in the university canteen consists mainly of organic matter, bread, and packaging. Composting on the UPV-EHU campus can treat about 80% of the organic waste generated by the canteen. This strategy of composting organic material in the canteen involves multiple stakeholders and joint efforts by university officials, canteen managers, and students. Thus, the integration of organic waste management and sustainable vegetation cover in the urban context becomes possible. With all this, the study of the solar efficiency was carried out over a whole year. The cooling efficiency for summer was found to be 75%. On the other hand, in winter the GR cooling efficiencies were found to be close to 90%. This means that the heating efficiency of this skin solution is only 10%. In other words, the temperature of the exterior surface will be much closer to the minimum possible temperature. This means a reduction in the temperature of the outermost part of the roof during sunny hours which will lead to a Buildings 2023,13, 1846 18 of 22 corresponding increase in the heating requirement and higher energy costs. In winter, the thermal performance of this skin solution is of no interest because of the cooling effect generated by evapotranspiration. This prevents solar radiation from heating the outermost surface of the building envelope. The results obtained confirm that in terms of energy consumption, an environmentally sustainable green roof would have a positive effect in summer, with this effect being notably more pronounced when water is present in the roof. The average heat flux through the saturated green roof was found to be about 10 W/m 2 lower compared to a traditional roof. There is a significant difference in the summer energy savings achieved by the green roof between the dry and wet scenarios, reducing energy consumption by 53.7% and 84.2%, respectively. This evidence suggests that it would be desirable to install an irrigation system in climates with low rainfall. On the other hand, the data show that in winter the differences between a vegetation cover and a non-vegetation cover are not significant. In the most unfavorable situation, the disadvantage for the green roof is minimal. In this case, the estimated energy consumption penalty (0.015 kWh/m 2 ) would be 10% of the summer gain. This is because during the day the plant’s evapotranspiration cools the outer surface of the roof, increasing energy consumption. However, during the night it acts as an insulator by reducing the convective heat loss through the roof, thus helping to keep the building warm. Spreading the practice of composting and the visibility of green roofs would be another important outcome of the project, including external visits and training to learn and control composting processes. In this way, chemical fertilizers could be replaced by the compost produced and a greater knowledge of green roof systems and direct contact with nature could be provided locally and as part of the university community. Finally, it should be noted that the advantages of implementing a green roof go beyond the economic sphere. Bearing in mind that the building where the green roof is to be installed is a public university, the most important aspect is probably its didactic and exemplary value. Author Contributions: Conceptualization, Z.A.-L. and N.R.-A.; methodology, Z.A.-L.; software, K.M.-E. and N.R.-A.; validation, K.M.-E. and G.L.-R.; formal analysis, K.M.-E.; investigation, N.R.-A.; resources, Z.A.-L.; data curation, Z.A.-L. and G.L.-R.; writing—original draft preparation, Z.A.-L.; writing—review and editing, N.R.-A. and G.L.-R.; visualization, G.L.-R.; supervision, K.M.-E.; project administration, K.M.-E.; funding acquisition, N.R.-A. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: The data related to the results of this study are available upon request from the corresponding author. Acknowledgments: This publication is part of the COMISTER (Caracterización de COMponentes de INStalaciones TÉRmicas) project PES22/93. Open Access funding was provided by the University of Basque Country UPV/EHU. Conflicts of Interest: The authors declare no conflict of interest. References 1. Haseeb, M.; Kot, S.; Hussain, H.I.; Jermsittiparsert, K. Impact of Economic Growth, Environmental Pollution, and Energy Consumption on Health Expenditure and R&D Expenditure of ASEAN Countries. Energies 2019,12, 3598. 2. Krellenberg, K.; Welz, J.; Link, F.; Barth, K. Urban vulnerability and the contribution of socio-environmental fragmentation. Prog. Hum. Geogr. 2017,41, 408–431. [CrossRef] 3. Chen, M.; Chen, L.; Cheng, J.; Yu, J. Identifying interlinkages between urbanization and Sustainable Development Goals. Geogr. Sustain. 2022,3, 339–346. [CrossRef] 4. Raji, B.; Tenpierik, M.J.; van den Dobbelsteen, A. The impact of greening systems on building energy performance: A literature review. Renew. Sustain. Energy Rev. 2015,45, 610–623. [CrossRef] 5. Chen, D.; Wang, X.; Thatcher, M.; Barnett, G.; Kachenko, A.; Prince, R. Urban vegetation for reducing heat related mortality. Environ. Pollut. 2014,192, 275–284. [CrossRef] Buildings 2023,13, 1846 19 of 22 6. Mihalakakou, G.; Souliotis, M.; Papadaki, M.; Menounou, P.; Dimopoulos, P.; Kolokotsa, D.; Paravantis, J.A.; Tsangrassoulis, A.; Panaras, G.; Giannakopoulos, E.; et al. Green roofs as a nature-based solution for improving urban sustainability: Progress and perspectives. Renew. Sustain. Energy Rev. 2023,180, 113306. [CrossRef] 7. World Cities Report 2020: The Value of Sustainable Urbanization; United Nations Human Settlements Programme (UN-Habitat): Nairobi, Kenya, 2020. 8. The 15 Circular Steps for Cities, 2nd ed.; European Investment Bank: Luxembourg, 2021. 9. Bianchini, F.; Hewage, K. How “green” are the green roofs? Lifecycle analysis of green roof materials. Build. Environ. 2012 ,48, 57–65. [CrossRef] 10. Shafique, M.; Kim, R.; Rafiq, M. Green roof benefits, opportunities and challenges—A review. Renew. Sustain. Energy Rev. 2018 , 90, 757–773. [CrossRef] 11. Zambrano-Prado, P.; Pons-Gumí, D.; Toboso-Chavero, S.; Parada, F.; Josa, A.; Gabarrell, X.; Rieradevall, J. Perceptions on barriers and opportunities for integrating urban agri-green roofs: A European Mediterranean compact city case. Cities 2021 ,114, 103196. [CrossRef] 12. Iaria, J.; Susca, T. Analytic Hierarchy Processes (AHP) evaluation of green roofand green wall-based UHI mitigation strategies via ENVI-met simulations. Urban Clim. 2022,46, 101293. [CrossRef] 13. Bruno, R.; Bevilacqua, P.; Arcuri, N. 10—Green roofs as passive system to moderate building cooling requirements and UHI effects: Assessments by means of experimental data. In Eco-Efficient Materials for Reducing Cooling Needs in Buildings and Construction; PachecoTorgal, F., Czarnecki, L., Pisello, A.L., Cabeza, L.F., Granqvist, C., Eds.; Woodhead Publishing: Sawston, UK, 2021; pp. 205–245. 14. Liu, W.; Engel, B.A.; Chen, W.; Wei, W.; Wang, Y.; Feng, Q. Quantifying the contributions of structural factors on runoff water quality from green roofs and optimizing assembled combinations using Taguchi method. J. Hydrol. 2021 ,593, 125864. [CrossRef] 15. Liao, W.; Drake, J.; Thomas, S.C. Biochar granulation, particle size, and vegetation effects on leachate water quality from a green roof substrate. J. Environ. Manag. 2022,318, 115506. [CrossRef] [PubMed] 16. Castleton, H.F.; Stovin, V.; Beck, S.B.M.; Davison, J.B. Green roofs; building energy savings and the potential for retrofit. Energy Build. 2010,42, 1582–1591. [CrossRef] 17. Parizotto, S.; Lamberts, R. Investigation of green roof thermal performance in temperate climate: A case study of an experimental building in Florianópolis city, Southern Brazil. Energy Build. 2011,43, 1712–1722. [CrossRef] 18. Kazemi, M.; Courard, L.; Attia, S. Water permeability, water retention capacity, and thermal resistance of green roof layers made with recycled and artificial aggregates. Build. Environ. 2023,227, 109776. [CrossRef] 19. Pérez, G.; Coma, J.; Solé, C.; Castell, A.; Cabeza, L.F. Green roofs as passive system for energy savings when using rubber crumbs as drainage layer. Energy Procedia 2012,30, 452–460. [CrossRef] 20. Schroll, E.; Lambrinos, J.; Righetti, T.; Sandrock, D. The role of vegetation in regulating stormwater runoff from green roofs in a winter rainfall climate. Ecol. Eng. 2011,37, 595–600. [CrossRef] 21. Susca, T.; Zanghirella, F.; Colasuonno, L.; Del Fatto, V. Effect of green wall installation on urban heat island and building energy use: A climate-informed systematic literature review. Renew. Sustain. Energy Rev. 2022,159, 112100. [CrossRef] 22. D’Agostino, D.; de Rossi, F.; Marigliano, M.; Marino, C.; Minichiello, F. Evaluation of the optimal thermal insulation thickness for an office building in different climates by means of the basic and modified “cost-optimal” methodology. J. Build. Eng. 2019 ,24, 100743. [CrossRef] 23. Vellei, M.; Ramallo-González, A.P.; Coley, D.; Lee, J.; Gabe-Thomas, E.; Lovett, T.; Sukumar, N. Overheating in vulnerable and non-vulnerable households. Build. Res. Inf. 2017,45, 102–118. [CrossRef] 24. Rodrigues, E.; Fernandes, M.S. Overheating risk in Mediterranean residential buildings: Comparison of current and future climate scenarios. Appl. Energy 2020,259, 114110. [CrossRef] 25. Rodrigues, E.; Fereidani, N.A.; Fernandes, M.S.; Gaspar, A.R. Climate change and ideal thermal transmittance of residential buildings in Iran. J. Build. Eng. 2023, 74. [CrossRef] 26. Jaffal, I.; Ouldboukhitine, S.; Belarbi, R. A comprehensive study of the impact of green roofs on building energy performance. Renew. Energy 2012,43, 157–164. [CrossRef] 27. Algarni, S.; Almutairi, K.; Alqahtani, T. Investigating the performance of energy management in office buildings by using a suitable green roof design to reduce the building’s energy consumption. Sustain. Energy Technol. Assess. 2022 ,54, 102825. [CrossRef] 28. Azkorra-Larrinaga, Z.; Erkoreka-González, A.; Martín-Escudero, K.; Pérez-Iribarren, E.; Romero-Antón, N. Thermal characterization of a modular living wall for improved energy performance in buildings. Build. Environ. 2023,234, 110102. [CrossRef] 29. Coma, J.; Pérez, G.; Solé, C.; Castell, A.; Cabeza, L.F. Thermal assessment of extensive green roofs as passive tool for energy savings in buildings. Renew. Energy 2016,85, 1106–1115. [CrossRef] 30. Cirrincione, L.; Marvuglia, A.; Scaccianoce, G. Assessing the effectiveness of green roofs in enhancing the energy and indoor comfort resilience of urban buildings to climate change: Methodology proposal and application. Build. Environ. 2021 ,205, 108198. [CrossRef] 31. Susca, T. Green roofs to reduce building energy use? A review on key structural factors of green roofs and their effects on urban climate. Build. Environ. 2019,162, 106273. [CrossRef] 32. He, H.; Jim, C.Y. Simulation of thermodynamic transmission in green roof ecosystem. Ecol. Model. 2010 ,221, 2949–2958. [CrossRef] Buildings 2023,13, 1846 20 of 22 33. Pérez, G.; Rincón, L.; Vila, A.; González, J.M.; Cabeza, L.F. Green vertical systems for buildings as passive systems for energy savings. Appl. Energy 2011,88, 4854–4859. [CrossRef] 34. Morakinyo, T.E.; Dahanayake, K.W.D.K.C.; Ng, E.; Chow, C.L. Temperature and cooling demand reduction by green-roof types in different climates and urban densities: A co-simulation parametric study. Energy Build. 2017,145, 226–237. [CrossRef] 35. Rakotondramiarana, H.; Ranaivoarisoa, T.; Morau, D. Dynamic Simulation of the Green Roofs Impact on Building Energy Performance, Case Study of Antananarivo, Madagascar. Buildings 2015,5, 497–520. [CrossRef] 36. Belgium: NERA Contributes Policy and Macroeconomic Analysis on the Circular Economy in a New Study from the Ellen MacArthur Foundation: Delivering the Circular Economy a Toolkit for Policymakers. TendersInfo News. 2015. Available online: https://ellenmacarthurfoundation.org/a-toolkit-for-policymakers (accessed on 20 June 2023). 37. Gajalakshmi, S.; Abbasi, S.A. Solid Waste Management by Composting: State of the Art. Crit. Rev. Environ. Sci. Technol. 2008 ,38, 311–400. [CrossRef] 38. Bruni, C.; Akyol, Ç.; Cipolletta, G.; Eusebi, A.L.; Caniani, D.; Masi, S.; Colón, J.; Fatone, F. Decentralized Community Composting: Past, Present and Future Aspects of Italy. Sustainability 2020,12, 3319. [CrossRef] 39. Brusselaers, J.; Van Der Linden, A. Bio-Waste in Europe—Turning Challenges into Opportunities; European Environment Agency: Copenhagen, Denmark, 2020. 40. Awasthi, M.K.; Duan, Y.; Awasthi, S.K.; Liu, T.; Zhang, Z. Effect of biochar and bacterial inoculum additions on cow dung composting. Bioresour. Technol. 2020,297, 122407. [CrossRef] 41. Sayara, T.; Basheer-Salimia, R.; Hawamde, F.; Sánchez, A. Recycling of Organic Wastes through Composting: Process Performance and Compost Application in Agriculture. Agronomy 2020,10, 1838. [CrossRef] 42. Cerda, A.; Artola, A.; Font, X.; Barrena, R.; Gea, T.; Sánchez, A. Composting of food wastes: Status and challenges. Bioresour. Technol. 2018,248, 57–67. [CrossRef] 43. Torrijos, V.; Calvo Dopico, D.; Soto, M. Integration of food waste composting and vegetable gardens in a university campus. J. Clean. Prod. 2021,315, 128175. [CrossRef] 44. Araújo de Almeida, M.; Colombo, R. Construction of green roofs via using the substrates made from humus and green coconut fiber or sugarcane bagasse. Sustain. Chem. Pharm. 2021,22, 100477. [CrossRef] 45. Kumar, S. Composting of municipal solid waste. Crit. Rev. Biotechnol. 2011,31, 112–136. [CrossRef] 46. Huang, D.; Gao, L.; Cheng, M.; Yan, M.; Zhang, G.; Chen, S.; Du, L.; Wang, G.; Li, R.; Tao, J.; et al. Carbon and N conservation during composting: A review. Sci. Total Environ. 2022,840, 156355. [CrossRef] [PubMed] 47. Tirkolaee, E.B.; Mahdavi, I.; Esfahani, M.M.S.; Weber, G. A robust green location-allocation-inventory problem to design an urban waste management system under uncertainty. Waste Manag. 2020,102, 340–350. [CrossRef] 48. Yu, K.H.; Zhang, Y.; Li, D.; Montenegro-Marin, C.E.; Kumar, P.M. Environmental planning based on reduce, reuse, recycle and recover using artificial intelligence. Environ. Impact Assess. Rev. 2021,86, 106492. [CrossRef] 49. Bui, T.; Tseng, J.; Tseng, M.; Lim, M.K. Opportunities and challenges for solid waste reuse and recycling in emerging economies: A hybrid analysis. Resour. Conserv. Recycl. 2022,177, 105968. [CrossRef] 50. Amaral, A.R.; Rodrigues, E.; Gaspar, A.R.; Gomes, Á. Lessons from unsuccessful energy and buildings sustainability actions in university campus operations. J. Clean. Prod. 2021,297, 126665. [CrossRef] 51. Gomez, T.; Derr, V. Landscapes as living laboratories for sustainable campus planning and stewardship: A scoping review of approaches and practices. Landsc. Urban Plan. 2021,216, 104259. [CrossRef] 52. Ampim, P.A.Y.; Sloan, J.J.; Cabrera, R.I.; Harp, D.A.; Jaber, F.H. Green Roof Growing Substrates: Types, Ingredients, Composition and Properties. J. Environ. Hortic. 2010,28, 244–252. [CrossRef] 53. Jakimiuk, A.; Matsui, Y.; Podlasek, A.; Koda, E.; Goli, V.S.N.S.; Vobˇerková, S.; Singh, D.N.; Vaverková, M.D. Closing the loop: A case study on pathways for promoting sustainable waste management on university campuses. Sci. Total Environ. 2023 ,892, 164349. [CrossRef] 54. Carrera, D.; Lombillo, I.; Carpio-García, J.; Blanco, H. Assessment of different combinations of substrate-filter membrane in green roofs. J. Build. Eng. 2022,45, 103455. [CrossRef] 55. Vijayaraghavan, K.; Joshi, U.M. Can green roof act as a sink for contaminants? A methodological study to evaluate runoff quality from green roofs. Environ. Pollut. 2014,194, 121–129. [CrossRef] 56. Xue, M.; Farrell, C. Use of organic wastes to create lightweight green roof substrates with increased plant-available water. Urban For. Urban Green. 2020,48, 126569. [CrossRef] 57. Krawczyk, A.; Domagała-´ Swi ˛atkiewicz, I.; Lis-Krzy´scin, A. Time-Dependent Changes in the Physico-Chemical Parameters and Growth Responses of Sedum acre (L.) to Waste-Based Growing Substrates in Simulation Extensive Green Roof Experiment. Agronomy 2021,11, 298. [CrossRef] 58. Young, T.; Cameron, D.D.; Sorrill, J.; Edwards, T.; Phoenix, G.K. Importance of different components of green roof substrate on plant growth and physiological performance. Urban For. Urban Green. 2014,13, 507–516. [CrossRef] 59. Davraz, M.; Koru, M.; Akda˘g, A.E.; Kılınçarslan, ¸S.; Delikanlı, Y.E.; Çabuk, M. Investigating the use of raw perlite to produce monolithic thermal insulation material. Constr. Build. Mater. 2020,263, 120674. [CrossRef] 60. Zhao, M.; Tabares-Velasco, P.C.; Srebric, J.; Komarneni, S.; Berghage, R. Effects of plant and substrate selection on thermal performance of green roofs during the summer. Build. Environ. 2014,78, 199–211. [CrossRef] Buildings 2023,13, 1846 21 of 22 61. Bellazzi, A.; Barozzi, B.; Pollastro, M.C.; Meroni, I. Thermal resistance of growing media for green roofs: To what extent does the absence of specific reference values potentially affect the global thermal resistance of the green roof? An experimental example. J. Build. Eng. 2020,28, 101076. [CrossRef] 62. De Luis Álvarez, A.; Menéndez Ruiz, A.; Ortuzar Iragorri, A.; Aranguiz Basterrechea, I.; Bilbao Ergueta, E.; Echevarria Astarloa, J.C.; Ojeda, P.R.; Castiñeira, J.V. Primera fase del proyecto de innovación para la sostenibilidad: Análisis y reducción de residuos alimenticios generados en una de las cafeterías universitarias de la Escuela de Ingeniería de Bilbao. In Actas del IX Congreso Iberoamericano de Docencia Universitaria; Universidad de Murcia, Servicio de Publicaciones: Murcia, Spain, 2016. 63. Smyth, D.P.; Fredeen, A.L.; Booth, A.L. Reducing solid waste in higher education: The first step towards ‘greening’ a university campus. Resour. Conserv. Recycl. 2010,54, 1007–1016. [CrossRef] 64. Gallardo, A.; Edo-Alcón, N.; Carlos, M.; Renau, M. The determination of waste generation and composition as an essential tool to improve the waste management plan of a university. Waste Manag. 2016,53, 3–11. [CrossRef] 65. Zucconi, F.; Bertoldi, M.D. Specifications for solid waste compost. Biocycle 1987,28, 56–61. 66. Spain Royal Decree 506/2013, of June 28, on Fertilizer Products. 2013. Available online: https://www.boe.es/eli/es/rd/2013/0 6/28/506/con (accessed on 20 June 2023). 67. Montejo, C.; Costa, C.; Márquez, M.C. Influence of input material and operational performance on the physical and chemical properties of MSW compost. J. Environ. Manag. 2015,162, 240–249. [CrossRef] 68. Azkorra-Larrinaga, Z.; Erkoreka-González, A.; Flores-Abascal, I.; Pérez-Iribarren, E.; Romero-Antón, N. Defining the cooling and heating solar efficiency of a building component skin: Application to a modular living wall. Appl. Eng. 2022 ,210, 118403. [CrossRef] 69. Natarajan, M.; Natarajan, M.; Rahimi, M.; Rahimi, M.; Sen, S.; Mackenzie, N.; Imanbayev, Y. Living wall systems: Evaluating life-cycle energy, water and carbon impacts. Urban Ecosyst. 2015,18, 1–11. [CrossRef] 70. Erkoreka, A. Modeling and Testing of Green Roof Using the PASLINK Methodology. Ph.D. Thesis, University of the Basque Country, Bilbao, Spain, 2012. 71. Sailor, D.J. A green roof model for building energy simulation programs. Energy Build. 2008,40, 1466–1478. [CrossRef] 72. ASHRAE. Fundamentals Volume of the ASHRAE Handbook; ASHRAE: Atlanta, GA, USA, 2005. 73. Moran, M.J.; Shapiro, H.N. Fundamentals of Engineering Thermodynamics, 6th ed.; Wiley: Hoboken, NJ, USA, 2010. 74. Stull, R. Wet-Bulb Temperature from Relative Humidity and Air Temperature. Am. Meteorol. Soc. 2011 ,50, 2267–2269. [CrossRef] 75. Spain Royal Decree 314/2006 Approving the Technical Building Code CTE. 2006. Available online: https://www.codigotecnico. org/pdf/Documentos/HE/DcmHE.pdf. (accessed on 20 June 2023). 76. Çengel, Y.A. Heat and Mass Transfer: A Practical Approach; McGraw-Hill: New York, NY, USA, 2007. 77. Omar, A.; Vigoderis, R.; Pandorfi, H.; Moura, G.; Guiselini, C. Green roof: Simulation of energy balance components in Recife, Pernambuco state, Brazil. Eng. Agrícola 2018,38, 334–342. [CrossRef] 78. Mentens, J.; Raes, D.; Hermy, M. Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landsc. Urban Plan. 2006,77, 217–226. [CrossRef] 79. Theodosiou, T. Green Roofs in Buildings: Thermal and Environmental Behaviour. Adv. Build. Energy Res. 2009 ,3, 271–288. [CrossRef] 80. Sfakianaki, A.; Pagalou, E.; Pavou, K.; Santamouris, M.; Assimakopoulos, M.N. Theoretical and experimental analysis of the thermal behaviour of a green roof system installed in two residential buildings in Athens, Greece. Int. J. Energy Res. 2009 ,33, 1059–1069. [CrossRef] 81. Heidarinejad, G.; Esmaili, A. Numerical simulation of the dual effect of green roof thermal performance. Energy Convers. Manag. 2015,106, 1418–1425. [CrossRef] 82. Kim, J.; Hong, T.; Jeong, J.; Koo, C.; Jeong, K. An optimization model for selecting the optimal green systems by considering the thermal comfort and energy consumption. Appl. Energy 2016,169, 682–695. [CrossRef] 83. Moody, S.S.; Sailor, D.J. Development and application of a building energy performance metric for green roof systems. Energy Build. 2013,60, 262–269. [CrossRef] 84. La Roche, P.; Yeom, D.J.; Ponce, A. Passive cooling with a hybrid green roof for extreme climates. Energy Build. 2020 ,224, 110243. [CrossRef] 85. Pastore, L.; Corrao, R.; Heiselberg, P.K. The effects of vegetation on indoor thermal comfort: The application of a multi-scale simulation methodology on a residential neighborhood renovation case study. Energy Build. 2017,146, 1–11. [CrossRef] 86. Kostadinovi´c, D.; Jovanovi´c, M.; Baki´c, V.; Stepani´c, N.; Todorovi´c, M. Experimental investigation of summer thermal performance of the green roof system with mineral wool substrate. Build. Environ. 2022,217, 109061. [CrossRef] 87. Ketut Acwin Dwijendra, N.; Muda, I.; Milanes, C.B.; Bharath Kumar, N.; Abosinnee, A.S.; Akhmadeev, R. How do green roofs affect per capita energy consumption in residential buildings under various climate conditions? Sustain. Energy Technol. Assess. 2023,56, 103127. [CrossRef] 88. Stella, P.; Personne, E. Effects of conventional, extensive and semi-intensive green roofs on building conductive heat fluxes and surface temperatures in winter in Paris. Build. Environ. 2021,205, 108202. [CrossRef] 89. Getter, K.L.; Rowe, D.B.; Andresen, J.A.; Wichman, I.S. Seasonal heat flux properties of an extensive green roof in a Midwestern, U.S. climate. Energy Build. 2011,43, 3548–3557. [CrossRef] Buildings 2023,13, 1846 22 of 22 90. Lundholm, J.T.; Weddle, B.M.; MacIvor, J.S. Snow depth and vegetation type affect green roof thermal performance in winter. Energy Build. 2014,84, 299–307. [CrossRef] 91. Juras, P. Positive Aspects of Green Roof Reducing Energy Consumption in Winter. Energies 2022,15, 1493. [CrossRef] 92. Teemusk, A.; Mander, Ü. Temperature regime of planted roofs compared with conventional roofing systems. Ecol. Eng. 2010 ,36, 91–95. [CrossRef] 93. Lazzarin, R.M.; Castellotti, F.; Busato, F. Experimental measurements and numerical modelling of a green roof. Energy Build. 2005 , 37, 1260–1267. [CrossRef] 94. Silva, C.M.; Gomes, M.G.; Silva, M. Green roofs energy performance in Mediterranean climate. Energy Build. 2016 ,116, 318–325. [CrossRef] Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.