Smart Green Prefabrication: Sustainability Performances of Industrialized Building Technologies

: There is an urgent need to increase the environmental, economic, and social sustainability of buildings. Indeed, construction has one of the lowest rates of sustainability among productive sectors, associated with high energy demand and pollutant emissions, frequent cost increase and time delays, and poor and unsafe working conditions. Building prefabrication is a construction tech ‐ nique that can enhance the sustainability of buildings, in terms of predictability, product and pro ‐ cess quality, and increased safety for workers. Recently, new approaches and concepts such as In ‐ dustry 4.0 (Construction 4.0) and circularity of resources emerged in the field of prefabrication to potentiate the benefits of off ‐ site construction. In this scenario, the scope of the work is to analyze the state of the art in the field of prefabricated building technologies in the light of these innovations and to evaluate their performances from a sustainability perspective. The work has been developed in two phases: (1) analysis of 13 case studies of prefabricated technologies in Europe; (2) compara ‐ tive assessment of their sustainability performances according to 21 qualitative parameters. Based on the results of the work, a set of guidelines is proposed as the outcome, i.e., suggested strategies and approaches for designers and industry professionals that can be used to enhance the sustaina ‐ bility of prefabrication.


Introduction
Since the beginning of the 19th century, building prefabrication has emerged as an alternative strategy to traditional construction. Prefabrication is a construction technique according to which parts of the building are assembled and completed inside a factory, then transported and installed on-site [1]. The degree of prefabrication varies from the assembly of structural components to the integration of finishes in prefabricated elements (fixtures, cladding); the highest degree of prefabrication are three-dimensional modular units, i.e., building sections completed in the factory with finishing and technical systems already integrated. The use of such techniques has been proven to have several benefits for the building process [2][3][4]:  Predictability, because construction activities are led within a controlled environment where it is possible to carry out tight quality controls over prefabricated components;  Time and cost reduction, because construction activities are no longer affected by external conditions (e.g., weather, site accessibility, etc.)  Increased safety because workers carry out their activities in a safe and protected environment;  Reduction of the impact of the construction site on the surrounding activities.
Stakeholders that are involved in prefabricated construction (public administrations, designer, private companies, etc.) have to deal with increasing concern about the sustainability of building products and processes, which have been addressed by some recent international policies and programs: Nevertheless, prefabricated construction is proving slow in achieving these goals, and the Architecture, Engineering, and Construction (AEC) sector has one of the lowest sustainability rates compared to other manufacturing activities [5].
From an environmental point of view, construction is responsible for one-third of global energy consumption and CO2 emissions [6,7]. In Europe, buildings account for 40% of energy consumption, and the percentage is expected to grow in the upcoming years [8]. In Italy, in 2001-2015, the construction sector saw increasing growth in electricity consumption and pollutant emissions [9].
Concerning economic sustainability, the sector is showing generalized inefficiency and unpredictability of products and processes [10]; this is a consequence of the high degree of complexity of activities and phases, competencies, and actors involved, resulting in frequent cost increases and delays. It is calculated that, on average, the time initially estimated for a project extends by 20% causing an exceedance in costs of up to 80% [11].
Finally, regarding social sustainability, the AEC sector shows the highest rates of accidents in the workplace [12] as well as the highest costs associated with work-related injuries [13].
Recently, some new concepts introduced in the prefabrication sector have demonstrated their potential to increase the sustainability of building products and processes. Firstly, the innovations engendered by the Fourth Industrial Revolution triggered the transition of prefabrication towards industry 4.0 models (smart factory). Industry 4.0 concept has been firstly conceived in Germany in 2011 at the Hannover Fair Event [14] and means the exploitation of cyber-physical systems for the automation of manufacturing. The term refers to a general approach that can be implemented within any production sector [15] with automotive and aerospace sectors being the first in consolidating its principles. In prefabrication, the concept goes under the name of Construction 4.0 and hinges on the automation and digitalization of design-manufacturing, i.e., a "file-to-factory" approach [16]. Such a method is enabled by the use of digital technologies such as artificial intelligence (AI), augmented reality (AR), the Internet of things (IoT), and building information modeling (BIM) [17,18]. In manufacturing, it also implies the use of high-precision equipment, such as 3D printers and computerized numerical control (CNC) machines [19,20]. This allows the establishment of direct communication between the different building phases and actors involved, optimizing resources (materials, time, costs) according to a lean approach [16,21,22].
Secondly, recent industrial and academic research developed innovative building materials, characterized by high performances and-at the same time-reduced life-cycle impact [23]. For instance, some experimental research exploited the use of "unconventional" materials as cardboard, cork, or composite materials with vegetable fibers for building applications [24][25][26]. However, the examples are not consolidated in practices or available on the market, and their effectiveness as building materials has yet to be validated through long-term experimentation.
In the described scenario, the scope of the work is to analyze the state of the art in the field of prefabricated building technologies in light of the aforementioned innovations and to evaluate their performances from a sustainability perspective. Specifically, the question addressed is as follows: Research question (RQ)-Which product-process innovations in prefabricated buildings enhance sustainability?
To answer this question, the work is developed in two phases: 1. Analysis of 13 case studies of prefabricated building technologies in Europe; 2. Evaluation and comparison of their performances according to sustainability qualitative parameters.
Finally, the result of the work led to the definition of a set of guidelines, i.e., suggested strategies and approaches that are intended for designers and industry professionals in the field of prefabrication that can be used to increase the sustainability of prefabricated construction.

Materials and Methods
The methodology adopted to answer the research question is divided into two phases ( Figure 1): 1. Analysis of 13 prefabricated building processes of modular units and prefabricated panel systems developed in Europe. The analysis has been carried out through three subsequent phases, namely: 1.1. Definition of the investigation sample: the sample includes 13 prefabrication technologies, divided into three macro-categories according to the classification of the MHCLG Joint Industry Working Group (UK) [27]. The prefabrication technologies analyzed are listed in Table 1 and their description is provided in Appendix A; 1.2. Definition of analysis criteria and preparation of a data collection sheet. A total of 11 qualitative criteria have been identified (Table 2) and divided into three investigation categories:  Technology, referring to construction features of the building;  Materials, referring to the type of resources employed;  Manufacturing, concerning production techniques and methods.
Criteria selected for the analysis refer to common characteristics of prefabricated technologies, that is to say, they do not depend on specific organizations and/or commercial strategies adopted by manufacturers. To facilitate data systematization a synthetic filing sheet model has been developed; the sheet is structured in four sections according to the criteria categories that are being analyzed (   Table 1, column (c). Information collected through the bibliographic research has been integrated by the direct experience of the authors who acquired, in the field, supplementary information that was necessary to complete the survey.
2. Assessment and comparison of 13 prefabricated technologies. The evaluation is developed according to 21 qualitative parameters of environmental, economic, and social sustainability (Table 3, column (b)). The parameters describe expected performances related to sustainability, i.e., product-process features that are proven to be directly linked to the achievement of sustainability goals (Table 3, column (a)) [3,22,25,[28][29][30][31][32][33][34][35][36]. For each technology, the achievement of the identified parameters has been verified through an analytical comparison of information and data collected in Phase 1. Hence, the results of the evaluation are reported in a summary matrix (Table 4) and discussed in section 3.
Finally, to summarize the research results, the work developed a set of guidelines for the field of prefabricated building technologies (section 4). The guidelines synthesize the strategies that, according to the results of Phase 1 and 2, have been demonstrated to enhance the sustainability of prefabricated building products and processes. The guidelines have been developed by the triangulation of information collected in Phase 1 and Phase 2, i.e., linking the current innovations of prefabricated technologies (analyzed in reading sheets) to the achievement of sustainability parameters described in Table 3, column (b).

MMC Category (a) [27]
Prefabrication Technologies (b) References (c) MMC Category 1: Pre-manufacturing, 3D primary structural systems A systemized approach based on volumetric construction involving the production of threedimensional units in controlled factory conditions before final installation. Volumetric units can be brought to the final site in a variety of forms ranging from a basic structure only to a structure with all internal and external finishes and services installed, all ready for installation T.01-Portable buildings [1,37] T.02-ISO Shipping containers [38,39] T.03-Light gauge steel modular units [40][41][42][43] T.04-Timber-framed modular units [40,42,43] T.05-CLT modular unit [40,42,43] T.06-Concrete modular units [40,43,44] MMC Category 2: Pre-manufacturing, 2D primary structural systems A systemized approach using flat-panel units used for basic floor, wall, and roof structures of varying materials which are T.07-SIP panels [40,45] T.08-Lightgauge steel panels [37,40,41] T.09-Timber-framed panels [37,40,42] T.10-CLT panels [37,40,42,46] T.11-Precast concrete panels [37,40] T.12-Cardboard panels [26,47,48] produced in a factory environment and assembled at the final workforce to produce a final three-dimensional structure MMC Category 4: Additive manufacturing, structural and non-structural The remote, site-based, or final workforce-based printing of parts of buildings through various materials based on digital design and manufacturing techniques (The category MMC 4-Additive manufacturing is not directly applicable to the concept of prefabrication, as it does not imply the assembly of components inside a factory. Nonetheless, for this work the technology T.13-3D printing is analyzed because the scientific literature promotes additive manufacturing as one of the 'enabling technologies' (Key Enabling Technologies, KETs) for industry 4.0 [15].) T.13-3D printing [49][50][51]

Degree of prefabrication
Describes the maximum degree of prefabrication of the building components, (i.e., the percentage of completion of the work inside the factory evaluated compared to the final building

Flexibility and integration
Analyzes flexibility features of the technological system (i.e., the capability to be integrated with technical systems and/or other technologies and to be adapted during the time: additions, selective demolitions, replacement, etc.)

Transport and installation
Analyzes the technological features related to the transport and installation of building components and parts, that is to say, the prefabrication strategies adopted to reduce transport and installation costs, identifying critical areas Describes the methodological approach and operational strategies adopted for design and manufacturing management Manufacture processing Analyzes the type of processes and equipment used for sub-components and/or components manufacturing and their assembly as well as the productive chain organization

Manufacturing environmental impact
Describes the environmental impact of manufacturing processing regarding energy demand and pollutant emission generated

Customization approach
Describes the approach underlying the product-process regarding the possibility to customize products (i.e., the degree of flexibility of the product-process compared to the need for personalization), as well as the commercial approach and the level of standardization of components (market offer) Figure 2. Reading sheet structure. Table 3. Sustainability goals and parameters used for assessment.

Results
The result of the first phase of the work is an updated catalog of current prefabrication technologies in Europe. The catalog consists of 13 analysis sheets (Appendix A), developed with a homogeneous and comparable structure that describes technological, productive, and material features of prefabricated technologies. The descriptive contents of the sheets summarize recent contributions and findings from scientific and industrial research.
Based on the investigation, in the second phase, we developed a qualitative assessment and comparison of sustainability performances achieved from each case study. As described in section 2 the evaluation is based on 21 qualitative parameters of environmental, economic, and social sustainability, i.e., performances directly related to the achievement of sustainability goals. Referring to each sustainability parameter, the main findings of the second phase are described as follows: Among the materials examined, steel (used in T.01, T.02, T.03, T.08) shows the best performances in terms of circularity. Indeed, the steel used for building components can be recycled and is recyclable at the end of its use, minimizing the need of processing raw material [37]. Besides, material circularity is intrinsically connected with the possibility to relocate and reuse building components and/or sub-components. From this perspective, steel technologies are those offering the highest degree of circularity; in fact, the high du-rability of the material and the use of dry-assembling (fastening) techniques allow repeated assembly and disassembly of modular units and/or steel-framed panels, maximizing building components circularity [1].
Nevertheless, the environmental sustainability of steel has to be read in the light of the high impact associated with the material production, as further described in SP. 10.
Similarly, concrete mixtures (used in T.06, T.11) can be obtained using recycled material and can be recycled at the end of the building use; however, its processing is also energy-intensive with associated high levels of pollutants emissions [40]. Moreover, concerning building circularity SP.03, i.e., the possibility to relocate concrete modular units or concrete panels, the analysis demonstrates that technology types T.06 and T.11 do not allow for multiple building uses, since modules and panels cannot be relocated but only disassembled and recycled at the end of the use.
Referring to prefabricated wooden technologies (T.04, T.05, T.09, T.10), the material shows a limited performance of circularity compared to the previous. In fact, despite the majority of suppliers being certified as sustainably managed sources, the material quality needed for building application requires the use of raw material [46]. Moreover, the overall circularity of wooden technologies is further limited considering SP.03, because wooden modular units and panels, once disassembled, cannot be re-assembled and relocated. That is a consequence of the type of fastening and sealing techniques used for assembly which compromises the structural integrity of components that cannot be reinstalled. Hence, wooden components resulting from disassembly can only be recycled, but the products obtained do not have the same properties, i.e., the material can be only downcycled.
The two best practices that emerge from the evaluation of material circularity are cardboard panels (T.12) and 3D printing (T.13). In the first case, prefabricated panels can be disassembled and relocated, and the paper used is supplied from recycled sources, and can be recycled at the end of the use [47]. However, the investigation shows that external treatments used for fire-proofing the panels can compromise the possibility for recycling; in this case, it is suggested that alternative technical solutions should be explored to verify legislation standards (fireproofing safety) while preserving future possibilities of recycling.
Instead, in the case of 3D printing (T.13), parameter SP.03 is not met because building components and/or parts cannot be reused once disassembled. Rather, circularity is strictly related to the type of material used as the binder. Recent applications have demonstrated the feasibility of using nature-based material as raw soil or natural fibers, which can achieve high building performances (strength, durability, thermal performances) and can be entirely recycled at the end of the use [51]. The majority of case studies demonstrate that design-manufacturing methods used for prefabrication can enhance product-process quality control compared to traditional methods of construction. Specifically, this possibility is always enabled by the use of fileto-factory approaches (Industry 4.0/Construction 4.0), i.e., the use of BIM and CAD-CAM (computer aided design-computer aided manufacturing) software to manage design and manufacturing phases. Thanks to the use of the aforementioned tools it is possible to directly transfer design specifications to production equipment, without losing information and minimizing the possibility of errors. Furthermore, quality control is enhanced by the use of CNC machines, which guarantee high precision and tight tolerances in the final product [16].
From a sustainability perspective, the achievement of SP.04 and SP.05 ensure product and process predictability, that is to say, control over performances, time, and cost of prefabricated buildings.
Concerning T.02-ISO Shipping Containers and T.07-SIP panels, this possibility is limited because off-site prefabrication for the mentioned technologies concerns structural components only (respectively, steel skeletal frame and composite panels) requiring additional assembly outside the factory to complete the building.
For T.12-Cardboard panels, the analysis shows a limited capacity to control both product and process quality; this is a consequence of a missing direct communication between design and manufacturing. Indeed, design is currently led by a computer-aided design (CAD) approach, while manufacturing is performed manually by workers, with a high margin of error in terms of product, time, and cost. Among the case studies, those that did not verify the SP.6 parameter are those adopting made-to-stock (MtS) or assembled-to-stock (AtS) commercial approaches (namely T.01, T.02, and T.07). These refer to stocking building components or building "kits" in the warehouse ready to be sold according to customer demands [40]. Although these approaches are justified by high market demand for portable buildings and containers (for other uses as construction sites or temporary emergency settlements), MtS and AtS have impacts on economic sustainability, since stocked material needs to be properly maintained. In this case, improved approaches are assembled-to-order and made-to-order, i.e., the production of building components according to customer demands. In such a way, the accumulation of unutilized material is avoided with advantages for both environmental and economic sustainability [21].
Regarding SP.7, the technologies that demonstrate better material optimization are steel (T.01, T.02, T.03, T.08) and cardboard prefabrication (T.12). Specifically, this is a consequence of the type of sub-components used as the input for manufacturing. Indeed, in the first case, steel coils are continuous strings that are shaped and cut according to panel dimensions, with minimum material waste. Similarly, for prefabricated cardboard panels, manufacturing is based on cardboard sheets purchased (from external sub-suppliers) according to panel dimension; that means once sheets are folded and glued wastes of material are minimized. The most intensive manufacturing processes are those associated with steel and concrete (T.01, T.02, T.03, T.06, T.08, T.11), which require high energy demand and produce high pollutant emissions. On the contrary, wooden-based technologies (T.04, T.05, T.06, T.07, T.09, T.10) show limited environmental impact related to manufacturing; moreover, the material itself has the capacity to stock CO2 and contributes to reducing the overall impact of the life-cycle [46]. Similarly, cardboard manufacturing has been proven to involve low-energy processing, even if specific investigations regarding the entire manufacturing process of cardboard panels are not available in the literature. As already proven by the literature presented in Section 1, prefabricated technologies demonstrate an increase in the speed of construction compared to traditional methods. Based on quantitative data, time reduction is emphasized for modular construction (MMC Category 1, T.01-T.06), varying between 50-70%, while is reduced to 20-30% for panelized systems (MMC Category 2, T.07-T.12). Nonetheless, when evaluating the overall technical performance of speed of construction, it must be taken into account that time reduction can be influenced by specific construction requirements, as better described in SP.12.
Concerning T.13 (3D printing), the achievement of SP.11 could not be verified because no quantitative data have been found in the literature to compare the speed of construction with traditional technologies.  Ease of construction:  SP. (12) Building components can be easily handled and installed by workers without using crane equipment  SP. (13) Stocking and installation of components do not require specific site protection measures against weather The possibility to install building components without using heavy-duty crane equipment ensures adaptability to different urban and social contexts, as well as reduces the total cost of construction from an economic sustainability perspective. However, only a few technologies can respond to this need, specifically, T.01, T.07, and T.12 (Portable buildings with a "flat-packed" option, SIP panels, and cardboard panels). In fact, in these cases, the lightweight building components make it possible for a few workers to assemble a one-story building without using complex equipment [45,47].
Conversely, innovative technologies such as 3D printing (T.13) require highly specialized professionals who can set and control printing machines, with a proportional increase in construction cost and the consequent reduction of economic sustainability [49].
Concerning SP.13, as already mentioned in SP.11, the analysis shows that the majority of the technologies (T.01, T.02, T.03, T.06, T.08, T.12, T.13) require the installation of a specific protective structure in the construction site, to preserve building components from weather, resulting in a more complex site and construction planning, and an increase in cost. For this reason, and especially for large-scale projects, an accurate organization and scheduling of construction activity is paramount, to optimize transport, times, and costs.  Economy of construction:  SP. (14) Building components are optimized for transport For the scope of this work, the unit costs of technologies have not been evaluated, since such parameters are strictly dependent on the installation context. The economy of construction is here related to the transport of building components, i.e., their optimization in the perspective of reducing costs. With these premises, the investigation demonstrated that panelized systems (MMC Category 2, T.07-T.12) better optimize transport from an economic perspective; in fact, the possibility of stacking panels minimizes the need for multiple transports, as well as the economic impact of transport from the factory to installation site [37].
Conversely, for prefabricated modular units (MMC Category 1, T.01-T.06) the need to transport empty volumes increases the economic impact of transport, especially for exceptional loads and long-distance locations. Moreover, the investigation shows that the bearing structure of modular units needs to be over-engineered to support dynamic loads during the transport, causing additional costs for the project. Regarding this criticism, an effective solution has been developed for technology T.01 (Portable buildings); portable units can be delivered in a "flat-packed kit of components", i.e., disassembled units that can be stacked as panelized systems to reduce the economic impact of transport [37]. Case study comparison shows that panelized systems (MMC Category 2, T.07-T.12) allow for enhanced adaptability in terms of the spatial configuration of a building (SP.16) since panels can be aggregated in different and flexible space layouts. Nevertheless, evaluating SP.18, i.e., the possibility for further changes in the functional program, it must be noticed that load distribution in panelized systems represents a constraint for further modification of spatial layout [1,37,40]. Such features have to be taken into account starting from the design stage to leave maximum space for adaptation; one effective strategy is to conceive space aggregation as a series of repeatable and independent units that can be added or removed according to the variation of user needs.
For this reason, modular buildings (MMC Category 1, T.01-T.06) perform better in terms of space adaptability. In fact, despite space configuration being initially limited by dimensional constraints, the structural functioning of modules, which is mutually independent, allows for further building adaptation; compatibly with technological limitations explained in SP.03, modules can be added and/or removed to accommodate changes in the space program without compromising the integrity of the building as a whole [1].
Regarding SP.17, i.e., the possibility to modify components after manufacturing, the investigation proves that the majority of case studies do not allow for further modifications (T.01, T.03, T.04, T.08, T.09). In the case of T.01 (Portable buildings), this is a consequence of product standardization as a commercial approach, while in the other cases results from intrinsic technical constraints. For instance, in CLT technology the structural behavior of the panel is closely related to its shape (dimensions, openings) and integrity, and only slight changes can be made to the final product. For concrete technologies, the shape and dimensions of panels and/or monolithic modules must be defined before casting, and again only minimum variations can be integrated during and after manufacturing. Such limitations not only require high control during the design phase, but also a tight collaboration and dialog with customers to ensure the final product corresponds to initial ideas.
Concerning manufacture adaptability (SP.19), the majority of case studies show the possibility to reconfigure the production chain to adapt final products. Customization varies from choosing within a catalog of pre-determined options (e.g., standard set of dimensions, range of finishing, etc.) to bespoke design solutions. This is enabled by a lean management model for production, as well as the use of an automatized design-production chain, which enables direct communication between different design specifications and processing equipment [18]. For T.12 (Cardboard panels), the parameter is not met because design-manufacturing is tailored for each customer; design specifications are elaborated for each project individually and panel manufacturing is currently led manually by workers that have to adapt the manufacturing process to executive design. Instead, for prefabricated concrete technologies (T.06, T.11), the possibility for easy reconfiguration is limited because of the need to adapt molds, which is a cost and time-intensive process. Despite all prefabrication technologies contributing to improving working conditions and safety, the majority of manufacturing processes still require substantial interaction with workers, and their contribution is mainly focused on sub-component assembly. This condition is exacerbated in T.12 (Cardboard panels), since panels are manufactured manually by workers, exposing them to a higher risk of injuries. In this scenario, technologies that satisfy SP.20 are limited to CLT prefabrication (T.05, T.10) and 3D printing (T.13); in these cases, workers' contributions are limited to machine setting, control, and intervention in cases of malfunction [50,51]. Moreover, recent IT technology for industry 4.0, and above all machine learning, have made it possible for equipment to run self-repairing actions, reducing the need for human interaction within manufacturing.  Inclusion and collaboration  SP. (21) Design adopts an "open system" approach to ensure customization and integration of different specifications (structural, energetic, costs, etc.) The adoption of an "open system" approach concerns those technologies that have a higher degree of customization, such as T.02-T.05 and T.08-T. 10. In these cases, the technological system is organized into "hard" components, i.e., standardized structural elements, and an open set of solutions to personalize the envelope (wall and roof), windows, external, and internal finishing. The interaction with different stakeholders (contractor, suppliers, clients, etc.) and between design phases (structural, architectural, energetic, etc.) varies according to the strategy adopted by the company, and, thus, the achievement of social sustainability performances related to "collaboration" could not be generalized to all the case studies.

Discussion
The sustainability of the AEC sector is an increasing concern for international policies and private and public professionals and it is sustained from investments focused on the reduction of the environmental, economic, and social impact of buildings. From this perspective, stakeholder design choices, implemented in different phases of the building process, are paramount to determine overall building sustainability.
Building prefabrication can help meet sustainability goals in construction and find renewed potential in light of recent concepts as Construction 4.0 and circular materials. These innovations in the field of prefabrication led to the development of new technological systems, with enhanced performances and quality. Nonetheless, designers and actors involved in the building process have to be guided, from an early stage, by proper decision-support tools to gain awareness of the effect of design and manufacturing strategies. The literature investigation did not provide an exhaustive map of existing prefabrication technologies that analyze the effect of technological, material, and manufacturing options for the sustainability of the building life-cycle. In fact, some recent research [3][4][5]24,29,32,35,36,43] identified qualitative and quantitative parameters affecting the sustainability of building processes; however, these studies focused on analyzing the effect of single design and manufacturing choices, without evaluating the mutual contribution of the strategies on the final sustainability of buildings.
From this perspective, the work contributes to the open debate with an updated analysis of prefabrication technologies in Europe that evaluates their sustainability performances considering the synchronous effect of technological, production, and use of materials strategies.
The first phase of the work developed an updated state-of-the-art investigation in the field of building prefabrication through 13 case studies analysis. Then, to evaluate their sustainability performances, the second phase produced a comparative assessment according to 21 qualitative sustainability parameters. The results of the work make it possible to answer the research question (RQ), i.e., to define which innovations and strategies enhance the sustainability of prefabricated buildings (Figure 3). The answer to the research question is not univocal, but is a complex set of connected design and manufacturing choices, the effectiveness of which have to be assessed in the light of the context and design requirements. For this reason, and given the complexity of the variables involved in buildings sustainability, to answer the RQ, a system of interrelated options was developed, summarizing strategies and design alternatives that have to be considered starting from the early design stage to achieve the highest degree of sustainability. The identified strategies are described in the form of suggested guidelines, and are linked to the achievement of the sustainability goals defined in Table 3, column a. The strategies can be grouped in four major areas of decision, each of which summarizes the main findings from the analysis:  S.1-Material and technology The use of prefabricated steel technologies help to achieve the sustainability goal of circularity of building components and material resources; for this reason, their adoption should be preferred for buildings with short but repeated cycles of use, such as transitional and/or emergency accommodation for student housing, temporary housing for workers, schools, and events. Such a strategy ensures maximizing steel durability and reduces the high environmental impact associated with production. On the contrary, the use of wooden and concrete prefabrication proves to have reduced circularity over building components and material resources; consequently, these technologies are demonstrated to be more suitable for long-term installation to exploit their potential before downcycling.
Finally, the use of cardboard shows high sustainability potential in a circular perspective, but more research efforts are needed to exploit its performance during the life cycle. Regarding this, open questions emerged from the analysis concerning its durability and the development of fireproofing solutions that allow paper recycling. 

S.2-Assembly and construction
Starting from the design phase, the integration of low-complexity assembling techniques proves to meet sustainability goals, such as economy, ease, speed of construction, adaptability, and integration. Moreover, an accurate design of the assembly and sealing methods potentiates circularity performances, allowing for multiple assembly and disassembly of building components and/or material selection for final recycling. Suggested improvements concern the development of assembly techniques that enable the circularity of building components (such as modular units or panels) and material resources, and particularly for wooden and concrete prefabrication that showed critical performances in this field.

S.3-Transport
The impact of transport plays a key role in the economic and environmental sustainability of prefabricated buildings, as it can increase the cost of construction and the amount of pollutant emissions. For this reason and given their impact, the use of modular buildings is more suitable for small-scale buildings requiring a limited number of units. For larger-scale projects, the use of modular buildings should be evaluated together with the availability of local manufacturers. Otherwise, an alternative is the manufacturing of sub-components that are further transported and assembled in a second location, close to the final installation site.  S.4-Industrialized manufacturing The transition of prefabricated technologies towards industrialized smart manufacturing (Industry 4.0/Construction 4.0) potentiates the benefits of prefabrication within design and manufacturing and guarantees the achievement of sustainability goals as product-process predictability and adaptability, reduction of material wastes, work safety and health for workers, inclusion, and collaboration between different actors. On the operational level, this requires a shift from traditional approaches to design and manufacturing, implying a substantial increase in the complexity of technical and procedural design variables, in terms of information, stakeholders, and competencies involved.
In this case, design has to embrace a design for manufacture and assembly strategy to facilitate prefabrication of building elements, i.e., to reduce changes from concept to executive design. In this scenario, emerging IT technologies for design-manufacturing management have innovative potential for construction, offering the possibility to control, simulate, and verify building performances alongside the process as well as to communicate design specifications between different phases. Among these, BIM-based (building information modeling) approaches emerged as a method to integrate design, performance simulation and verification, and manufacturing-related information. Moreover, the proposed strategy is to develop a tight collaboration between designers, manufacturers, and suppliers starting from the early phases of design, to streamline the design-manufacturing process (cost and time reduction), optimize design specification, and minimize wastes.
The identified guidelines represent a set of suggested strategies to be adopted from designers and manufacturing professionals in the prefabrication sector to address the sustainable transition of the AEC sector. Indeed, starting from the investigation of current prefabrication technologies in Europe, the work established a link between the implementation of the described strategies and the achievement of sustainability goals.

Conclusions
This research defines a decision-support tool for designers and industry professionals involved in the prefabrication sector. The results of the evaluation led to the identification of a set of guidelines that support stakeholders in evaluating the effects of design and manufacturing choices on the final sustainability of the building process. Moreover, the work identifies effective strategies to enhance the sustainability of the building process considering specific design and manufacturing requirements. Furthermore, starting from the qualitative assessment, directions for future research in specific technologies have been identified to improve critical aspects detected during the investigation.
In the first stage of this work, a qualitative research approach has been adopted to provide a scientific framing of the complexity of the variables involved. Starting from the research results, further development of the work concerns the evaluation and comparison of sustainability parameters according to quantitative data. The implementation requires the definition of assessment quantitative parameters, starting from existing literature [4] as a base, and their application to case studies, i.e., prefabricated building designs that can be compared. The goal is to contribute to recent research works defining quantitative assessment benchmarks to evaluate the environmental, economic, and social sustainability of current prefabricated technologies available in Europe.
A further implementation of the work involves the development of a web catalog based on the technologies inventory (Appendix A); the tool will be connected to the opensource libraries made available from manufacturers, to provide information to designers and stakeholders about the performance and sustainability characteristics of their technologies. This represents a further step toward providing building stakeholders (design, manufacturers, decision-makers) with a decision-support tool to evaluate the sustainability of their choices starting from the early phases, as well as to project their effects within the building life-cycle.

Conflicts of Interest:
The authors declare no conflict of interest.