The application for innovative methods and materials for greater sustainability in residential buildings in the UK: “a bibliometric review”

The provision of sustainable housing should factor energy efficiency, the use of sustainable building methods and materials and address the current associated barriers. This consideration triggered an ongoing study in UK which has so far conducted a bibliometric analysis of 275 papers using the VOSviewer software. More papers have been published in the last 5 years compared to the previous 15, and the UK is less engaged in successful collaborations in comparison to other economically developed nations. The lesser use of innovation in the UK has been identified. Also, the uses of sustainable methods and materials in residential housing are not fully exploited for achieving energy efficiency. A future research direction emerging is the potential to study the combined impact of using innovative materials and methods to achieve greater energy efficiency in UK housing provision. Through technological innovations and the need to move from fossil fuels (for example, through the use of heat pumps) a new approach can move the UK closer to achieving net-zero in housing provision and operations.


Introduction
Innovation is a product or idea offering new methods, originality and advanced thinking [1].Sustainability in construction refers to the creation of projects which have positive outcomes for both the environment and society [2].There is undoubtedly a need for more sustainable and innovative materials and methods to be used in the construction industry.For too long, focus on energy efficiency without necessarily considering sustainability has been a problem [3].Whilst it is possible to achieve sustainability through highly energy-efficient houses, it is often an occurrence that energy-efficient houses are not necessarily sustainable [4].Energy efficiency and sustainability are related concepts but have different distinct focuses.Sustainability focuses more broadly around environmental social and economical considerations, compared with energy efficiency which simply refers to the amount of energy required to achieve a certain output or service level.Generally speaking, energy efficiency should be more considered a component of sustainability but the element of sustainability encompasses a wider focus on goals beyond simply energy requirement [5].It is therefore important to consider ways in which residential buildings can achieve both energy efficiency and sustainability with one way of doing so, using innovative and sustainable methods [6].This, accompanied by the use of recyclable and renewable materials, which are naturally available to us in the environment, can contribute to both innovation and sustainability within the built environment Limitations such as costs, availability of materials, along with education and building speed have created challenges, but with research, care and persistence, there are benefits to be gained when building houses sustainably [7].
Akabogu [3] and Menna et al. [4] both state that there is a need for more sustainable housing that does not just focus on energy efficiency but also the methods and materials which are used to achieve this.There are already sustainable materials which are naturally and readily available and can be utilised but, as identified by Kolosok [8] these are simply not being used.Current research in sustainable materials shows that, whilst they are available, barriers exist to their use.Akabogu [3] discusses the difficulty in obtaining sustainable materials due to their low production and, even when available, the financial cost for doing so is considerably higher than using traditional methods.A concerning statistic also revealed that 63% of building organisations do not use sustainable materials when constructing houses [9].Reducing carbon levels is, however, paramount if Net Zero is going to be achieved and the consideration of modern methods over traditional ones should be encouraged and promoted, although there will be barriers to overcome whilst doing so [10].Generally speaking, sustainability oriented around four key pillars which are climate action, circular economy, nature and resources and people and communities [11].However, when it comes to considering sustainable approaches to the actual building of houses then fundamentally, the approaches circle around what can be applied to make that property sustainable, these can be broken down into three approaches which are; the materials, methods and technology which are used when building.
Previous systematic and bibliometric reviews have been carried out; however, there has been a limited focus on the methods and materials directly being used.Bhyan et al. [12] carried out a systematic review on life cycle sustainability using a bibliometric review; Hu [13] focused more on residential sustainability through the environment in the elderly; whilst Hoseini et al. [14] focused on sustainability through the use of cleaner energy.Whilst all of these studies are steps in the right direction of increasing overall sustainability, they still do not address sustainable building materials and methods within residential buildings.Other reviews in looking at the natural world for creating more sustainable houses have been carried out, such as that by Godfaurd et al. [15].However, this focused more on biomimetics, which is about the biological processes to elements in the natural world which can then be utilised.There was no focus on materials and methods but rather the environment instead.Cabeza et al. [16] began to look at more sustainable construction and the affordability of it, but identified gaps within research of sustainable building materials which need exploring further to better contribute to making them more affordable.From this, it is possible to conclude that there is a need for a review into sustainable and innovative building materials in residential buildings.It is essential that these gaps in this area are researched further to achieve the goal of reducing carbon emissions.Thebuilt environment is responsible for over a third of all emissions and residential housing is a huge contributor to these due to the use of fossil fuel reliance, such as gas, subsequently houses need to move towards a more sustainable future [6].In order to do this, however, it should be cost effective and not create a heavy financial burden on the homeowner or in fact, companies and organisations who are building these houses.

Aim and objectives
The aim of this review was to identify current gaps in the area of innovative and sustainable methods and materials in residential housing to invite future research.
The purpose of the review was to investigate the level of research which has been carried out in the area of sustainable and sustainable methods and materials in residential buildings.The review sought to identify key areas within the topic, subsequently identifying gaps within the research that create the opportunity for further exploration.The review offers a visualisation for what specific areas are being focused on within the topic, which countries are carrying out research in this area and what prominent studies in the field of study have been cited the most, to see which specific author(s) are leading in this area of research.When completed, the gaps that are identified allow for further research around the topic of innovative and sustainable methods and materials in residential housing.
The key objectives of this review are as follows: 1) Identify gaps in the existing research in sustainability that can invite further investigations into these areas in future research for residential housing sustainability.2) Identify which publication sources are most commonly used in this field of research.
3) Determine which countries are leading research in the subject field and if there is scope for the UK to be doing more on the Net Zero acceleration 4) Analyse key words within the published journals and see if there is a trend when it comes to sustainable and innovative materials.5) Identify the most prominent studies in this field of study which have been cited the most.This will help identify elements of sustainability research which are referenced and searched more.6) Provide recommendations that need to be carried out relating to further research in the area of sustainability within residential buildings.

Methods
Literature reviews play an essential role in academic research allowing for the gathering of existing knowledge and the examination of the field's current state [17].Typically, the two most common types of reviews carried out in academic research are systematic and bibliometric, both of which have advantages and disadvantages [18].Systematic reviews allow for a deep analysis of research identifying what is already known but importantly, identifying potential gaps in research which can allow researchers to then explore those areas further [19].When conducting the initial search of data, however, if too many search results are yielded, this can often make the systematic review method more difficult from a time perspective and, as such, a bibliometric analysis may offer a better alternative [18].Understandably, the choice of carrying out a bibliometric review does not allow for the same detailed review of a specific paper publication(s) that a systematic review offers.However, bibliometric reviews do allow for a more broader focus across a wide range of papers.Despite this, bibliometric analysis allows the researcher to further explore their reading and reviewing of these publications.Bibliometric analysis is a popular and rigorous method which allows the analysis of large amounts of data which can also create a visualisation of authors and countries who may be leading in a certain area of research, so that patterns can be identified if certain keywords, authors and countries are focusing on certain topics within the field of study [20].As a result of this, given the initial large amount of papers identified, a bibliometric review was chosen over a systematic review.

Data collection
The data collection initially started with a keyword search, which used the Scopus database in this study.Scopus is the largest database of peer-reviewed literature, containing over 20,000 peer-reviewed journals, and is often considered a favoured choice over other databases such as Web of Science [21].This makes Scopus a good starting point when carrying out an analysis.To be relevant to the research topic area, the following keywords were inputted into Scopus: "sustainable" OR "innovative" AND "materials" OR "methods" AND "residential housing" The search string was as follows: TITLE-ABS-KEY (sustainable OR innovative AND materials OR methods AND residential AND houses) AND PU.
As the subject focus is around sustainability of more modern approaches which can be referred to as innovative, the search string was decided to include sustainable materials or methods as these can then be grouped into approaches.The inclusion of innovative was to ensure that new approaches that may not be well researched were also introduced into the search results.This search initially yielded 1399 results.There was, however, some exclusion data which then needed to be carried out, which brought this initial number down.This criteria was journal articles that were within the last 20 years (2003-2023).The way in which Scopus produces results meant that 2023 was included as a year to show papers to the end of 2022.Finally, searches that were English were also added to the criteria; this then returned 420 results.The next step was to then cut down and filter through topics that were not relevant to the subject.This was done manually by identifying key topics which should be omitted from the study.One example of this was an article which focused on cruise ships' sustainability, which was not relevant to housing.Equally, topics around computer science and medicine were excluded, limiting instead to engineering, environmental science, energy and materials science.After filtering through the set of results, the final return was 275 results left to be analysed.With this final amount of results produced, even after filtering, still being high, a bibliometric analysis approach was taken.

Results collection
After the collation of results from the Scopus search, the next step was to use a software for the analysis.The software chosen for this scenario was VOSviewer.VOSviewer is a free to use computer programme used for bibliometric analysis.However, in comparison with other bibliometric analysis softwares, VOSviewer offers special attention to graphical representation of bibliometric maps, being able to offer large maps with the ability to produce a co-citation map of up to 5000 journals [22].
In order to meet the objectives set out for this study, the number of papers published from 2003 to 2022 was collated, the countries of which those papers were published and finally, the keywords of topics from the author title.The papers published were analysed to see (1) if there was more focus in this area in recent years due to an increased focus in sustainability with the commitments to Net Zero and (2) the current knowledge and research of the subject matter.The countries of paper publication was studied to see which countries are researching sustainable materials and methods more than others and what lessons can be learned from them.Finally, the keywords by author title was another criteria to be examined in order to see if there were links within this.For example, sustainability and energy efficiency or residential housing and sustainability (Fig. 1).

Number of publications by year (2003-2022)
The number of publications steadily increased from 2013 to 2017, before a slight decrease in 2018 and then a rise again in 2019 to 2022.The likely increase in publications is probably due to the increase in interest in sustainability and in particular due to targets surrounding the Net Zero commitment, as the Paris Agreement was drafted in 2015 before being signed in 2016.As of 2023, there are now 194 countries signed up to this agreement which is set out to tackle climate change and create targets for each country to reach Net Zero [23].More papers have been published in the last 5 years (2018-2022), with a total of 140 compared to the previous 15 years (2003-2017) in which only 110 were published.This indicates more research is being carried out in the area of innovative and sustainable materials and methods in residential housing in recent years.Now that the world are committing to the Paris agreement, it is likely further research will be carried out.Most publications on the subject came in 2022 with a total of 36 papers.This shows a trend that in recent years, the level of research around this topic is on the rise, which shows promise to those looking around sustainability through the use of innovative and sustainable methods and materials in residential housing.With the Paris agreement signing, resulting in countries set on reducing their carbon levels by 80% in comparison to 2004 levels [24].This is likely creating more of a need for research in sustainability given the commitment to reach Net Zero.If future policies around Net Zero and building start to come in, the level of research in sustainability will likely continue on this trend.This should be encouraged and links between sustainability and residential housing through the use of innovative or sustainable materials or methods should be made better.

Publications by source
When analysing the sources from which the most publications and citations came from, the Journal of Energy and Buildings has the most citations, with 337.However, this was only from 10 documents published.Sustainability (Switzerland) had the most documents published, but had 208 citations.This can show the impact a particular article published is having in the field of study; for example, a particular article that is published in the Sustainability (Switzerland) journal, may be of high interest and relevance to other researchers, therefore enabling other authors to use it and make reference to it in their own research.Table 1 below shows the top 10 cited sources.
As can be seen in Table 1 Sustainability (Switzerland) has the largest amount of publications, although not as heavily cited as other journals, it does have a considerably higher amount of publications in comparison with other sources with the nearest being the Journal of cleaner production, which had 15 fewer published documents.Energy and buildings had the most citations, with 337, but with only 10 documents published.The Energy and Buildings impact factor is 6.7 which is considered very good [25].The Journal of cleaner production has an impact factor of 11.072.The Sustainability (Switzerland) journal has an impact factor of 3.9 as of 2022.It is often found across most research that high impact factor journals have a tendency to be more heavily cited and acknowledged by researchers in comparison with other journals in the same field [26]

Publications by country
When studying the countries that have published papers, the UK appear to have quite a high number of publications (joint 3rd), with 16 articles published as seen in Table 2.However, as can be seen in the visualisation (Fig. 2), there is very little collaboration with other countries.Stronger collaboration appears to exist between the United States and Canada and China, and the United States and Turkey, who also collaborate with each other.Italy had the most citations, with 464 from 10 publications, whilst China had the most published articles out of all countries.It is important to consider the amount of citations due to the impact that the publication is having in the field of study.Publishing multiple documents which are not being cited does not necessarily mean that the particular article is not relevant.However, multiple citations will indicate that the study and that authors work is being used more and referenced more frequently.For the purpose of this study, the countries which were more heavily cited were identified, which may indicate the influence that particular countries research may be having on others who are researching in the same field of study.It is also good to then have a visualisation of which countries may be leading in that area of research.
Collaboration between authors is considered to be important and can be even more helpful when those authors are from other countries, allowing for research into what other countries in the same field of study are doing [27].As can be seen in the visualisation, China shows as the largest collaborator, with links to the United States, Australia, India and Turkey.This shows international collaboration which was mentioned by Kong et al. ( 2019) as important as this can allow an insight into what may and may not be working in other countries.A potential example of this is that of heat pump technology.When looking at Scandinavian countries, they significantly lead the way in heat pump technology [28].The UK set an ambitious target of installing 600,000 heat pumps per year by 2028 with the plan to phase out traditional boilers operating off mains gas [29].More recently, however, in September 2023, this plan became very unlikely to materialize as the Government announced it would delay these plans with no new date set [30].Potential collaboration between Scandinavian countries and the UK may be insightful for the UK to learn about how the Scandinavian countries are having such a high success in the gas boiler phase out and high implementation of heat pump technology instead.Equally, the potential collaboration with other countries should be encouraged in order to learn from and promote better practice for the UK.As seen in Table 2, the UK ranks 4th in terms of cited documents and joint 2nd in terms of published documents, but in the visualisation in Fig. 3, the UK does not appear, thus indicating a lack of collaboration with other countries in this field of study.

Keyword co-occurrence
Keyword co-occurrence allows for the identification of primary topics which are being discussed in the area by offering a visualisation of frequently co-occurring keywords or topics within the literature [31].Keyword co-occurrence analysis breaks down the literature into multiple clusters that then represent research within that field.The analysis identifies if two keywords have a strong co-occurrence if they are appearing in articles together [32].Each node in the visualisation represents a keyword, with the node's size representing that keywords occurrence in the literature.Keywords that then co-occur more frequently are positioned closer to each other within the visualisation.Having being guided by previous bibliometric review studies [33] which subsequently followed guidance on best practice [34].The minimum number of keyword occurrences was set at a threshold of 10.Further justification of this allowed for a greater understanding of key words links without offering an overwhelming and unclear visualisation of words which were not as popular within the search, equally, this would not offer as many links to other keywords.Of the 2838 keywords, 40 met the threshold for analysis.The node size and colour was then analysed to explore the main relationships and research hotspots within the literature.These were clustered using auto portioning within the VOSviewer software.Cluster 1: (Energy) Identified in red is the largest of the clusters, containing 13 words.The keywords in this cluster consist of: Housing, intelligent buildings, energy consumption, energy conservation, residential energy, thermal insulation, energy efficiency, energy utilization, heating, ventilation, walls (structural partitions), investments and optimization.This cluster appears to have a particular focus around energy and energy efficiency and what factors contribute towards this.For example, heating, ventilation, thermal insulation, walls and consumption would all be expected to be closely related and impact energy efficiency and subsequently energy consumption.The keywords within this cluster would be expected to be found together as most will coincide with one another.What is identified, however, is that the distance to other clusters shows gaps in research for innovation of materials and methods.As can be seen, building materials (in the green cluster) appear to have no link with thermal insulation and intelligent buildings, yet intelligent buildings and the construction industry does have a link.This perhaps invites research into building materials that can be used for thermal insulation which subsequently will then link to energy efficiency and better sustainability.Heating within the cluster also does not appear to have links to materials and the construction industry, despite the fact that heating, just in the UK alone, is responsible for 14% of emissions produced, mainly due to over 19 million homes currently using gas boilers [35].Links between building materials to energy efficiency needs to be explored further.
Cluster 2: (Impact on the environment) is the joint 2nd largest of the clusters, containing 12 words and is identified in green.The keywords in this cluster consists of: Building materials, construction industry, construction, global warming, environmental impact, life cycle assessment (LCA), environmental management, residential building, carbon dioxide, greenhouse gas, embodied energy and gas emissions.This cluster has a focus on environmental impact, such as that of greenhouse gases and global warming, but also how these are linked with building materials, residential buildings and the construction industry.With the built environment contributing to over a third of all global emissions, it is essential that the construction industry plays a greater part in reducing these emissions [36].Furthermore, in the UK alone, 40% of emissions produced come from residential households [37].There are evident links between these elements, proving the construction industry and residential buildings are linked with global warming.Furthermore, building materials, which are also within this cluster, need to be explored further as the construction industry using more sustainable building materials will have a great environmental impact and likely equate to better sustainability.
Cluster 3: (Sustainable development) also contains 12 words and is identified in blue; the keywords in this cluster consist of: Residential development, urban planning, surveys, planning, residential location, sustainability, houses, architectural design, climate change, sustainable development, building and decision making.Understandably, the planning and decision making of sustainable development can lead to greater sustainability in housing.There are evident links within these areas to other clusters as well, which shows just how strong an impact that sustainable development can have on overall sustainability.Cluster 4: (Build types) is the smallest of the clusters containing 3 words and is identified in yellow.The keywords in this cluster consist of: Buildings, apartment houses and apartment buildings.This cluster focuses on types of buildings particularly apartments.This indicates research around sustainability within apartment buildings but in addition to this, as can be seen within the visualisation, the yellow cluster sits quite neutral and, in the middle, showing closer links to all other clusters.
Regarding the node size, sustainability, sustainable development, residential buildings and energy efficiency were the largest.Given the original search on Scopus included key words relating to sustainable materials and residential buildings, it is not surprising that the visualisation demonstrates this.Innovation, however, is not included in the visualisation at all.Equally, methods are not included within the visualisation.With regards to the frequent co-occurrence, residential buildings and energy efficiency were close together, along with energy efficiency being closely linked with thermal insulation and energy conservation, energy consumption and thermal storage.When breaking this up further, there are then links to energy consumption and conservation, which would be expected in line with energy efficiency affecting energy consumption and utilization.
Gaps are identified with innovation and methods missing from the visualisation and equally, a link between energy efficiency and building materials being quite far away from each other within the visualisation.There is a link with building materials and sustainability, which demonstrates this is an important factor when considering sustainability in housing.However, this is not linked with energy efficiency, which leaves this area open to be explored further.There is a gap to invite further research in innovative materials and methods in residential housing.As can be seen from the nodes in the visualisation, building insulation and sustainable design displayed considerably smaller nodes and did not have links to sustainability.This gap needs to be bridged in order to focus on houses that can be both energy efficient and sustainable.This could be achieved by further research being carried out in the area of sustainable and innovative methods and materials that can be used in residential housing to not only increase energy efficiency but also so that the house is sustainable as well.The focus around sustainable design and building insulation could be better linked to the delivery of sustainable and energy efficient homes.

Author citations
Carrying out an analysis on the authors through the VOSviewer software allows a streamlined approach to seeing which authors have the most citations in that particular area by other authors.In this example, authors who were highly cited by others in the same field of work in sustainable OR "innovative" AND "materials" OR "methods" AND "residential housing".The local citation analysis gives an indication that researchers are citing other authors from within the same field of research.This, in turn, shows a high level of relevance within the field to then be used in another authors piece of research which may be related or slightly different to the topic.Upon analysing the results, the top 10 authors had a combined 299 citations between them, with Zhang being the most cited (43 times).The measure shows that in the 275 articles yielded, Zhang had been cited in 43 of the other articles within the collection.Zhang has recognised the importance of global warming contribution within the built environment and has dedicated research into practices of low carbon towns and cities in China.Having published over 300 documents with over 14,000 citations, Zhang looks towards lower carbon production within the built environment within China.Zhang has looked into urbanization and, as per the 2016 paper on sustainable urbanization, focuses on achieving social, economic and environmental sustainability [38].This same level of research in other countries would be strongly welcomed, as well as collaboration to learn from Zhang's findings in research they have carried out in China.
The majority of the top-cited authors originate from China, which falls in line with the countries with the most publications (as seen in Table 3).This is showing that China is carrying out more research in the area of sustainability.Given that China is currently the largest producer of Carbon emissions, equating to more than a quarter of the worlds entire emissions, China is now taking more action to combat this and drive a more sustainable future for the country [39].As a result of this, it is perhaps unsurprising that authors in China are taking a much higher interest in the area of sustainability now, which is welcomed.Being home to the 2nd largest climate tech hub in the world, technological advances and research in sustainability by China is accelerating their commitment into achieving greater overall sustainability and reduction in global emissions [40].Consequently, with such advancements in technology and greater research into the area of sustainability, a welcoming of collaboration between authors in other countries and Chinese authors would be beneficial here.Papers on the topic have been heavily cited others and would give opportunity for authors in other countries to work with leading researchers in this field of study.Author collaboration should always be encouraged, in particular on topics which are of high public interest.With a lot of focus around the world now being around sustainability due to global warming, researchers coming together must be encouraged.The collaboration between authors has been highly successful in sharing practices that may or may not work in other countries [27].Franceschet and Costantini [41] concluded that collaborative research has been associated with higher productivity and enhances knowledge and research techniques, giving opportunity for researchers to learn how others work and approach their research but to also arrive with enhanced visibility of final results.

Conclusion
This study aimed to identify current gaps in the area of sustainable methods and materials in residential housing.The analysis showed that there is a clear link between energy efficiency and sustainability within residential housing; however, materials and energy efficiency appear to have either little or no link.Despite this being in the original search string, there is no mention of either innovation or methods (as identified in the keyword analysis).These areas need further research to focus on implementing more sustainable materials and methods into residential housing.A further review of sustainable or innovative building materials may be advised here to identify what materials may be readily available and are being used.Whether this be insulation, or another type of material could prove helpful in future research around the research area.The United Kingdom, despite having several publications within the 275 articles, appears to have very little collaboration with other countries compared with others that do so.Given the current position of the UK in residential property sustainability, this needs to be encouraged as collaboration may offer insight into what other countries are trying to implement within their residential houses.Further research into what these countries are doing could be used as an educational tool for what areas the UK may need to investigate, as they strive to decarbonise all buildings.There is an increasing amount of interest in this area of research currently, with an upward trend year on year since 2018.As more focus on Net Zero becomes apparent, more research is expected to be carried out, which is welcomed.This study is limited to the Scopus database, but future studies could explore more databases.Different databases may have introduced more studies into this mix and equally, provide a different set of results and consequently, whilst there should be an expectation of similarities, there may be some different results and findings than purely basing the findings from what were identified from only using Scopus.Other databases such as web of science, scholar, CORE and science direct may be considered in the future.The study does however justify the rationale behind choosing Scopus.
Due to the identified targets the UK have and the sign up to the Paris agreement combining principle and legal commitment to Net Zero by 2050, there needs to be more research carried out within the UK.Other countries for example, Norway, are leading the way in heat pump research and technology, it may therefore prove beneficial for the UK to collaborate with such countries to identify how they are successfully implementing heat pump technology.China is now accelerating their research in sustainability, and this is evident with the amount of research papers published, cited and collaboration with other countries showing willingness to the commitment of Net Zero.As a planet, and with so many countries signed to the Paris agreement, there is a need for researchers across the world to come together, share their experiences and research with one another, learn from each other and ultimately share best practices to achieve the common goal.

Fig. 1
Fig. 1 Year by year amount of publications from 2003 to 2022 (Scopus)

Table 2
Top 10 countries with most citations in the research area

Table 3
Top 10 most cited authors in the research area Author Citations