Review and Analysis of Models for a European Digital Building Logbook

: The concept of a Digital Building Logbook (DBL) was ﬁrst introduced with the European strategy ‘Renovation Wave’. It is considered as one of two fundamental parts of which the Building Renovation Passport is composed: the DBL and a Renovation Roadmap. As the implementation of the DBL is a European priority, this paper reviews the existing literature and analyses the most developed European Digital Building Logbook models. The analysis includes iBRoad, ALDREN, X-tendo, and the Study on the Development of a European Union Framework for Buildings’ Digital Logbook, from the perspective of seven key aspects: References used as a starting point for the model deﬁnition; Identiﬁcation of the relevant stakeholders in the DBL; Identiﬁed potential user needs; Proposed structure of indicators; Data sources; Potential functionalities; and Operation and use. The results show that important advancement has been made, although there is still no consensus about crucial subjects, such as the indicators to be collected or how to collect and use them. This is probably due to the fact that the ﬁnal functionalities (objective and scope) that the logbook should provide are not fully clear.


Introduction
European directives and strategies, such as the 'European Green Deal' and the 'Renovation Wave', point out the importance of the building sector in achieving the climate goals set by the European Union for 2050. In this sense, building decarbonization is a priority [1]. However, to achieve the goals set by EPBD (EU) 2018/844, a higher renovation rate of the existing buildings is required, moving from the current rate of 0.4-1.2% to a rate of 3%. There are many barriers that prevent the renovation rate from growing. They are of different nature: technical, financial, social/behavioral, and institutional, as identified in the literature [2][3][4][5]. Among the technical barriers, the lack of skilled workers with energy-efficient materials and renovation techniques, as well as the gap between predicted and actual energy savings achieved due to renovation processes; the fragmentation of the construction sector and the lack of dialogue between the involved stakeholders; the lack of information; and the lack of consideration of users' needs and expectations were mentioned in the literature. Regarding financial barriers, the long pay-back times of renovation interventions, the high investment needed to face a renovation process, the lack of attractive financial tools to make operations viable or the impossibility to access them, and the high risk perceived by the potential investors make the renovation rate remain low. In addition, social/behavioral barriers, such as long decision-making processes, especially in condominiums; the lack of awareness about renovation benefits; the concern about suffering disturbances during site works; and the lack of knowledge about the matter are of great importance because the building owner/user is called to be the booster of renovation processes. Finally, among the institutional barriers, the long administrative procedures needed are perceived as a discouraging burden [2][3][4][5][6]. Although national-scale building logbook initiatives already exist, there is not an official common model for a European building logbook so far. As the implementation of the DBL is a European priority to trigger buildings renovation, there are several research groups that have been recently working on the definition of this common tool.
This paper presents a review of the most developed European Digital Building Logbook models, with the aim to summarize and compare the available information. The overview includes iBRoad [21], ALDREN [22], and X-tendo [23] projects as well as the Study on the Development of a European Union Framework for Buildings' Digital Logbook (Study EU DBL) [24]. This paper is structured as follows: Section 1 introduces the concept of Building Renovation Passport and Digital Building Logbook and explains their origin. In Section 2, the methodology followed in this review is described. Section 3 provides an explanation of the selected European Digital Building Logbook models. In Section 4, a comparison of the selected models is carried out. Finally, conclusions and recommendations for the next steps are made in Section 5.

Methodology
The methodology for the review is composed of the following steps: 1.
Identification of initiatives on Digital Building Logbooks and selection of models depending on their scope (European scale), ambition level and maturity of the proposal.

2.
Identification and study of the key aspects in the definition of the DBL model. The following parameters are used to identify the key aspect: 1.
References used as a starting point for the model definition.

2.
Identification of the relevant stakeholders in the DBL.

7.
Operation and use.

3.
Comparison of the models.

Selection of European Digital Building Logbook Models
Four research work outcomes have been studied and compared in this paper due to their European-scale scope, ambition, and level of definition. Three of them are outcomes of H2020 projects: iBRoad, ALDREN and X-tendo, whereas the fourth is the result of the Study on the Development of a European Union Framework for Buildings' Digital Logbook carried out for the European Commission.
In the following paragraphs, a brief overview of each one is shown.

iBRoad-Individual Building (Renovation) Roadmaps
The iBRoad project ( Table 2) aims to boost the renovation of single-family houses through the implementation of Individual Building Renovation Roadmaps that guide the building owner providing tailored step-by-step renovation plans (iBRoad-Plan) in combination with a building logbook (iBRoad-Log) that acts as a repository of aspects such as energy performance, executed maintenance and building plans. The iBRoad approach is conceived as an evolution of EPCs and energy audits, and it has been tested in Bulgaria, Poland, and Portugal [21, 25,26].

ALDREN Project-Alliance for Deep RENovation in Buildings
The objective of the ALDREN project (Table 2) is to encourage all the stakeholders involved in the renovation processes to foster and undertake deep renovations by overcoming market barriers and preparing the ground for investment. The ALDREN project proposes a voluntary and modular framework that implements the European Voluntary Certification Scheme (EVCS) to assess the energy performance of buildings with a European common base. The framework is composed of four standalone modules that allow gathering several indicators-Energy rating and target, Energy verification, Comfort and well-being and Cost value risk-plus two reporting tools: a European Voluntary Certificate (EVC) and a Building Renovation Passport (BRP), integrated by the ALDREN BuildLog and the ALDREN RenoMap. The ALDREN methodology has been tested across 20 buildings in France, Spain, Slovakia, the United Kingdom and Italy [11,22,27].

X-Tendo Project-eXTENDing the Energy Performance Assessment and Certification Schemes via a mOdular Approach
The X-tendo project ( Table 2) aims to promote next-generation EPC schemes by introducing a toolbox with ten innovative features-innovative indicators and innovative data handling-that will help to improve usability, integrity, transparency, and harmonization of energy certification, and is targeted to public authorities and implementing agencies.

Study on the Development of a European Union Framework for Buildings' Digital Logbook
This study (Table 2), compiled in three reports and prepared by R2M Solution, VITO and the BPIE for the European Commission, aims to set a widely accepted definition of the DBL, as well as to provide the basis for the implementation of an EU-scale DBL model. With this aim, consultation among the stakeholders, several interviews with experts of the construction sector and a detailed analysis of forty international building logbook models have been carried out. As a result of this desk research and fieldwork, the main gaps in the existing initiatives have been identified, and a set of actions to address them and prepare the ground for further research has been proposed [18,19,24].

Comparison of the DBL Initiatives
Despite the similarities of the four studied proposals regarding their scope and objectives, they have different approaches when it comes to some key aspects. In the following paragraphs, they are identified.

References Used as a Starting Point for the Model Definition
The first difference among the models relies on their starting point. The iBRoad-Log first reference is the EPC (Table 3), especially the Portuguese model, established in 2007, which is considered as one of the more complete ones in Europe. Other BRP and DBL initiatives from European countries, such as Portal Casa+ and Woningpas, were identified and analyzed, and the lessons learned were applied to the iBRoad-Log [31]. Qualitative interviews with public authorities and targeted surveys with potential users of the DBL were also carried out in Bulgaria, Poland, and Portugal to gather useful information [32].  (Table 3) also started from desk research of previous initiatives [19]. The desk research was accompanied by an online survey to gather stakeholder inputs regarding the type of data that should be included in the logbook, its functionalities, and data governance issues. Finally, through interviews with several experts, more in-depth insights were collected [18].
Additionally, in the same line, the X-tendo project (Table 3) developed an analysis of good practices regarding existing EPC databases and registers (as its aim is to provide a framework of new-generation features for energy performance assessment and certification), as well as a study of DBL existing initiatives. Moreover, BPIE and VITO, two X-tendo partners, participated in the development of the Study on the Development of a European Union Framework for Buildings' Digital Logbook; hence some lessons learned were applied to this project [29,[33][34][35]. Additionally, a digital survey was conducted to gather information from potential users [36,37].
The ALDREN BuildLog was conceived from a slightly different starting point (Table 3). Even though several BRP models were identified and studied, the analysis of existing databases related to building characteristics and stocks, such as Eurostat, the Building Stock Observatory (BSO), EPC registers and EU project databases outcomes, and of different certification schemes, such as BREAM, LEED, VERDE, etc. was highly relevant to gather information for the definition of the DBL template [11,38].
To sum up, there is not a common approach in the four DBL models. Nevertheless, the study of existing BRP/DBL initiatives was one of the first steps in all the cases. Additionally, the analysis of EPC models/registers and surveys with stakeholders were important references for three of the studied proposals.

Identification of the Relevant Stakeholders in the DBL
Identifying the stakeholders that are involved in the DBL is crucial, not only to take into consideration the specific needs of each one but also to detect the main sources of information and to adapt the logbook interface and structure to final users.
At this point, three different visions have been identified among the DBL proposals. First, the iBRoad-log is the most ambitious one regarding the range of stakeholders interacting with the logbook. Not only the actors that can take advantage of the DBL or provide data are important for it, but also some collectives that can trigger its implementation, such as Non-Governmental Organizations (NGOs), the media, or consumers associations [25,31,39,40].
The second approach, from the Study on the Development of a European Union Framework for Buildings' Digital Logbook, also considers a large number of stakeholders, focusing on the entire set of actors of the construction sector, public institutions and investors mainly [18].
Finally, the ALDREN BuildLog and the X-tendo logbook consider a shorter number of stakeholders, and the importance of experts and auditors is highlighted [34,35,41].
Through the comparison of the stakeholders considered in the four initiatives, we can conclude that the most important agents are the building's owner and occupants, public authorities, and financial institutions since they are the only ones identified by all of them. They are highlighted grey in Table 4.

Identified Potential User Needs
Identifying the user needs and the functionalities from which the stakeholders can benefit is a key for the success in the implementation of the DBL. In this sense, the greater the benefits for users, the lower the perception that the logbook is one more useless administrative burden. As explained in Section 4.1, three of the studies carried out a participative process to gather information from different stakeholders of the building's value chain.
In order to properly prepare the testing process of the iBRoad concept that was going to take place in Bulgaria, Poland and Portugal, a study was conducted by Ipsos to gather information about the users' profile, needs and preferences [32].
The study comprised three parts in each country:

1.
A round table with eight participants, lasting 90 min.

2.
Interviews with three public authorities' members.

3.
Around 500 targeted surveys with potential users.
Within these targeted surveys, five topics were assessed: general attitude towards energy renovation, views from buyers, views from homeowners, perspectives on the renovation roadmap and perspectives on the logbook (Table 5). During the interviews with public authorities members, the analyzed topics were: what information about residential buildings is being collected and if it is sufficient; if energy renovation is an important political priority and what are the main barriers to setting up programs promoting residential renovation; if more building data would be useful in designing and implementing renovation schemes and what would be the value of a logbook; and what would be the potential value of a building renovation roadmap.
Within the ALDREN project, several webinars and workshops were carried out with the aim of sharing the ALDREN framework. The first training workshop [42], which took place in November 2018, was an interactive event with open discussion and polling sessions with the REHVA network. During the session, valuable information was gathered about the Building Renovation Passport and digital logbook [43].
Within the X-tendo project, a digital survey was conducted in April 2020 by Ipsos Belgium to gather information about the needs, views, and expectations of potential users (mainly homeowners, landlords and tenants of residential buildings) of EPCs and their linked innovative features investigated by the X-tendo framework when buying, selling, renting, or renovating a property. The survey was conducted in five projects implementation countries, including Poland, Portugal, Greece, Romania, and Denmark and 2563 people participated, with at least 501 respondents in each country [36]. Although some differences between countries were found, the main responses were consistent across countries; thus, a high replicability potential for the logbook was identified.
Regarding the building logbook, the following questions [37] were launched to the participants: 1.
"If you decided to buy or rent a property, how important would it be that a document or folder with all the building-related documents (dwelling's condition, maintenance activities etc.) is present for the property?" 2.
"Did you receive a folder or document with all the building-related documents (dwelling's condition, maintenance activities etc.) when you bought the property or started renting it?" The answers were classified according to building types, tenure status and renovation status.

3.
What information would they like to find in the logbook? (Table 5)

4.
Some additional inquiries are available at the national level in reports 2.4 of the X-tendo project [37] and 4.2 [35].
Finally, during the Study on the Development of a European Union Framework for Buildings' Digital Logbook, several collaborative actions were organized.
During the first part of the study, individual interviews took place with 32 stakeholders from different groups of the construction value chain; an online survey with 93 respondents from 19 countries was run; a webinar with 68 attendees was organized, and a stakeholder meeting with 66 participants took place [24].
The online survey, where several groups from the entire construction value chain had representation, consisted of 17 inquiries that can be consulted in [18], gathering information about the background and profile of the respondents, their connection and knowledge about the DBL and, finally, technical issues, such as their point of view about the data fields that the logbook should incorporate (Table 5), its potential data sources, main functionalities, update needs, and ownership and liability issues. Questions about the role of the EC in the design and implementation of the DBL and actions to be addressed to facilitate its deployment were asked in the surveys and in the individual interviews with the experts.
The main findings from the online survey were compared with the ones from Report 2 [19] through Krippendorff's alpha method to investigate their degree of agreement, i.e., the gap between stakeholders' expectations and what is implemented in real initiatives [24].
In May 2020, a webinar gathering 68 attendees from different stakeholders' groups was organized. In the event, the results of the activities that had taken place until that moment were presented and discussed, including the results of the previous survey and the definition of the DBL, lately established [24]. A new poll was then launched containing inquiries about the key benefits that the DBL should provide, the key features for the development of a successful tool and the main barriers for the logbook to overcome [19].
Finally, the first part of the study was concluded with a stakeholder event with 66 attendees that took place in July 2020 to receive inputs about the identification of priority actions to be implemented by the EC and to explore synergies with other EU initiatives.
During the second part of the study, some experts were selected to be interviewed to assess the development of the last part of the final report (Chapters 5 to 7) and, finally, a second and last stakeholder event with 96 attendees took place in November 2020, to make an overview of the project's main results and gather stakeholders' feedback [24]. Table 5 summarizes potential users' views about the information they would like to find in a building logbook according to their importance. It must be noted that the way the inquiry was stated and the format of the possible answers make the percentages obtained in the surveys of each project doubtful to be directly compared. For this reason, we first identify the most important topics in each survey and afterward, we make comparisons. The 40% more important topics of each survey are highlighted grey in Table 5. When comparing the results, four data topics were pointed out as the most important ones in two surveys: "building's features/description," "technical specifications of walls, windows, insulation, etc.", "energy use, water consumption and energy bills," and "Energy Performance Certificate." Other prominent subjects were: "basic information about the house," "building material inventory," "condition of walls and roof," "energy renovations completed," "design and plans on building interventions," "building plans," "specifications and designs of heating system and other equipment," and "age of equipment." It should be noted that the only topic that appeared in the three surveys was "building plans," which obtained a medium relevance in all the cases.

Proposed Structure
Generally, a digital building logbook consists of a series of indicators about a building, classified in different modules or categories. Nevertheless, each DBL proposal presents a particular scheme with different levels, indicators, and categories.
iBRoad and X-tendo share the same approach regarding the structure of the logbook, although they differ in the indicators that they propose. On the one hand, regarding the structure, the X-tendo model follows a similar vision to the iBRoad-log [31,35]. They propose a fixed-flexible structure, meaning that it counts on different levels of granularity: there is some fixed information (common at European level, levels 0 to 2) and flexible information (that can be adapted to accommodate to national and regional needs, levels 3 onwards). On the other hand, regarding the proposed indicators, in the iBRoad-log, they are classified into 5 modules: general and administrative information; building construction information; building energy performance; building operation and use; and SMART information, making a total of 68 indicators (at level 2). The X-tendo logbook, at the same level, counts on 216 indicators, classified into 8 categories: administrative information; general information; building descriptions and characteristics; building operation and use; building performance; building material inventory; smart readiness; and finance.
The final version of the ALDREN Buildlog consists of 6 modules (2 more in the ALDREN RenoMap) with their own subcategories and protocols for data gathering, entry and calculation: building picture; energy rating and target; energy verification; comfort and well-being; cost value and risk; and documentation and BIM. Although all the modules have been defined, only the 88 indicators for the category "building picture" are available now. Unlike the other logbook models, the modules of the ALDREN BuildLog were designed to work separately, with the aim to provide different levels of data access for various stakeholders in each category. It is important to point out that this scheme was designed for non-residential buildings, mainly hotels and offices, which makes the set of indicators differ slightly from the other DBL proposals [38,41].
The Study on the Development of a European Union Framework for Buildings' Digital Logbook presents a set of 83 data fields or indicators classified into 8 categories: administrative information; general information; building descriptions and characteristics; building operation and use; building performance; building material inventory; smart readiness; and finance. The indicators that compose each category are a result of the desk research, and their relevance depends on their context. In addition to the definition of the set of indicators, the following parameters are also specified for each one: the source of the indicator, whether it is core or complementary, the building type, whether it is suitable for new or existing buildings if it is static or dynamic and the easy of collection [18].
To easily compare the four data models, the indicators of each one were classified by categories defined by the authors of this paper (Appendix A). These categories are general and administrative information, construction information and materials, building technical systems and smart readiness, energy performance, operation and use, and finance (Table 6). In parentheses, the percentage of indicators in each category can be found. The category with the highest number of indicators in each model is highlighted dark grey. The category with the second-highest number of indicators is highlighted light grey. Figure 1 shows the importance of each category for the DBL models according to its number of indicators. Some adjustments have been made to the original indicators to be able to assimilate the indicators from the different models, and this has affected the accounting of indicators.  Through the analysis of Table 6 and Figure 1, we can observe the great difference that exists among the approaches of the four logbook models. There is no consensus about the indicators to be collected. In general, the category with a higher number of indicators, due to its huge relevance in the X-tendo logbook is "building technical systems and smart readiness," with 145 indicators, followed by "general and administrative information," with 105, "construction information and materials", with 90, "energy performance, operation and use," 71 indicators and, finally, "finance," with 27 indicators. Figure 1 shows the great difference that exists among the approaches of the four logbook Through the analysis of Table 6 and Figure 1, we can observe the great difference that exists among the approaches of the four logbook models. There is no consensus about the indicators to be collected. In general, the category with a higher number of indicators, due to its huge relevance in the X-tendo logbook is "building technical systems and smart readiness," with 145 indicators, followed by "general and administrative information," with 105, "construction information and materials", with 90, "energy performance, operation and use," 71 indicators and, finally, "finance," with 27 indicators. Figure 1 shows the great difference that exists among the approaches of the four logbook models. There is no consensus about the indicators to be collected.
Although it can be thought that the more indicators collected, the more useful the DBL is, it is not always true because, in most of the member States, problems gathering information from the logbook sources are frequent. Thus, paper-based information is still common, along with outdated as-built data [44].

Data Sources
To set up a DBL, it is not only necessary to define the data to be collected to fill the proposed data fields, but also to know who owns the needed information, whether it is available in opensource or not, if there is any limitation factor, such as data protection issues, and how the information can be linked. Different approaches have been identified in the studied DBLs, but all of them agree on the importance of incorporating existing databases in building information.
Within the iBRoad project, an effort to identify the main sources for the iBRoad Build-Log in Portugal, Bulgaria, Germany, and Poland was made, and several possible sources were defined: public authorities, energy sector, financial services, construction industry, research and academia, the building user, real estate market, utility companies, qualified experts, energy auditors, consumers association and other national associations [25]. The specific contribution of each one can also be consulted in [31]. In the same report, a method to integrate data from external databases is also explored.
In contrast, the main source for the ALDREN BuildLog is the ALDREN Auditor, who creates the DBL database through a detailed inspection of the building and the collection of all documentation from the owner and other existing databases [41]. Nevertheless, other sources can contribute to collecting some specific information, such as existing databases (Eurostat, the Building Stock Observatory, EPC registers, previous EU projects, other certification schemes) and data gathered by means of ICT technologies [38].
In the same way, external databases, such as EPC registers, are pointed out as important sources for the X-tendo logbook. In the near future, data coming from the Smart Readiness Indicator, material passports and ICT technologies (such as sensors and IoT) will also contribute to generating high-quality information. On the contrary, information submitted by the owner is considered less reliable than the one from public authorities and official databases [34,35].
Similarly, for the Study on the Development of a European Union Framework for Buildings' Digital Logbook, the DBL collects different types of data from different sources. These sources include existing public and private databases at the European or national level (databases of interest are identified in [24]) but also information gathered through smart equipment connected to the building, from the homeowner and auditors, utility companies, the construction industry, third-party service providers, public authorities, among others [18,24].
In conclusion, the most popular data sources according to the four DBL initiatives are the building users, the construction industry, public authorities, financial institutions, qualified experts/auditors, utility companies, existing databases and ICT technologies ( Table 7).

Potential Functionalities
The functionalities of the logbook refer to the services built around it [18,35]. Although there is no consensus regarding all the functionalities the DBL should provide (see Table 8), all the proposals agree that the main function of the tool is to serve as a repository of the relevant information of a building, especially general, administrative, and constructive data. Four more functionalities were pointed out by the four logbook models [18,24,25,33,38], including the "assessment of the energy performance" and "smart readiness"; the "connection to the renovation roadmap"; and the "link to external databases." These five functionalities are highlighted dark grey in Table 8, as the most important ones already identified. Table 8. Functionalities of the Digital Building Logbook. The functionalities considered by all the DBL initiatives are highlighted dark grey, and those considered by three of them are highlighted light grey.

Functionality
iBRoad-Log ALDREN BuildLog X-Tendo Logbook Study EU DBL Repository and checklist of building data X X X X Operation and maintenance plan X X X Energy performance assessment X X X X Health and well-being assessment X X Building analysis and status X X X Alerts and reminders X X Link to the renovation roadmap X X X X Link to external databases X X X X Smart readiness assessment X X X X Lifecycle costing X Comparison with similar buildings X X Integration with BIM/3D models X X X Link to financial incentives X X Best practices repository X Traceability of building materials X X Bringing together building sector stakeholders X X X Improved overview of the building stock (benchmarks) X X Construction project management tools X RES potential (benchmarks) X In the next level, four functions were considered relevant for three of the logbooks: containing an "operation and maintenance plan"; providing the "building analysis and status"; "integration with BIM/3D models"; and "bringing together building sector stakeholders." These four functions are highlighted light grey in Table 8.
In contrast, "health and well-being assessment," "alerts and reminders," "life cycle costing," "comparison with similar buildings," "link to financial incentives," "best practices repository," "traceability of building materials," "improved overview of the building stock," "providing construction project management tools," and "establishing RES potential," were considered potential functionalities for only one or two logbook proposals.

Operation and Use
Once the data template and sources for the DBL are defined, it is time to design the way it will work. At the moment, three different approaches have been identified. On the one hand, data can be gathered and introduced manually to a worksheet. On the other hand, the information can be automatically collected and displayed from existing databases. A third option is a hybrid approach, where the previous methodologies are combined.
The iBRoad-Log is an example of the last approach, where three types of data integration were envisaged:

1.
Through an initial load, the building-related information is inserted from existing databases. In this process, the data is sent by the external entities in text format and is later inserted into the chosen database.

2.
Some information is inserted during the lifecycle of the building manually by authenticated users, such as the homeowner or energy auditors.

3.
Finally, some information from third parties is necessary for the implementation of the solution, such as users' authentication/validation information or some buildingrelated data [31].
The ALDREN BuildLog settled in two versions-digital extended, composed by an Excel file, and paper portable by printing a Word file-is composed by worksheets that currently are fed manually by trained experts. Nevertheless, a direct link to active databases and other sources has been considered and may be developed in future research. The ALDREN BRP is complementary to EPCs and other certification schemes; thus, it needs to be permanently updated and linked with these tools [38].
The creation of the ALDREN BRP follows a five-step procedure [41], where the first three correspond to the creation of the DBL:

1.
First interview between the auditor and the homeowner and selection of the modules for the logbook (the selection of the modules depends on the needs and investment).

2.
Building inspection carried out by the auditor, survey with the owner and collection of all documentation. 3.
Calculation, by the auditor, of the selected modules and integration of the indicators.
Finally, the digital and paper versions of the BRP are generated and delivered to the owner. The data architecture of the X-tendo logbook will be described in forthcoming reports. However, it will be based on the link between different existing databases by using common protocols and interfaces [35]. At the moment, the same three approaches for data linkage of the Study on the Development of a European Union Framework for Buildings' Digital Logbook are being considered and will be further studied.
Finally, following the criteria exposed in the Renovation Wave, the Study on the Development of a European Union Framework for Buildings' Digital Logbook understands the DBL as a gateway that allows data storage and exchange between several data repositories (EPC registries, Level(s) framework, smart readiness indicator, etc.). However, to enable interoperability and consistency between databases, it is necessary to establish a semantic data model of the core DBL elements and the use of common languages [24]. At this point, three approaches have been considered to accomplish the linkage of the databases [18]: The DBL as a database with physical storage.

2.
The DBL as a digital gateway to information. 3.
Hybrid versions.
To further explore this topic, the study recommends carrying out a technical study on technical linkage mechanisms and guidelines that should include the following tasks:

1.
Technical requirements to allow the linkage of databases. 2.
Data properties.

3.
Creation of new market opportunities.

Conclusions
The Digital Building Logbook is a digital repository of all the relevant data on a building collected throughout its lifecycle. This paper identifies and compares the existing European scale DBL initiatives: iBRoad-Log, ALDREN BuildLog, X-tendo logbook and the Study on the Development of a European Union Framework for Buildings'Digital Logbook according to seven parameters: References used as a starting point for the model definition; Identification of the relevant stakeholders in the DBL; Identified potential user needs; Proposed structure; Data sources; Potential functionalities; and Operation and use.
Concerning the stakeholders involved in the DBL, based on the studied initiatives, it can be concluded that the most important agents are the building's owner and occupants, public authorities, and financial institutions.
Through participatory processes with potential users of the DBL, their main needs and priorities regarding the logbook were identified. According to the users, four data topics were pointed out as the most important ones: "building's features/description," "technical specifications of walls, windows, insulation, etc.", "energy use, water consumption and energy bills," and "Energy Performance Certificate." Other subjects, such as "basic information about the house," "building material inventory," "condition of walls and roof," "energy renovations completed," "design and plans on building interventions," "building plans," "specifications and designs of heating system and other equipment," and "age of equipment," were considered relevant too.
A Digital Building Logbook consists of a series of data fields or indicators about a building, classified in different modules or categories. To compare the structure of the four DBL proposals, the indicators of each one were classified by the same categories, defined by the authors of this paper: (1) General and administrative information, (2) Construction information and materials, (3) Building technical systems and smart readiness, (4) Energy performance, operation and use; and (5) Finance. The analysis shows great differences among the structure of the indicators in the four logbook models. Whereas in the iBRoadlog, most of the indicators (65%) are in categories 1 and 4, in the ALDREN, they are in 1 and 3 (78%), in the X-tendo logbook in 2 and 3 (62%), and in the Study EU DBL in 1 and 2 (58%). The fifth category had the lowest number of indicators in three of the models. Additionally, the absolute number of indicators varies from 68 in the iBRoad-log to 214 in the X-tendo logbook. The numbers of common indicators proposed by the four models (4 out of 438) and by three of the models (21 out of 438 indicators) are very low, representing only 6% in total. Therefore, we can conclude that there is no consensus about the indicators to be collected.
Identifying the sources that collect and may provide the data for the DBL is also crucial for the success of the tool. According to the studied initiatives, the most popular data sources according to the four DBL initiatives are the building users, the construction industry, public authorities, financial institutions, qualified experts/auditors, utility companies, existing databases and ICT technologies.
The functionalities of the logbook refer to the services built around it. In this sense, all the proposals agree that the main function of the tool is to serve as a repository of the relevant information of a building. Besides, four more functionalities were pointed out: the assessment of the energy performance and smart readiness, the connection to the renovation roadmap, and the link to external databases.
Regarding the operation and use of the logbook, none of the proposals has defined a procedure to automatically gather, store and share the information.
Thus, the first step towards the definition of a European Digital Building Logbook has been taken thanks to the emerging initiatives. Nevertheless, there is still a long way to go to synthesize all the information collected and translate it into a cost-effective, useful model for all the involved stakeholders. In fact, there is no consensus about crucial subjects, such as the indicators to be collected or how to collect them. This is probably due to the fact that the final functionalities (objective and scope) that the logbook should provide have not been fully decided, nor have they been linked to the indicators.
The authors of this paper think that the indicators should be reduced to the minimum number in order to achieve the maximum simplification of the use of DBL, avoiding information noise and potential obstacles that may prevent massive data collection. In order to identify which are the essential indicators to collect, the authors think that the DBL functionalities should be more clearly defined. The DBL should contain a useful data set for the definition of a building renovation roadmap, which guides stakeholders and triggers renovation works [38]. As buildings get renovated, the progress towards the building sector decarbonization must be monitored, as stated in Directive (EU) 2018/844 [14]. The decarbonization process should follow the European goals established in the different strategies, such as the Renovation Wave, the Green Deal, etc. We propose to simplify the indicators set, walking the road backward, from the European goals to the DBL indicators [45] (Figure 2). We will first identify the European goals for the decarbonization of the building sector and then link progress indicators to specific goals. Afterward, we will identify how renovations should be to fulfill the goals. This will be useful to identify the functionalities of the DBL. Finally, DBL indicators will be selected for these functionalities. Future research will focus on developing this scheme (Figure 2).
Regarding the operation and use of the logbook, none of the proposals has defined a procedure to automatically gather, store and share the information.
Thus, the first step towards the definition of a European Digital Building Logbook has been taken thanks to the emerging initiatives. Nevertheless, there is still a long way to go to synthesize all the information collected and translate it into a cost-effective, useful model for all the involved stakeholders. In fact, there is no consensus about crucial subjects, such as the indicators to be collected or how to collect them. This is probably due to the fact that the final functionalities (objective and scope) that the logbook should provide have not been fully decided, nor have they been linked to the indicators.
The authors of this paper think that the indicators should be reduced to the minimum number in order to achieve the maximum simplification of the use of DBL, avoiding information noise and potential obstacles that may prevent massive data collection. In order to identify which are the essential indicators to collect, the authors think that the DBL functionalities should be more clearly defined. The DBL should contain a useful data set for the definition of a building renovation roadmap, which guides stakeholders and triggers renovation works [38]. As buildings get renovated, the progress towards the building sector decarbonization must be monitored, as stated in Directive (EU) 2018/844 [14]. The decarbonization process should follow the European goals established in the different strategies, such as the Renovation Wave, the Green Deal, etc. We propose to simplify the indicators set, walking the road backward, from the European goals to the DBL indicators [45] (Figure 2). We will first identify the European goals for the decarbonization of the building sector and then link progress indicators to specific goals. Afterward, we will identify how renovations should be to fulfill the goals. This will be useful to identify the functionalities of the DBL. Finally, DBL indicators will be selected for these functionalities. Future research will focus on developing this scheme ( Figure 2).