A Comprehensive Analysis : Sustainable Trends and

Established by the U.S. Green Building Council (USGBC), the Leadership in Energy and Environmental Design (LEED) became a sustainable leader of green building rating systems in American and many other countries. In Vietnam, LEED is expected as a potential solution in improving the sustainable quality of buildings for residents and solving the housing/infrastructure demand with a limit in resource consumption and minimizing negative environmental impacts. The study analyzed the awarded LEED 2009 credits by investigating the data of 36 of the total 42 LEED BC+D 2009 certified projects in Vietnam. The results of the investigation indicated the awarded credits were significantly implemented in Vietnam. These results were converted based on the summary updated on LEED version 4 of the USGBC report, to become a useful guideline for green building cost‐efficiency strategies. Additionally, it also served as reference data for the Vietnamese public agency to update their green regulations based on the specific characteristics of Vietnam.


Introduction
Besides the rapid economic growth, environmental pollution and the lack of housing demand, the research of sustainability has become a top concern for developing countries [1]. Especially in 2019, the study of Climate Central showed that a large area of Vietnam could be drowned due to rising sea levels in 2050, and a quarter of Vietnam's population will be profoundly affected by climate change [2]. Along with the limitations of developing countries such as seeking large budgets to address environmental issues, it is essential and necessary to find a way to promote sustainable development. Obviously, the construction industry not only accounts for a large proportion of Vietnam's production, but buildings also contribute a large part to the consumption of electricity, water, and other research resources [3]. Thus, green buildings had flourished in recent years after recognizing their potential social and economic benefits in solving many negative environmental issues and problems [4]. Moreover, the increase in electricity prices and recent urban environment incidents in Vietnam also became a strong motivation for real estate developers to choose green building certification for their future projects [1,3]. This trend can be reflected by the increase in the number of projects participating in green building certification in Vietnam in recent years, especially LEED certificates with notable numbers.
LEED and other green building certification systems provided technical guides and a framework to promote sustainable construction practical activities by balancing the elements of environmental protection, energy/water consumption, the benefits of user and community and operations, and maintenance solutions [3,5]. While promoting sustainable construction and bringing many benefits to building residents and the community, several problems related to the applying of LEED are also identified [6], such as encountering challenges for the project team and cost increasing [3]. Considerable challenges of implementing a LEED building project were the expensive quests, higher construction cost [7] and non-experience extra works for the construction team [5]. The additional charges might come from hiring LEED consultants, Commissioning Authority (CxA), air-testing and flush-out test contractors [8], which are entirely unfamiliar and expensive to the Vietnamese building project [3]. However, several studies indicated that LEED projects may have little incremental costs (2.5 to 9.4%) but will significantly reduce the operating costs (about 31%) of the building [9]. The study by D. Langdon (2007) and J.O. Choi (2015) also confirmed that this cost impact could be completely limited by their previous similar project experience and commitment during project implementation [5,10]. The research of Y.H. Ahn (2015) showed that it is crucial to apply the Integrated Construction Process (ICP) to collect the knowledge and ideas of all key stakeholders for successfully implementing green buildings with an acceptable cost [4].
In addition, C.P. Cheng's (2015) study mentioned that the better judgment of target credit selection and the appropriate green design technologies largely depend on the experience of managers from previous LEED projects [11]. However, construction projects are unique, and it is often difficult to participate in similar projects, especially when the number of certified LEED projects is very limited in Vietnam at present. Besides that, the new projects applying for LEED certification form 2018 have to comply with the unique requirements of LEED version 4 with many considerable changes, which might create several difficulties for the project team [3]. As a result, a useful guide for the green building project in selecting LEED credits that are appropriate to the project goals, from the synthesis and analysis of previous projects, would bring many significant benefits to the LEED projects and promote the sustainable construction. Thus, this study was conducted to create a useful guideline for the project and building developer for identifying the suitable project LEED credits/goals and its risk by investigating and analyzing all similar projects that have received LEED certification in Vietnam. This paper aims to: (1) Identify the significantly awarded credits at LEED projects in Vietnam.
(2) Investigate the significant different credits between the different LEED certificate levels and provide the recommendations for credits required for future LEED projects. (3) Review and summarize the significant differences between LEED 2009 and LEED v4 to combine with the analysis results and make more suitable guidelines and recommendations.

Background of Research
"The most widely used green building rating system in the world," "The framework to create healthy, highly efficient and cost-saving green buildings," and "The globally recognized symbol of sustainability achievement" are the definitions of the U.S. Green Building Council (USGBC) about the "Leadership in Energy and Environmental Design" (LEED) rating system [8]. Additionally, LEED is the meaningful attempt of USGBC to create a global rating system, which guides technically to design and deliver the green building. Thus, green building projects that comply with the LEED requirement not only promote sustainable construction but also increase the project life cycle costeffective health benefits to users [3,6,12]. LEED is considered as the most globally popular green building rating tool because of its overlooked variations between different climatic and environmental conditions, rules, standards, and laws. The selection of LEED credits dramatically affects the results of the project, but it is more difficult due to the rising tide of cost, schedule, and LEED experience. Thus, understanding the relationships between decision-making factors and characteristics of LEED on various credits is essential for project stakeholders and regulatory authorities [3,6]. As a result, many pieces of research are conducted to guide better efficiency of the green building projects and encourage sustainable construction. In particular, a significant number of them were conducted to support the design process and identify appropriate LEED credits for the project during the early phase, when the design changes are much higher in efficiency and less costly [3,4]. Cheng and Ma (2015) studied the relationship between LEED credits and green building technologies/sustainability design strategies to improve the effectiveness of LEED credits selection [11]. M.A. Jun's paper (2015) presented a methodology for identifying the target LEED credits based on project information and climatic factors [13]. J. Ma's study (2016) analyzed LEED credit achievements in previous projects using data-driven techniques and provided useful instruction so as to help the project team to understand and identify the appropriate LEED credits [14]. Jack C.P. Cheng's paper (2015) proposed a case-based reasoning (CBR) approach to find out the suitable case study base on the similar previously certificated green building projects and suggestions on target LEED credits [11]. F. Jalaei and A. Jrade's study (2015) (BIM) explains how this integration would assist project teams in making sustainability-related decisions [15]. P.H. Chen and T.C. Nguyen developed an application by interacting web map service (WMS) technology and BIM to pre-calculate LEED awarded points of the "Location and transportation" category [16]. The research of S. Pushkar (2018) pointed out that several data analysis studies of previous LEED-certified projects have to consider the problem of pseudoreplication [17]. Recently, when Wu et al. (2017) [6] and Wu et al. (2018) [18] analyzed previous LEED certification project data, they used aggregated processes to achieve extremely low P-values. Da Silva and Ruwanpura (2009) investigated LEED Canada certificated building data and found out that material (MR) and energy and atmosphere (EA) criteria are not common at LEED projects in Canada [19]. Cell and Beata (2009) presented that the lack of consideration in the design phase of a green building project may have an adverse impact on the implementation phase [20]. J.-Y. Park's study (2017) developed an optimization algorithm that aims to determine the LEED awarded points that can be achieved with minimum cost [12]. Madanayake and Ruwanpura (2012) suggested a method for determining appropriate LEED credits based on cost, productivity, and environmental impact information [21]. Juan, Gao, and Wang (2010) addressed a selection of green building technology strategies by optimizing the trade-off between costs and achievements in different green building rating systems [22]. Castro-Lacouture, Sefair, Flórez, and Medaglia studied the factors influencing the selection of sustainable materials in the LEED project [23]. Boschmann and Gabriel proposed (2007) the implementation and development of green technologies directly related to a particular LEED credit [24]. Pushkar (2018) identified the significant differences in the choice of implementing LEED credits among several countries, including SS, WE, EA, EQ, and ID [25]. In other words, it is highly useful for analyzing the influence and development of green buildings in developed countries to benchmark the Green Building regulation system of developing countries [26].
In summary, the previous studies have provided the LEED credits choosing by using various methods of selection supporting the appropriate credit through the assistance of BIM, available previous LEED building data, or decision-making strategies technic and so on. However, there is no previous study which offers limited guidance for developing countries like Vietnam. Therefore, an analysis of certified LEED projects is needed to promote the development of sustainable construction in Vietnam by providing direction of goal-setting to designers and developers. In addition, the developers of the Vietnam Green Building Council also have grounds to adjust their policies more appropriately for promoting sustainable construction in Vietnam.

Comparison of LEED 2009 and LEED Version 4
Every few years, the USGBC offers a new version of LEED to capture new trends in the construction industry, and embraces new opportunities for encouraging sustainable construction. [27]. There are several versions of LEED being used in Vietnamese existing buildings, but the 2009 LEED version has occupied for the majority of current certified projects and is applied to projects registered before 2018. The previous versions of LEED, which are LEED v2.2 and LEED 2009, both have six main categories, including sustainable sites (SS), the water efficiency (WE), the energy and atmosphere (EA), the material and resources (MR), the indoor environmental quality (IEQ), and the innovation in design (ID) [6,25]. LEED 2009 consists of 6 major sets of criteria with 12 prerequisite credits and 42 optional credits, and credits also have different performance strategies [11]. The newer version is the LEED v4 and LEED v4.1 that was announced in 2014 and mandatorily applied to all newly registered projects from 2018 [8]. Currently, most current projects in Vietnam are certified LEED 2009, while new projects need to comply with LEED v4 requirements. To apply for future projects, the results of this analysis should be considered by checking the differences between LEED 2009 and LEED v4.
The updated version, LEED v4, still has the same total score (110 pts), but the SS (26 pts) criteria group is divided into two smaller criteria groups, which are "Location and Transportation (LT)" (16 pts) and SS (10 pts) [8,25,28]. The updated Location and Transportation criteria were published in LEED v4, including some Sustainable Site credits of LEED 2009, such as protection and green transportation parking, land usage and so on. Meanwhile, the new SS criteria group includes light pollution reduction, site usage and protection, rainwater, and heat island credits [28]. In fact, the assessment of SS issues is very similar to the assessment of LT+SS issues [25]. In LEED v4 EA, MR, EQ criteria, credits are still included as in LEED 2009 but there are some changes in the detailed requirements and more recommended green strategies [28]. Only one new credit, acoustic performance, was introduced in the LEED NCv4 [25]. According to the USGBC report [28], although there is not much change in the score distribution between categories, several credits have been added and some of its requirements have been majorly changed in the new version. Svetlana Pushkar's research evaluates that the majority of LEED version 4 is similar to the previous LEED 2009 and that it is feasible to modify the algorithm for LEED v4 extended credits [25].

Scope of the Study
The objective of the study is investigating LEED-certified buildings data to find the association between each LEED credits compliance performance and LEED certificate targeted level of the project. From the results, we provided a guideline to the project team in making decisions on their green strategies and the LEED credits decision that will be more cost-effective. The study also aims to identify differences in green building projects in Vietnam and other developed countries as a basis for adjusting Vietnam's existing green building standards such as LOTUS (Vietnam Green Building Council) and TCVN 09-2017 (Vietnam Ministry of Construction). Therefore, the population of the study is all LEED projects in Vietnam, but the number of projects is minimal and mostly LEED NC projects. The target of this study is collecting and analyzing all existing LEED NC projects (November 2019). Furthermore, similar to the other countries' conditions, the number of LEED v4 projects in Vietnam is also minimal [3], so data of the research is obtained from LEED 2009 projects in Vietnam.

Data Collection
Project data was collected from the official website of the U.S, Green Building Council. In an effort to promote and disseminate the LEED standard system globally, the USGBC has published numerous documents as well as data on their successful projects. At the time of the study, a total of 42 LEED 2009 NC projects were certified, of which only 36/42 projects had complete data on the credits received. Despite the limited number of samples, the study was conducted over the entire population, and the statistical results of the data were accepted as a normal distribution according to the central limit principle [29] (p. 208).

Characteristics of the Vietnamese Green Building
Factors of project characteristics, location, and climate are critical to the selection of appropriate investment strategies. The average annual temperature in Vietnam is from 22 °C to 27 °C. Every year, there are about 100 rainy days with an average rainfall of 1500 mm to 2000 mm and the air humidity is around 80% [30]. The majority of projects (28/36) investigated in this study are located in the southern part of Vietnam, which has a humid tropical climate with a low amplitude of heat fluctuation during the year, with high temperature and humidity. Therefore, most LEED projects are not equipped with heating equipment. Furthermore, these projects are also mainly factory projects or office buildings, where the developer is responsible for operating their project, and the energysaving strategies bring direct efficiency to their operating costs. Post-tropical humid weather with heavy rainfall and the diversity of native species also offer many advantages in developing LEED strategies related to greenery and irrigation. The sunshine hours in many areas of Vietnam are quite high, with the number of sunshine hours being about 1500-2000 h [30]; also, the average annual radiant heat is 100 kcal/cm 2 which provides many advantages for solar energy solutions. However, similar to many developing countries, the shortage of various green materials also makes it more difficult to implement LEED credits for material criteria.

Data Analysis
In this study, 2009 LEED credits are divided into two groups for analysis due to their characteristics. The first group is the credits that only awarded a maximum of 1 point, meaning that the project team can only decide to implement or not. The research will then investigate the credit of group 1 based on the frequency of awarded credits for LEED-certified projects in Vietnam. In the second group, the number of points awarded per credit is greater than 2, and the project may achieve different levels of compliance to request based on the project's conditions. For group 2, the credit achievement degree (CAD) will be revised from W. Peng's (2017) [6] study, and be calculated based on the following Equation (1):

100%
(1) where CO is the point that credit obtained and TC equals to total points of credits. As such, the CAD represents the level of fulfillment of LEED's requirements.
In this study, the statistical tests used to analyze LEED awarded credits from collected data include the Mean-Value Ranking test, One-sample t-test, Kolmogorov-Smirnov test, Wilcoxon-Mann-Whitney and non-parametric effect size index (Figure 1). At first, in order to identify common LEED credits, the study conducted the ranking techniques for first and second group credits. In the second step, the One-sample t-test was used to determine the significant LEED awarded credits in the previous Vietnamese LEED projects. The given hypothesis H0 is "the frequency of awarded credit is less than 50% in the Vietnamese LEED projects" with the confident index of 95%. In the third step, the normal distribution test is implemented because the number of samples of LEED silver, gold LEED, and LEED platinum projects is limited. Furthermore, the results of the Kolmogorov-Smirnov test showed that data is not a normal distribution, and LEED data are presented in ordinal scales [17]. To investigate the significant differences between the two groups with different LEED certificate levels of certification and consider the problem of pseudoreplication, the statistical tests used are based on suggestions of S. Pushkar's (2018) study [25]. Following the non-parametric tests of S. Pushkar's (2018) study, Cliff's δ and WMW tests [29] were used to compare the two unpaired groups. The data are presented as the median ± interquartile range (IQR, 25th-75th percentile) [17]. Cliff's δ is used to measure the substantive significance (effect size) between two non-paired groups. Cliff's δ [31] (p. 495) is calculated following Equation (2) where x1 and x2 are the points in 2 compared groups respectively; n1 and n2 are the sizes of the sample groups, and # is the index times. The WMW test was used to determine the statistical difference between the two unpaired groups. It should be noted that WMW tests can be applied in two forms: approximate or extracted. If the sample size is more than 9, an approximate WMW test has been used, and the extracted form for all else [32] (p. 56).
Finally, the Neo-Fisherian significance assessments (NFSAs) are used to interpret the signs and magnitudes of the statistical effects [17]. Based on NFSAs, precise P-values were evaluated and shown according to a three-valued logic as follows:  "Positive": The credit seems to be a difference between two compared groups.  "Negative": The credit does not seem to be a difference between two compared groups.  "Suspended": The credit regards the difference between two compared groups.

The Popularly Awarded LEED Credits in the Vietnamese Projects
The critical objective of this study is to investigate the credits that are commonly implemented at LEED projects in Vietnam. As a result, the study suggested a list of LEED credits that should be considered for future projects, which were shown through the analyzed result of ranking Technic methods (in Table 1) and t-test (in Table 2). Please see the detailed description of the LEED credits and the corresponding symbol in Appendix A As mentioned above, the 1st group are the credits that the project can only achieve or not. Thus, the result of ranking technics of the 1st group, which were shown in Table 1, illustrated the ranking based on credits' frequency. If the credits' frequency is similar, the credits have a smaller standard deviation index that would be prioritized. Similar to the first group, Table 2 showed the ranking of the second group of LEED credits based on the CAD index; the average points the creditors have set are also presented. At LEED projects in Vietnam, the most popular 1st group credits include IDc2 (97.22%), EQc4.2 (94.44%), and SSc1 (91.64%) compared to the opposite side of rarely awarded credits such as SSc3 (0%), MRc6 (0%), and MRc7 (0%). In the second group of credits, the high popularity credits are SSc4.3 (100%), SSc4.4 (97.22%), MRc5 (94.44%), WEc3 (93.75%), and WEc2 (91.67%), and the low popularity credits were MRc1 (10.19%), and MRc3 (0%). We suggested that the inexperienced LEED project teams should select the credits to consider for their projects based on the rankings in Table 1. As shown in Table 2, the results of the Kolmogorov-Smirnow test determine that most of the data collected are not a normal distribution. Therefore, the t-test is only performed for the whole sample (number of samples >30), and its results are also presented in Table 2. The t-test was conducted to identify the significantly awarded credits at LEED projects in Vietnam with a comparative value of 50%, and 95% of the confident index was set. The results of significantly awarded credits at LEED projects in Vietnam are:

The Difference between LEED Project Target Goal Levels
USGBC classifies buildings by four levels of LEED certification, including LEED certificated, LEED Silver, LEED Gold, and LEED Platinum. The higher the certification level, the more the project has to comply with LEED's strict requirements, which means they have to spend more on sustainable investments and additional work. Investigation of LEED credits that differ between these different project groups on project goals is based on the results of the Wilcoxon-Mann-Whitney test and the Cliff's delta values. The approximate WMW test results and Cliff's delta values of LEED credits between the groups of silver LEED and gold LEED projects (n = 12) are shown in Tables 3a and 4a. Similarly, Tables 3b and 4b presented the results of the extract WMW test and Cliff's delta between the groups of the LEED gold and LEED platinum projects (n = 7). Table 3. The results of the Wilcoxon-Mann-Whitney test and Cliff's delta indexes for the 1st group credits. (a) Between the groups of LEED Silver and LEED Gold certification projects; (b) between the groups of LEED Gold and LEED Platinum certification projects.  The summary of results of Tables 3a and 4a, which were the result of comparing two groups of LEED Silver projects and the group of LEED Gold projects showed the five credits had significant differences, including SSc7.2, EQc4.1, EQc 5, EQc 8.1, and EAc1. In addition, other credits have the "suspended" status, such as SSc8, EQc3. 1, EQc3.2, EQc4.2, EQc4.3, etc., can be considered as a secondary choice for getting the LEED Gold certificate. When comparing the difference in credits achievement between the LEED Gold projects group and the LEED Platinum projects group, Tables 3b and 4b illustrated the five credits which were identified with significant differences, including SSc4.2, SSc7.2, EAc1, EAc2, and IDc1. The credits SSc5.1, SSc.2, EQc1, EQc5, etc., were also considered for the second choice for getting the LEED Platinum certificate. When comparing Tables 3 and 4, there were some duplicated credits such as EAc1, EQc5, EQc7.1, EQc7.1, EQc8.1, and SSc7.2. These credits are all part of the second set of credits, so project teams need to consider the different levels of achievement of these credits to achieve higher levels of credits. Table 5 is referenced from Jae-Yong Park's study (2017), and compared with the results of this study, the following points should be noted. The popular LEED credits of Vietnamese projects were focused on Costless-Easy and Cost-Easy credits groups. Additionally, several Cost-Hard group credits are popular such as WEc2, EAc1, EAc2, EQc4.3, with the reason being that these credits have a large percentage of points in LEED and most of the LEED projects in Vietnam are LEED silver or gold. A lot of Costless-Hard credits have not been implemented, reflecting the limited experience of LEED consultants in Vietnam. Therefore, some of the credits that are costly but still implemented, such as SSc5.2, SSc7.2, WEc1, WEc3, EQc4.1, EQc4.2, need to be carefully studied and judged before being applied to the projected practice. In addition, LEED specialists need to study more and understand clearly the costless and uncommon credits group to offer appropriate solutions. For example, SSc2 and SSc4.1 credits relate to community connectivity planning, and it may not be feasible for many projects because of the current urban conditions in Vietnam. The construction waste management, MRc2, is currently only implemented by well-known contractors, so the Vietnamese government should have stricter regulations to promote them. As a result of uncommon materials with sustainability certificates, the credits for materials are not offered in the Vietnamese LEED project. The credits of SSc8, EQc8, EQc2, EAc4, and SSc3 had not been implemented in many projects, and this predicted that the idea contribution meetings did not work well. Therefore, the roles of LEED experts and the integrative design process need to be addressed in the future.

Converting Results to LEED Version 4
Besides the significant changes, as mentioned in the literature review section, some of the credits are combined or renamed, which included the following changes, such as the SSc7.1 and SSc7.2 credits are combined into SSc5; the EQc4.1, EQc4.2, EQc4.3 credits are grouped into EQc2; and most credits have changed names. Not only changing the names and aggregation of credits, but the content of these credits also had specific changes that the project team needs to refer to before deciding on the selection of sustainable credits for their LEED project in the future. Thus, after comparing and converting common LEED credits of LEED 2009 projects in Vietnam, the list of LEED v4 credits proposed by this study includes:

Conclusions
The important contribution of LEED in driving green building in Vietnam is evidenced through the increase in the number of recent LEED projects registered in recent years. More studying and a deeper understanding of project objectives are needed for the development of green buildings without much additional cost. Understanding the development characteristics of green buildings in Vietnam is also significant for the Vietnamese government to establish appropriate Green Building guidelines and regulations. Therefore, the main contribution of this study to the investigation of the credits earned by LEED buildings in Vietnam, provide benchmarks for future LEED projects to improve. In summary, the key points are:  The results of significantly awarded credits at LEED projects in Vietnam are identified and the recommended LEED v4 credits are SSc1-Site Selection; LTc6-Bicycle facilities; SSc3-Open space; SSc5-Heat island reduction; EQc3-Construction indoor air quality management plan; EQc2-Low emitting materials; INc2-LEED Accredited Professional; LTc8-Green vehicles; LTc7-Reduced parking footprint; WEc1-Water Efficient Landscaping; WEc3-Water Use Reduction; WEc2-Innovative Wastewater Technologies; EAc2-Optimize energy performance; and MRc3-Building product disclosure and optimization-sourcing of raw materials.  The significant different credits between the LEED Silver and the LEED Gold projects are SSc5 (SSc7.2), ECc2 (EQc4.1), EQc3 (EQc5), EQc7 (EQc8.1), and EAc2 (EAc1).  The significant different credits between the LEED Gold and the LEED Platinum projects are SSc1 (SSc4.2), SSc5 (SSc7.2), EAc2 (EAc1), EAc5 (EAc2), and INc1 (IDc1).
In addition, many limitations of research need to be mentioned. First, the number of LEED certificated projects in Vietnam is minimal, and detailed statistical analyses for each type of credit, or buildings in each region, are also not conducted. Thus, project teams need to have deeply researched similar projects because of the uniqueness of construction works. Research data was also collected from LEED v2009, and further research is needed based on data from new LEED v4 projects when the number of LEED v4 projects in Vietnam is large enough.  Stormwater design-quality control 1 SSc1 Site assessment 1

EAc1
Optimize energy performance 19 Wc3 Outdoor water use reduction 2

EAc2
On-site renewable energy 7 Wc4 Indoor water use reduction 6

EAc5
Measurement and verification 3 EAc2 Advanced energy metering 1

Material and resources 14 EAc4
Renewable energy production 3

MRc2
Construction waste management 2 EAc7 Optimize energy performance 18

MRc3
Materials reuse 2 Material and resources 13

Mrc1
Building life-cycle impact reduction 5

Mrc2
Building product disclosure and optimization-environmental product declarations 2

Mrc3
Building product disclosure and optimization-sourcing of raw materials 2 Indoor environmental quality 15

EQc1
Outdoor air delivery monitoring 1

Mrc4
Building product disclosure and optimization-material ingredients 2