Development of a Plug-In to Support Sustainability Assessment in the Decision-Making of a Building Envelope Refurbishment

1. Introduction

Existing scientific studies provide clear evidence of the role played in the climatic crisis by the built environment, in that it emits 40% of greenhouse gas emissions worldwide [1]. Future tendencies indicate extreme climatic situations and a substantial depletion in biodiversity if no measures are implemented for their reduction [2]. Radical changes are therefore required in order to alter the ways of designing and conceiving our buildings and the built environment.

In the European context, strategies, such as the Green Deal [3] and the Renovation wave [4], propose the progressive and absolute reduction of carbon emissions in order to achieve carbon neutrality by 2050, and suggest an increment in the building renovation and rehabilitation, as well as the increment of the data digitalisation along the building life cycle. These ambitious objectives imply the decarbonisation of all the economic sectors, including the construction and building sector, for both the embodied and operational carbon footprints. The embodied carbon footprint is related to the materials and products that are installed in the building, and includes processes such as manufacturing, construction, and transportation [5]. In the current context in Spain, instruments such as the Climate Change Law [6] encourage the use of materials with the lowest embodied carbon footprint possible. However, these measures do not include a quantitative procedure, nor do they involve the calculation of the embodied carbon. This calculation and its declaration are now being integrated in several European Countries, such as Sweden [7] and the Netherlands [8]. Its expansion across European countries has been planned for development in the next few years. Indeed, the new version of the Energy Performance of Buildings Directive, EPBD [9], proposes the calculation of the carbon emissions of the whole life of buildings for both the embodied and operational carbon footprints.

In order to calculate the whole-life carbon emissions, the Life Cycle Assessment (LCA) methodology is crucial, since the "carbon footprint is the sum of greenhouse gas emissions and greenhouse gas removals in a product system, expressed as CO₂ equivalent and based on this methodology (LCA) using climate change (CC) or global warming potential (GWP) as the only impact category" [10]. However, to implement effective measures for the reduction of carbon emissions from the building sector, it is also necessary to make these measures affordable (from the economic point of view) and positive for the community (from the social point of view). Hence, the scientific community [11,12] recognises the value of methodologies such as Life Cycle Sustainability Assessment (LCSA) in addressing the triple-dimension sustainability assessment. The LCSA methodology combines three methods based on the impact quantification throughout the building life cycle: Life Cycle Assessment (LCA) (environmental dimension); Life Cycle Costing (LCC) (economic impact); and the Social Life Cycle Assessment (SLCA) (social dimension) [11]. Given the increasing necessity to address building sustainability from a holistic perspective (by integrating the environmental, economic, and social dimensions), the building design phase is crucial, since it is the phase where it is the easiest and cheapest to incur design changes [13]. A study demonstrates that the design phase has a great potential to achieve accurate LCSA results during the early design stages in design tools such as Building Information Modelling (BIM) [14]. For instance, during this stage, it is possible to estimate more than 80% of the CO₂ emissions (environmental dimension), 60% of the costs (economic dimension), and 70% of the working hours (social dimension) of the total results for product and construction impacts (A1-A3, A5 information modules ISO 21931-1[15]) in the detailed design stage.

In recent decades, multiple studies [16,17,18,19,20,21,22,23,24] have demonstrated the increasing development of life cycle-based methods in the design process and in design tools such as BIM. The main developments focus on simplifying the assessment process and workflows, and on integrating different life cycle information modules (system boundaries) for the assessment of new buildings and building refurbishment. The application of life cycle-based methods to building refurbishment presents several particularities [25]. For example, not only must the life cycle inventory focus on the new materials and components [26], but it must also consider existing materials and components to correctly model the quantities of materials. This implies the control of building information in relation to new and refurbished elements. The number of existing studies [27,28,29] focused on the application of the life cycle approach to building refurbishment in BIM remains limited and includes the assessment of the economic and/or environment dimension(s), without considering the social dimension (S-LCA). For example, Dauletbek and Zhou [27] focus on a refurbishment of an existing residential building using BIM-enabled LCA and simplified LCC “considering environmental compatibility, energy efficiency, and profitability based on real construction and energy consumption data”. Tushar et al. [29] develop a BIM and LCA workflow to compare different precast materials for a building retrofit. Kim [28] conducts LCA studies based on different energy standards, and uses BIM to formulate refurbishment alternatives through a case study. The study also shows the correlation between LCA and LCC. Moreover, it should be borne in mind that the level of automation in the BIM-LCA application to these existing studies remains low. For instance, Dauletbek and Zhou [27] use WEBlca, a web application for the LCA that has not been integrated into the BIM methodology. Tushar et al. [29] use a combination of tools including Tally [30] for the LCA calculation, FirstRate5 for the regression analysis, and @RISK palisade for the Monte Carlo simulation.

On the other hand, the existing BIM-LCSA tools and advances [31,32,33] focus on new buildings. For instance, Figuereido et al. [32] develop a framework for sustainable material selection by integrating various software (such as Tally and Excel) and focus on a new building case study, which includes all the building elements in the LCSA and is limited to the A1 to A4 information modules. In Soust-Verdaguer et al. [31], one of the main limitations is the control of the building elements that comprise the system boundaries. The plug-in neither allows the identification of the existing and new building elements, nor that of the elements that belong to the building element systems. The BIM objects library is limited to the structure system elements, and therefore fails to consider the recommended thermal transmittance values of the envelope to achieve the required operational energy performance.

The authors’ previous studies are based on the development of assessment tools focused on new buildings [31], and hence this present study proceeds with the improvement of the latter’s development and explores its potential for the selection of building material in building envelope refurbishment.

To the best of our knowledge, no study has yet included the LCSA in the decision-making of building refurbishment stage and supported results in BIM in real-time. This study is therefore based on a plug-in development to conduct the LCSA of a BIM model to support the decision-making in the refurbishment process. The plug-in development aims to provide a solution to the existing limitations in the LCSA in BIM.

2. Materials and Methods

In order to achieve the aforementioned goals, this study proposes a tool development following the steps described in Figure 1 and based on the methodological framework validated in several scientific publications [18,31,33].

The tool development took previous development [31] into consideration to conduct the building structure evaluation based on the LCSA in BIM. The procedure to adapt said tool to the envelope building system consisted of a methodological adaptation of the LCSA to the building refurbishment assessment and a BIM-Object library enrichment. The methodological adaptation aimed to include the detected particularities to apply the LCSA to a building refurbishment as described in [26].

The BIM-TBL database is a BIM-Object library that includes alphanumeric information for the implementation of the LCSA methodology at the building element scale. The data referring to the environmental dimension (Carbon Footprint), the economic dimension (euros), and the social dimension (working hours) are organised according to the structure of the IFC Building Elements (IFC4) classes [34], and include the building elements that comprise the envelope system. The BIM model for the evaluation of the building includes the information of the building geometry and the quantification of the building elements thereof, at an intermediate level of development (LOD). The plug-in includes the LCSA calculation procedure in the BIM models and the visualisation of results in real-time developed in the native BIM modelling software: Autodesk Revit [35]. Lastly, the results and their discussion include the data extracted from the use of the plug-in to evaluate the BIM models and the building design alternatives. A brief discussion of the results in terms of operational and embedded aspects is also included.

2.1. Tool adaptation to the building refurbishment process

The plug-in definition focuses on reducing the effort, by simplifying processes and, and increasing the level of automation in the LCSA calculation in the building envelope refurbishment, thereby helping the designer visualise the best design solution (while considering environmental, economic, and social dimensions).

The tool development assumes that the definition of the BIM model geometry to be employed for the evaluation presents a relatively low level of development (LOD) [36]. Therefore, in order to apply the LCSA to the building envelope system, the alphanumeric information on the BIM objects that make up the BIM model incorporate information of a more detailed nature regarding the materials and products.

This enables the rapid evaluation of construction alternatives so that the designer is aided in their definition of the best solution that would subsequently be incorporated into the BIM model. In order to minimise the effort invested in the identification of the elements and to facilitate the verification of the system boundaries, the plug-in includes a limited list of building elements that comprise the envelope system, including the façade, windows, doors, roofs, and floors (see Figure 2).

To enable a quick and direct interaction of the user with the BIM model and the LCSA calculation, the plug-in includes a button for the manual selection (“Select Elements” button) of the building elements in the BIM model in 3D view. Moreover, to adapt the plug-in development to the specific aspects of the building refurbishment life-cycle techniques, the authors include, apart from the building elements filter (Figure 3 left-hand side), which is used to select the building elements that can potentially be included in the envelope, a field (“IsEnvelope”) to confirm those specific elements that are included in the envelope. For example, if the slabs are not in contact with the ground or the doors are not exterior. Moreover, in order to adjust the system boundaries to the building refurbishment LCA particularities [25], the plug-in displays the “Phase” of the construction process, which could be “Existing” or “Refurbish”, to filter the existing and new elements.

In Spain, specific tools and methods are employed for the calculation of the operational energy and that of the national regulations compliance [37]. Building parameters are considered, such as internal loads and operational conditions, climate zone, exterior conditions on inner and outer surfaces, building elements, thermal transmittance of materials, thermal bridges, the void factor, and transmission and radiation in opaque enclosures and the ground. From all these factors, the parameter that most influences the embodied impacts is that of the definition of the building elements and materials, especially regarding the thermal transmittance and opaque and transparent enclosures. Therefore, the building design in BIM can be limited to utilising only recommended transmittance values and recommended proportions of openings and transparent enclosures. To simplify the integration of the operational energy calculation in BIM, the plug-in includes a library of predefined solutions for building refurbishment (including walls, doors, windows, and roofs) that comply with the thermal transmittance recommended values [38] for the climate zone of the case study.

2.2. Case study description

The case study is an energy rehabilitation of a multi-family building of 36 social housing dwellings located in Camas (Seville), which was conducted in 2020 and promoted by the Housing and Rehabilitation Agency of Andalusia [39]. The real total area of the building is 2,686 m^2.. However, for the evaluation of the model and verification of the methodology employed, only a portion of the building was selected, as shown in Figure 4. Since the case study is a building envelope refurbishment, the verification of the usefulness of the tool focuses on the analysis of several construction alternatives.

The information to be included in the BIM model of the building has been classified in accordance with the LOIN concept [40], wherein geometric information and alphanumeric information of the objects that comprise a BIM model are recognised.

2.3. Tool application to the case study

The methodology used for the evaluation of the case study consists of the following steps (see Figure 5):

Step 1: Evaluation of solutions for the walls: the study compares three alternatives to determinate the best solution for this type of building element and adopts this solution before continuing onto the following step.

The three alternative construction solutions that have been selected for the rehabilitation of the façade are the following: Exterior Thermal Insulation System (ETICS), ventilated façade, and interior insulation (see Table 1). On the other hand, each of these solutions has been evaluated using two alternative materials for the insulation layer: EPS and Mineral Wool. The transmittance values for the three solutions are all the same (0.30 W/(m²K)). This value was used in the operational energy simulation to comply with the energy regulations [38] and is lower than the oriented values for this climate zone and type of element (climate zone B, element wall U value= 0.38 W/(m²K)). The supplementary data includes a detailed description of the materials and thicknesses.

Step 2: Comparison of solutions for the windows and doors of the case study, whereby the best solution is determined and adopted for the evaluation of the last element. For this purpose, three types of carpentry were considered for the windows, and two solutions for the exterior doors (steel and aluminium) and a combination of both solutions (see Table 2 and Table 3).

Step 3: Incorporation of the roof into the evaluation of the sustainability of the envelope to obtain the results of the complete envelope system. Finally, Table 4 shows the horizontal envelope taken into account for the roof.

The feasibility of this tool for the evaluation of the case study is largely due to the speed and simplicity of the procedure, which only requires the assignment of the codes corresponding to the BIM objects to be evaluated according to the characteristics specified in the BIM-TBL library [31,33] which are integrated in the plug-in.

2.3.1. Life cycle sustainability assessment applied to the case study

The scope of the building sustainability assessment is defined in the first instance by the type of assessment of the three main dimensions: environmental, economic, and social. Moreover, by considering the methodological aspects and assumptions in order to conduct the LCSA applied to the design phase of buildings in BIM, as previously addressed in [14,18,33], the scope of this assessment covers the following aspects: the definition of elements to be included in the assessment, the definition of the building life cycle phases to be included in the assessment, and the definition of the impact categories and indicators to be used for the sustainability assessment (Section 3.5).

2.3.2. Definition of building information included in the assessment

Figure 6 shows the information modules included in this study, following the criteria defined in previous work [14,18,33], together with the data availability and the relevance of the information modules [14]. Since the case study is a refurbishment and following the criteria established in previous work [25], the sustainability assessment focused on the new materials and processes incorporated in this phase, and excluded those existing in the building. For the evaluation, the building elements employed were those detailed in the previous section that are included in the BIM model in the Refurbished/Rehabilitated phase. The study compared the embodied aspects of each solution, including the information modules A1-A3, A5, B2, B3, B4, and C1 for the new materials, products, and components incorporated in the building envelope refurbishment.

2.3.3. Definition of building elements included in the assessment

The building assessment is based on the building envelope system and includes the IFC element classes listed in Table 5. The definition of the BIM objects that comprise the building corresponds to an LOD of approximately 200-300 [36]. This means that the elements included in the evaluation are modelled following the criteria detailed in Table 5, including the geometric information of the BIM model.

2.3.4. Definition of impact categories and indicators

Considering the three dimensions (environmental, economic, and social) the assessment included the following impact categories and indicators: Global Warming Potential (GWP) in CO₂ equivalent emissions, Costs (euros), Working hours (hours).

One of the contributions of the methodological development of the LCSA [16] and the BIM-TBL database is the harmonisation in the structure of environmental, economic, and social data given by the BCCA [41]. This database uses a systematic structure [42] to organise the information of the building and its elements, as well as the materials and processes that are necessary from the product phase, construction, use to the end-of-life phase.

For the evaluation of the case study, generic BIM objects were used. These BIM objects include information on environmental, economic, and social impacts of general BIM elements from the BIM-TBL library and include generic data on environmental impacts (kg CO₂ eq.) extracted from the ecoinvent v3.7.1 database [43] and systematically organised through the BCCA [43]. Economic (euros) and social (working hours) data was extracted from the BCCA [43].

3. Results and discussion

3.1. Case study LCSA results

After applying the evaluation procedure to all the technical solutions detailed in Section 2.3, the best solution was that which includes ETICS -EPS in the façade, wooden windows, and aluminium for the main door and exterior doors (see Figure 7).

From the results obtained, the EPS solution for the exterior walls was indicated as the best performing option in the environmental dimension. However, when analysing the material variation between the EPS and mineral wool ETICS solution, it is possible to see that the impacts per kg of mineral wool for the manufacturing process are lower than those of EPS. The results extracted from the ecoinvent v3.7.1 database [42] for the GWP impact category (IPCC 2013) show that the impacts per functional unit (kg) of EPS are 3.541593 kg CO₂ eq. and of mineral wool these are 1.306912 kg CO₂ eq. Due to the differences in the density of the two materials, the results of the environmental assessment turn in favour of EPS when similar thicknesses are installed as thermal insulation in the building to obtain equivalent thermal transmittance values.

This demonstrates the importance of using assessment tools and methods that are close to the real materials and quantities installed in the building. Other types of comparisons that simply compare impacts (e.g., per kg) without considering the totality of the characteristics of the materials installed in the building could lead to erroneous decisions. On the other hand, it should be borne in mind that the results of this assessment are limited to the inclusion of one impact category (GWP) which is mainly focused on the product and construction phase (A1-A3 and A5). In this respect, it is understood that the order of the alternatives evaluated could be modified if, for example, another life cycle phase is included or the results of the benefits of recycling or reuse of the materials and products utilised (module D) are communicated.

The evaluation of the window solutions (Table 6) shows that the wooden solution yields the best results for all three dimensions (environmental, economic, and social), since it presents the lowest CO₂ emissions and costs, and can generate the most local working hours.

Finally, the results were calculated for the three dimensions including the best solutions for the façade, windows, external doors, and roof. The multidimensional (environmental, economic and social) assessment gave equal weight (33.3%) to all dimensions. Table 7 shows the three best solutions for the façade, windows, and doors.

3.2. Advantages of the tool developed

The results show that the triple dimension assessment (environmental, economic, social) of the building envelope refurbishment can be conducted in real-time using a BIM model. The tool enables the comparison of various materials and technical solutions in real-time, enabling the logical and orderly organisation of the evaluation of the elements that comprise the building envelope, thereby facilitating the calculation of the LCSA. However, the normalisation of the results, and the weighting of the three dimensions (environmental, economic, and social) has manually implemented.

It was possible to perform the assessment and attain coherent values for the information modules included. For instance, the values obtained for the environmental dimension are consistent when compared with other similar work [44]. In Hollberg et al. [44], the reference values obtained for the case of external walls vary between 0.82 to 3.82 kg CO₂ eq. per square metre per year. In this study these values are lower (0.38 kg CO₂ eq. per square metre per year) due to the fact that it is a building refurbishment, and hence the structure system and other building systems are excluded from the system boundaries. Other studies [27] that were focused on comparing two scenarios for refurbishment of multi-residential buildings assumed different U-values for the external walls: 0.23 W/m²k and 0.62 W/m²k. The results for the study vary from 5.74 to 3.79 kg CO₂- eq. per kg of building material per square metre per year, considering all the building systems that comprise the building envelope.

The study demonstrates the adjustability in the definition of the system boundaries of the building for a building refurbishment LCSA, and its correlation with the 3D-view BIM model. The tool focuses on the envelope design at the material and geometry level, which can help optimise the building performance. Therefore, by focusing the building envelope design on the material thermal transmittance parameters, a reduction is facilitated in the effort required in the operational energy calculation. This can help towards optimising the embodied impacts, while leaving other variables that affect the calculation of operational energy (such as form factor and occupancy) at fixed values.

3.3. Limitations and future developments

Limitations in the scope of the assessment and of the information modules included have been detected. For example, several information modules, such as C3, C4, and D, are not included in the case study validation. Although the integration of the building elements has been limited to the building elements implemented in the real building, the authors aim to maintain the scope of the study close to the expected theoretical values for operational energy demand (33.07 kWh/m²yr.) and to provide a fair comparison of the solutions.

The results demonstrate that the EPS solution is better than that of mineral wool if solely the Product, Construction, and Deconstruction processes (C1 module) are considered, which is also aligned with other studies such as [45,46,47]. However, waste processing and benefits beyond the building system are not included. Other studies [48], that have focused on the material circularity analysis, provide evidence that the flame-retardants in polystyrene materials in the existing stock make polystyrene-based materials less suitable for recycling than mineral-based insulation.

The study focused on the use of predefined combinations of materials and precalculated thermal transmittance in accordance with the recommended values for the climate zone of the case study (Seville, B4 according to the CTE [38]). Future studies can include the dynamic calculation to verify a wide range of options for the building envelope, or could include a more comprehensive number of values for the various climate zones in Spain [38]. Future research could therefore be focused on its automatic integration into the plug-in development. For example, as highlighted in Soust-Verdaguer et al. [31], multi-criteria decision methods such as TOPSIS (Technique for Order of Preference by Similarity to Ideal Solution) [49] and AHP (Analytic Hierarchy Process) [50] could support automatic weighting and performance of a multi-criteria assessment in the plug-in.

4. Conclusions

The application of the plug-in and the methodology developed to carry out the quantitative evaluation of sustainability during the design phase of buildings have enabled the identification of the best construction solutions, materials, products, and processes to be used in building refurbishment, without the necessity of attaining high levels of detail in the BIM models. The results presented herein provide evidence of the special consideration that should be taken into account in the application of the building refurbishment LCSA in BIM. The main contribution of the tool lies in the flexibility in the use of the data regarding the BIM model to conduct the LCSA, in order to comply with the requirements of the application of the LCSA for building refurbishment.

Moreover, the study proposes a design methodology not only to reduce the GWP of buildings and the building life cycle costs (e.g., materials that help to minimise maintenance costs and require the least number of replacements over the life cycle of the building), but also to increase the social benefits, following element optimisation based on its relevance in the building geometry design. From the results attained herein, the integration of various dimensions (economic, environmental, social) in the building design demand special attention, especially in building refurbishment where the return of the investment needs to be assessed not only from the economic dimension (savings in energy consumption), but also from the environmental (embodied and operational impact optimisation) and social dimensions.

5. Patents

The software is under the authorship and patent registration of the University of Seville.