1 Introduction

1.1 Future of Saudi Cities Program

The Future Saudi Cities Program (FSCP) is one of the most promising projects, with distinct goals and outcomes that are entirely aligned with the Saudi vision 2030. The fundamental goal of this program, which is being carried out in close collaboration with the municipalities of 17 major Saudi cities (see Fig. 1), is to create developed, environmentally sustainable buildings/cities with sufficient green infrastructure according to the standards of the international sustainability requirements. The cities were chosen based on their diverse population sizes, geographic distribution, and a variety of factors based on capacities and economic potential to achieve a more balanced urban development among Saudi’s cities [52].

Fig. 1
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The 17 cities in the (FSCP) with different regional levels of urbanization [52]: Arar, Sakaka, Tabouk, Haiel, Qatif, Buraydah, Madinah, Riyadh, Dammam, Al-Hasa, Taief, Jeddah, Makkah, AlBaha, Abha, Jizan, Najran

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Among the 17 cities selected in FSCP, Taif city is considered one of the most attractive high-altitude cities in Saudi Arabia in terms of its high population, diverse environment, geographical characteristics, and nice living conditions. Therefore, Taif city becomes an important resort for the people of the Kingdom and the Gulf States as well as the visitors of Makkah al Mukarramah. Recently, Taif city has become the official summer capital of Saudi Arabia as well as the Arab countries. However, Taif city has major environmental and health problems, such as sick buildings syndrome, heat island effect, climate changes, pollutants, and low percentage of pressure and oxygen in the air [23]. These environmental and health problems in most of Saudi’s cities can hinder the enhancement of Sustainable Development Goals (SDGs) and the hopeful achievements of FSCP. Consequently, some important initiatives should be announced to overcome most of the Saudi's cities associated life problems.

1.2 Buildings’ sustainability and sustainable development goals

At the United Nations (UN) Sustainable Development Summit in New York in September 2015, on future global sustainability, a political pact was signed. Many nations around the world have acknowledged and accepted the framework that the United Nations’ 2030 Agenda sets. This framework includes 169 Targets and 17 Sustainable Development Goals (SDGs), explained in details in [69], and briefly mentioned in Fig. 2.

Fig. 2
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The 17 Goals of the Sustainable Development (No Title n.d.)

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Recently, the World Green Building Council (WGBC) has announced strong support to the UN-SDGs by encouraging the buildings’ sustainability industry worldwide. Sustainable buildings can contribute towards achieving the Sustainable Development Goals, as it can be seen in the info-graphic (as shown below in Fig. 3) and designed by WGBC (No Title n.d.). The 2030 Agenda and its Sustainable Development Goals (SDGs) are proving difficult to fully integrate into building projects (Res. 2019). The sustainable buildings can provide foundations towards meeting some important SDGs (No Title n.d.), in more details, the sustainable buildings can contribute to 10 goals of SDGs; namely:

Fig. 3
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Information on how green buildings are contributing to the SDGs (No Title n.d.)

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SDG_3: Good health and wellbeing, SDG_6: Clean water and sanitation, SDG_7: Affordable and clean energy, SDG_8: Decent work and economic growth, SDG_9: Industry, innovation, and infrastructure, SDG_11: Sustainable cities and communities, SDG_12: Responsible consumption and production, SDG_13: Climate action, SDG_15: Life on land, and SDG_17: Partnerships for the goals.

These 10 points of contribution to sustainable buildings in achieving the SDGs have been recently received a great attention in the sustainable development research of many countries. Most of the previous recent investigations attempted to link the sustainable development goals, in general, with the local goals strategies and initiatives of some countries interested in sustainable development, e.g. [18, 25, 30, 48]. Some other investigations examined the impact of the various elements of sustainable development, including, for example, green buildings technology in achieving the well-known sustainable development goals. These investigations concluded that the need to expand sustainable facilities, in general, and sustainable residential buildings, in particular, because of their direct and strong impact on achieving many Sustainable development goals and their access to what is known as the future sustainable cities, e.g. [14, 36, 37, 47, 60].

1.3 Sustainable buildings elements and rating systems

Briefly, a green or sustainable building is a building designed, constructed, and operated to improve the quality of life in the environment where it is located. To achieve this target, it is necessary to accomplish a high level of efficiency in four main categories, namely, energy, materials, water, and health by preserving precious natural resources and reducing negative impacts on our climate and natural environment. The concepts of sustainable buildings with a detailed discussion of processes quality that improve buildings’ sustainability can be found in [15]. Furthermore, a comprehensive review of sustainable buildings can be found in [24, 74], where the current status and the future agenda have been declared. The current sustainable buildings researches are focused on the performance of the main categories of sustainable buildings and how to improve the different elements included, e.g. [6, 14, 72]. The worldwide sustainable building movement culminated in the establishment of countries’ Green Building Councils to encourage sustainable design, construction, and operation of buildings e.g., United States Green Building Council (USGBC), Hong Kong Green Building Council (HKGBC), Indian Green Building Council (IGBC), and Saudi Green Building Council (SGBC). These international councils have seen the important need to establish sets of standards for sustainable buildings, known as rating systems, to cover the common attributes such as energy, water, material efficiency, enhancement of indoor environmental quality, waste reduction as well as maintenance and operations optimization [29, 59]. Most of important rating systems have been discussed and compared in [64], e.g., Leadership in Energy and Environmental Design (LEED), Building Research Establishment’s Environmental Assessment Method (BREEAM), Building Environmental Assessment Method (BEAM), Comprehensive Assessment System for Built Environment Efficiency (CASBEE), Green Mark (GM), Green Building Index (GBI), Estidama, which was developed by the Abu Dhabi Urban Planning Council (ADUPC), Green Pyramid Rating System (GPRS) Egypt, and Recently MOSTADAM rating system of Saudi Arabia [12, 13]. Most countries used LEED rating system; however, the investigations have shown that most of the available rating systems are not convenient to the different countries’ characteristics and requirements. Therefore, many regional rating systems have been recently developed that may respond to the countries’ local context besides keeping the privacy and principles of local society. Consequently, the regional rating systems can differ in main categories and their items weights and their certification process. The currently known sustainable building rating systems include about 20 overlapping criteria that represent the key areas of evaluation. However, there are six common green building rating criteria for sustainable buildings, namely, Energy, Water, Material Efficiency, Indoor Environmental Quality, Sustainable site, and Innovation. Consequently, any new regional rating system should include such criteria to evaluate the sustainable buildings’ performance. However, the weight of each criterion can be different in comparison with other international rating systems. The recently announced Saudi Mostadam rating system contains 9 criteria with total credit 100 points. The Mostadam rating system is nearly like LEED v4.1 rating system; however, the weight of the included criteria in both systems is different. The comparison between the two rating systems is shown in Table 1 and in Fig. 4, as well.

Table 1 Mostadam rating system categories compared with those of LEED v4.1
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Fig. 4
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Comparison between the weight of criteria included in Mostadam and LEED

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1.4 Green buildings in KSA

The increasing adoption of green building technology in Saudi Arabia coincides with the alignment of planning a sustainable approach for the future. This led to concerted efforts to advance the progress and development of smart cities, within the framework of Vision 2030. In 2009, the King Abdullah University of Science and Technology (KAUST) in Jeddah became the first institution in the nation to be certified under the Leadership in Energy and Environmental Design (LEED) program. Recently, 2020, King Salman Energy City (SPARK) obtained the LEED Silver Level Certificate in Leadership in Energy and Environmental Design, as the first industrial city in the world to obtain the “Leadership in Energy and Environmental Design” program certificate according to environmental standards and key sustainability measures, which include: water efficiency, reducing carbon dioxide emissions, improving indoor environmental quality, energy savings, stewardship of resources and effectively dealing with their impacts. In the Arab world today, Saudi Arabia is responsible for 15% of green construction projects, with the highest concentration of LEED-certified gold and platinum buildings (No Title n.d.). Moreover, Saudi Arabia has many projects under construction across its current and future cities that are either LEED certified, or LEED registered. However, most of these projects are either giant government projects such as the projects of the city of NEOM in the Red Sea, or commercial, industrial or educational buildings. Consequently, residential buildings had a little chance to be sustainable certified due to the numerous barriers included [42]. Most of the important barriers included are unsupportive government policies, lack of skilled personnel and cultural and market.

The design of the residential buildings in Saudi Arabia is strongly related to the principles and privacy of Saudi society and it is not simply to be retrofitted to be sustainable buildings following the criteria's items in any rating system. Therefore, renewed non-traditional initiatives should be announced to help the existing as well as the new residential buildings to follow the sustainability standards. One of the most future challenging initiatives in sustainable buildings is the applications of passive strategies for different purposes in residential buildings, e.g., for cooling, heating, thermal comfort, lighting, etc.

1.5 Passive sustainability strategies in residential building

Arguably the most important aspect in creating a sustainable building is the implementation of the available natural resources in enhancement of sustainability criteria, this is known as a passive building design. The term “passive design” refers to a kind of architecture that takes advantage of the weather to keep a suitable temperature range in the building. Passive design can reduce or eliminate the need for supplemental heating or cooling devices, which account for roughly 70% of energy consumption in most of the Saudi residential buildings [31]. Improving thermal comfort by using passive thermal design has recently received a great attention due to the wide range of applications in different types of buildings, including the passive environmental design of an eco-house in the hot-humid climate (No Title n.d.), the principles of passive cooling and heating [4, 21, 63], and thermal efficiency of female secondary school buildings during warm seasons as a result of passive design techniques [71]. It is found that, most of such previous investigations have focused on the well-known passive strategies, such as: orientation, shading, insulation, ventilation, windows, glazing, and thermal mass.

Although the green roofs technology is considered as a passive thermal comfort strategy, the related investigations, especially in the residential buildings, are relatively small due to the associated problems [58, 70]. More recently, according to the new technology implemented in the green roof’s technology, the investigations of such passive thermal strategy became noticeable [1, 65]. Moreover, the effect of green roofs technology on the enhancement of buildings' sustainability through the different items included in the rating systems criteria makes green roofs technology as one of the key indicators for obtaining a certified sustainable building. In the following section, a brief literature review on green roofs technology is presented.

2 Literature review of green roof technology

Green roofs technology can be considered as an innovative solution for most of environmental and health problems in sustainable cites as well as high-altitude areas [62]. The benefits of green roofs showed that it can play an important role in making cities more secure, sustainable and resilient to local climate change [19]. Moreover, green roofs can become a real solution to many of the environmental and health problems such as: noise reduction [61], Climatic Change Mitigation and Adaptation Strategies [9, 35], carbon dioxide reduction [57], reducing air temperature and enhancing outdoor thermal comfort ([67], greywater recycling in buildings [50], reducing the internal temperature of the rooms in buildings [8, 66]), sustainable development in residential buildings [28], flood mitigation system [41], energy saving techniques in buildings [7, 10, 11, 22, 68]. All these benefits of green roofs make it as an innovative device technology that can be applied for greening existing as well as new buildings’ roofs [32]. This conclusion has been verified through different recent investigations that measured the people awareness of green roofs technology [3, 56, 75].

The concepts of green roofs are compatible with nature, sustainable, environmentally friendly, and aesthetic by using natural resources efficiently. However, green roof design costs are higher than any traditional roof insulations or the standard photovoltaic system used for covering buildings’ roofs [36, 37]. This opens new prospects for a model of green technology in smart cities with exciting aesthetic effects and opens a wide range of research applications in different kinds of buildings, e.g., residential, commercial, and educational buildings. In the following sections, a review of the most important issues related to green roof technology is presented.

2.1 Green roof types

Extensive Green Roofs (EGR), designed for environmental solutions, Intensive Green Roofs (IGR), also known as rooftop gardens, and Semi-Intensive Green Roofs (SIGR), which combines elements of both extensive and intensive systems, are the three different basic types of green roofs. These systems are used to harness the environmental benefits of a green roof as well as a diverse garden within a manageable maintenance budget [33]. Naturally, there is some cross over between these categories, e.g. Blue-green roofs; that combining the blue roof technologies to maximize water storage [27], and Biosolar-green roofs; that combining with solar energy to supply renewable energy and deliver biodiversity [55]. Figures 5 and 6 show the different types of green roofs, where Table 2 shows their characteristics.

Fig. 5
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Different basic types of green roof [http://www.greenrooftechnology.com/green-roof-types]

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Fig. 6
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Additional types of green roof [https://livingroofs.org/introduction-types-green-roof/]

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Table 2 Different characteristics of the different types of green roof: https://livingroofs.org/introduction-types-green-roof/
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Through a sustainability-based multi-criteria analysis, the factors influencing designers in the choice of a building roof systems and the role played by each one in the decision process were investigated [34]. This method offers helpful ideas for identifying strategies that can help a greater adoption of greening systems for urban resilience. Mainly, there are four important factors for green roof product selection: the suitable materials for the roof, the designed irrigation system, the choice of waterproofing material, and the maintenance of the green roof. These important factors are critical to the success of any green roof project [73]. Moreover, in other types of investigations, the effects of plant selection on thermal performance of green roofs during the summer are evaluated [39].

2.2 Pros and cons of green roofs

Generally, there are various advantages to having a green roof in sustainable buildings. Some of these advantages are obvious, while others are not. Moreover, there are certain drawbacks to having a green roof, but they are relatively few and can be partially eliminated. Looking at the benefits and drawbacks of a green roof in sustainable buildings, that enables us to see if it is valuable and worth.

The advantageous of green roofs can be stated as:

  1. 1.

    Sound insulation

    A green roof acts as an additional layer of insulation, absorbing up to 30% of environmental noise pollution [26].

  2. 2.

    Temperature regulation

    Green roof modules help to keep a building’s temperature under control. In winter, your home or business will be warmer, and in summer, it will be cooler.

  3. 3.

    Aesthetics

    Green roofs often play a big part in the overall design and it can be considered as aesthetically pleasing architecture and visually appealing.

  4. 4.

    Roof protection

    Generally, installation of green roofs can protect building’s roof from UV rays and acidic rain.

  5. 5.

    Improved air quality

    Green roofs plants can absorb pollutants and emit oxygen, improving air quality and helping the environment.

  6. 6.

    Rainwater retention

    Green roofs can absorb a large amount of rainwater, helping to prevent localized flooding.

  7. 7.

    Biodiversity

    Green roofs can help to promote biodiversity by attracting wildlife.

  8. 8.

    Buildings rating

    Green roofs installation can be considered as an excellent starting point to improve the building’s rating with international as well as national rating systems. This will be discussed in detail in the following sections.

However, the disadvantageous of green roofs can be stated as:

  1. 1.

    Installation cost

    The installation cost of green roofs is the main disadvantage. Green roofs are more expensive to build than regular flat roofs because the underlying structure may need to be reinforced to handle the additional load.

  2. 2.

    Maintenance

    Landscaping and gardening costs must be considered even with simple plants.

  3. 3.

    Damage and leakage

    Green roofing systems, unfortunately, are prone to damage and leaks. Plant roots can break through the waterproof membrane, causing roof leaks and structural damage.

Although green roofing solutions have few drawbacks, the benefits vastly exceed the drawbacks. Whether an extended, semi-intensive, or intensive green roof has been chosen, one will undoubtedly profit from its many advantages.

2.3 Green roof design

The selection of efficient and sustainable components of green roofs is important and critical if it is environmentally friendly. The performance and benefits of green roofs have been the focus of previous review publications. However, in more recent article [17], an extensive review of the key layers of green roofs such as: the waterproof and anti-root membranes; the protective, filter, and drainage layers; the substrate; and the vegetation. A waterproofing membrane, an anti-root barrier, a protective layer, a water storage and drainage layer, a filter layer, substrate (growth media or soil), and vegetation are among the components that make up a green roof (plants). Figure 7 illustrates these components. More details about the green roof technology can be found in [54]. Moreover, the impact of the green roof on both internal air temperature and energy needed for achieving thermal comfort has been investigated in the area of Arabian countries, e.g. [2, 5, 36, 37, 51].

Fig. 7
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Green roof layers [17]

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3 Material and methods

3.1 Green roof technology and buildings rating systems

The Kingdom of Saudi Arabia has developed several policies and plans to advance the aims of sustainable development, including the transformation of the built environment into a vital component of sustainable cities and societies. Additionally, Saudi Arabia aspires to be the greatest investor globally in the field of sustainable construction. As a result, developing sustainable buildings has drawn a lot of interest lately. In order to attain the requirements of sustainable building in a way that is acceptable with the norms and privacy of Saudi society, Saudi Arabia has developed the “Mostadam” rating system for existing and new structures. This was done in accordance with Saudi Vision 2030. Under the direction of the nation’s Ministry of Housing (www.mostadam.sa), the rating system MOSTADAM was created and is now widely used. In general, Leadership in Energy and Environmental Design (LEED), an internationally known green building certification system, and the “Mostadam” grading system are comparable. The “Mostadam” rating system's categories are listed in Table 1, along with the overall point value for each category.

There are five certification levels given by Mostadam in accordance with the total points obtained for a particular Saudi project, as per the “Mostadam” rating system. The certification levels for the “Mostadam” rating system are displayed in Table 3. In principle, this indicates that each material, element, gadget, etc. must be certified in accordance with the sustainable building requirements for a structure to be certified in accordance with the “Mostadam” rating system. Additionally, it should be noted that the “Mostadam” rating system’s most crucial categories for the certification of sustainable buildings are Energy, Water, and Health & Comfort. Table 4 provides more information about the sub-points of each category and their credit points.

Table 3 Levels of certification awarded from LEED and Mostadam rating system
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Table 4 The sub-point of Mostadam rating system for residential buildings
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The scientific explanations of the “Mostadam” categories and their credit titles can be found in our previous researches [12, 13].

It is now necessary to examine the contribution of green roof technology to the various categories and associated sub-items in the “Mostadam” grading system in more detail. In other words, it is important to understand how many points a sustainable project might earn if it uses green roof technology.

The method adopted in the present research is based on relating the advantages of the green roof technology and its environmental effects to the different categories of the “Mostadam” rating system using the same quantitative measure adopted in our previous research [13].

The green roof main characteristics can contribute to the following categories of “Mostadam” rating system and their Credit Titles: Site Sustainability, Water, Energy, Health and Comfort, Education and Innovation, Policies Management & Maintenance. This approach can be applied to any kind of green roof as their main characteristics are widely similar.

4 Results and discussion

It is simple to determine which category of the “Mostadam” ranking system is related to the benefits of a green roof in a sustainable construction. Utilizing green roof technology has numerous advantages for one's health, the environment, emotional impact, thermal and aesthetic comfort, and electric bill. Additionally, for better indoor environmental quality, solar radiation is not permitted to pass through the building's roof, as shown by most international rating systems for sustainable buildings.

Through the following table, Table 5, green roof technology can contribute to the categories of the “Mostadam” ranking system. According to the “Mostadam” scoring system, a sustainable building can receive up to 32 points merely by utilizing green roof technology. The Green Roof-Mostadam Contribution Matrix is the name of this table.

Table 5 The total points obtained by applying green roof technology according to Mostadam Rating System. (Green Roof- Mostadam Contribution Matrix)
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It can be seen that; the application of green roofs technology can boost the sustainable building ratings by contribution to the major categories of any rating system with about 32% credits total based on Saudi MOSTADAM rating system. This encourages stakeholders to apply green roofs in residential buildings supporting the sustainable development goals of the Saudi government’s ongoing initiatives that advocates for more sustainable and resilient cities considering vision 2030.

5 Potential and challenges of green roofs in Saudi Arabia

With over 2,150,000 km2 of territory, Saudi Arabia is the largest Arab nation in Western Asia and is situated in the so-called Sunbelt. Its latitude and longitude are roughly 24.7° and 46.73°. High daytime temperatures and low nighttime temperatures are the main characteristics of Saudi Arabia’s climate. Except for the southwest, where a semi-arid climate predominates, the country’s climate generally follows the pattern of a hot desert climate, as shown in Fig. 8. Saudi Arabia experiences an average winter temperature of 22 °C and an average summer temperature of 45 °C. Around 29 °C is the typical spring and fall temperature. The heat increases after daybreak and lasts until nightfall. The pre-known installation data of green roofs is significant and should be taken into consideration when incorporating the technology into building design.

Fig. 8
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(Red) desert climate regions, (yellow) semi-arid climate regions [41]

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The mean amount of annual rainfall seems to be homogeneous at all regions of Saudi Arabia except at a few regions, such as Tabouk and Sharorah [49]. Therefore, the green roofs technology can be considered as an important strategy for rainfall water management in Saudi Arabia [38].

Although most of the earlier research on the green roof technology were positive, there are still a number of obstacles in the way of its widespread use in Saudi Arabia’s construction industry. These difficulties can be summed up in terms of maintenance, economic factors, and awareness problems.

The application of green roof technology and its deployment in the building industry are largely dependent on the user's awareness of it, as is the case with any new technology. The decrease in electrical energy costs might not be sufficient to persuade the user of this new technology simply. Additionally, it was observed that most building owners have a tendency to install several restrictions on the buildings' roofs, e.g., dish antennas, water tanks, A/C condenser, and other non-used materials [20], as shown in Fig. 9.

Fig. 9
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Green roofs restriction in residential buildings of Saudi Arabia [2]

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Therefore, increasing user conviction can be accomplished by raising awareness of the outstanding advantages of green roof technology in terms of visual comfort, environmental pollution, healthy conditions, indoor environmental quality, and productivity enhancements.

Although of the different challenges of green roofs technology in Saudi Arabia, a pilot project has been performed in the King Abdulaziz Center for World Culture in Dahran over more than 38,000 Square Meter which completely covered the occupied space, as shown in Fig. 10. Most of the challenges faced such project are the water leak under roof, and the sandy wind found in such area.

Fig. 10
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The King Abdulaziz Center for World Culture: https://cleverbuildings.files.wordpress.com/2012/05/render-123213123.png?w=500&h=241

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In general, these are issues that cannot be avoided, and discussion of the new green building tool will remain theoretical and unfounded without relating to these, e.g. Life cycle energy analysis is an approach that accounts for all energy inputs to a building in its lifecycle and Life cycle cost analysis is defined as “the total discounted dollar cost of owning, operating, maintaining, and disposing of a building or a building system” over a period of time [16]. Life cycle cost analysis is an economic evaluation technique that determines the total cost of owning and operating a facility over a period.

The UNSW recently completed a very broad research program on UHI mitigation in Saudi Arabian cities (Santamouris et al. 2022; Urban Heat and Mitigation Potential in Riyadh). The research has assessed different strategies, including the use of vegetation. The conclusions are not straightforward, and vegetation’s advantages certainly cannot be taken for granted under the specific conditions. Non-indigenous vegetation will demand massive irrigation, indigenous vegetation may have little or no effect being adapted to minimizing water loss through evapotranspiration, and certain plants may prove to have a negative impact in both air temperature mitigation and air quality, for more details one can see [40].

6 Conclusion

The present paper indicates that the green roofs technology can help in achieving most of the sustainable development goals in the Saudi society by contributing to the sustainable buildings rating systems with high percentage. The adopted methodology, in the present study, is considered as a condensed assessment method that is based on linking the main characteristics of the green roofs to the main categories of Saudi “Mostadam” rating system. Thoroughly, most important perspectives of green roofs in Saudi Arabia have been discussed and explored.

It is found that green roofs technology can boost the rating of sustainable building by contribution to the major categories of MOSTADM rating system with about 32% credits total. The weighted average of the results can be used as a first indicator of the decision made by built environment stakeholders to pursue green building technologies. Accordingly, other green building technologies, including rainwater harvesting systems, can also be evaluated.

Generally, the obtained results encourage stakeholders to install green roofs on residential buildings, thereby assisting the Saudi government’s ongoing initiatives to promote more sustainable and resilient communities in light of Vision 2030. The future research should be directed to the associated challenges of green roofs technology in Saudi Arabia, such as: Climate changes effects, sandy wind, and buildings users’ awareness.