Research trend on sustainable architecture: a bibliometric analysis emphasizing on building, material, façade, and thermal keywords

Studies about sustainable research remain increase recently due to the global trend and movement on greener lifestyles. Keywords on building, material, façade, and thermal represent the current issue in sustainable architecture. This study aims to trends in sustainable architecture research with specific keywords and periods. Publishing period between 1976-2020 with particular limitation applied to emerge 859 publication documents to be analyzed. Scopus’ on-site analysis system and VOSviewer were employed to generate graphical and visual analysis on research trends. The analysis results generate five significant research clusters: energy, façade technology, measurement, thermal, and climate. Besides groups, the analysis also indicates that most of the countries involved were from Europe. Hence, the conclusion of this study reveals potential research on a similar topic in tropical countries which demanding sustainable building encounters a humidity challenge. This study fills the gap in the bibliometric analysis regarding sustainable architecture with specific keywords and publishing period.


Introduction
The high demand for green building concepts brings a new environmentally-friendly culture to the construction industry. The government and stakeholders are expected to take this issue seriously and commit to everlast sustainable construction to establish a better environment, social, and economic [1].
Complex decision yet thoughtful design to the environment is enormously required to address the sustainable living standards [2]. In research about the interpretation of sustainable building, Berardi [3] clarifies a building predicated as a sustainable building if it contributes whole sustainability domains holistically. However, the green building movement with all those promising benefits, in any case, finds difficulties in realizations, particularly in developing countries. Lack of information, unclear regulation, and insufficient marketing strategies for green products lead to sluggish green building movement in developing countries [4]. Hence, the government and the green building council should encourage stakeholders to promote the concept through certification, regulation, and reward [5].
Pertain to the green building; the material is the inseparable part that contributes to establishing sustainability. The research discussed the contribution of material to promote green building with earthy material usage [6]. The expedient in material usage also considerably reduce the waste material; thus, it is an addition to promote sustainability in material aside from earthy material [7]. The involvement of recent technology, Building Information Modelling (BIM), supports the sustainability in material usage [8] that comprises information on volumes and material selected for each element in building [9], and waste management support [10]. Materials are also entangling to the greenhouse effect, which  [11]. Glass as typical material for highrise building enveloped wall facade feasible contributes to causing waste energy effect. Some issues addressing glass envelope walls comprise poor insulation and rely on artificial ventilation [12].
Hence, the envelope wall as a highrise building façade is always appealing to research. To prevent the thermal transfer but remain the glass as envelope wall is a challenge for architects. Such tricky actions to obtain thermal comfort encompass double layer wall glass with sure angled sun blinds and air-circulated space between [13]. Meanwhile, wall materials remain updated with the latest technology and formula to prevent the thermal transfer, such as glazing and glass prismatic [14]. Research regarding building, material, façade, and thermal shown desired theme to study. Thermal comfort, heat transfer mitigation building, and pursuing a better environment are the purpose of those research, and the trend of this research will remain updated in the future. Bibliometric analysis is legal research using big data on Scopus, Web Science, Dimensions, or PubMed.
Keywords mentioned above in the title are about the architectural or construction industry. Li et al. [15] illustrated two-decade publications about green building sourced from the Web of Science core collection database. Their research revealed that thermal comfort and natural ventilation are appealing issues for green building themes [15]. Lima et al. [16] conducted a bibliographic analysis using VOS Viewer to generate potential future research regarding the construction industry. Analyzing big data from literature databases is fruitful to bridge historical gaps, understand the research evolution, and predict future research [15,16]. Then, this research aims to generate a mapping and visualized research regarding the keywords building, material, façade, and thermal in the Scopus database. Thus the outcome of the research enables future researchers to envision the potential and intense research theme.

Methodology
This study employs bibliometric analysis to provide knowledge mapping and visualize the maps graphically [17]. Bibliometric analysis likewise illustrated the geographical source and identified leading authors and affiliations involved in particular research keywords [18]. The supply of data uses in the study acquired from Scopus comprises citation information, abstract, and keyword. A preliminary step uses four keywords determined by authors during the colloquizing process and explores the Scopus database using the following keywords: building, material, façade, and thermal. The keywords utilized are explained in the introduction chapter, which are imperative keywords for current research in sustainability in the modern urban life of architecture. Scopus database reveal 984 document results in search of those keywords accessed on June 13, 2021. The number of documents exposed encompasses publications from 1976-2021, numerous subject areas, various source types, and two final publication stages.
The exclusion sets limit the documents before exporting the data for meta-analysis purposes. The limitation (L) encompass year (L1), subject area (L2), publication stage (L3), and language (L4) (table1). After searching and screening the document results, the data are exported to the CSV Excel file, including citation information and abstract & keywords. Thus, CSV Excel file extracted processed using VOSViewer to generate meta-analysis of 859 publications. The employment of VOSViewer allows the researchers to understand the meta-analysis visually by presenting label view, density view, cluster density view, and scatter view [19].  (2018) work, which revealed glass innovation for façade, and obtained the most cited document outright [23]. Nevertheless, in the following years, the number of publications was lessened insignificantly (figure 1). Actually, in the middle of 2021, the publications along 2021 capped 68 documents. TIt indicates that publications in the whole year of 2021 possibly reach a higher amount than in 2018. According to the area of the research publication with four keywords found in Scopus, table 2 indicates engineering possesses the most subject followed by energy, material science, and environmental science. Table 3 shows that Energy and Building is the journal most regarding this research area, looking at the screened documents.

Countries involvement in research
European countries dominated the ten most occupied countries in research about those four keywords. China and the US were the only country outside the European continent that belong to ten. Italy leads the highest involved country equipped with 121 research documents on Scopus. Researches in Italy are mainly focusing on the achievement of thermal comfort in building by the utilization of vegetation, natural ventilation, and material innovation [24,25,26]. Meta-analysis through VOSViewer presents the network between countries involved in research with keywords: building, material, façade, and thermal.

Keyword analysis
Author keyword applies in this study to recognize the relation between keywords and indicates the clusters. This analysis requires five minimum number of occurrences of a keyword. Thus, VOSviewer visualizes 74 thresholds ( figure 5). Meta-analysis of selected Scopus documents generates 9 clusters indicated by different colours. Manual labelling is required to designation the clusters based on the items contained (Table 4). Five clusters indicated the most influential cluster and have inter-cluster linkage. The prominent cluster is red, labelled as energy, possess the most keyword in the research area. Energy efficiency holds the hottest item in this label and broadest linkage area outright-energy efficiency is tightly associated with thermal concerns, façade, and material. The second cluster, the green dots labelled as Façade Technology, contains façade and shading innovations to mitigate heat transfer. The blue dots, the third cluster labelled as measurement because of assessment and simulation contained. Fourth, cluster the yellow dots labelled Thermal since they are closely associated with natural ventilation and thermal comfort. The fifth cluster is purple dots labelled as climate as a consequence of urban heat island is the main issue in this cluster.    (Table 6). Authors belong to the list signify their frequent in this research area based on documents they published on Scopus.