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Efforts to reduce greenhouse gas (GHG) emissions across various sectors are on-going to overcome the global climate crisis induced by global warming. The construction sector is a significant contributor of GHG emissions due to the complexity of its diverse processes and the extensive use of various materials. Consequently, simplifying construction processes and adopting low-carbon materials and processes through the rigorous review of material carbon footprints is urgently needed. This study focused on bathrooms (wet areas), which are characterized by complex procedures, the use of diverse materials, and the significant carbon emissions and material waste often resulting from high defect rates. We conducted a comparative analysis of the carbon reduction effects between the conventional wet construction method and the modular construction method specifically for bathroom construction. The analysis involved selecting materials, assessing their suitability against performance standards using a mock-up evaluation, and evaluating the construction applicability of modular bathrooms. Furthermore, through a Life Cycle Assessment, it was confirmed that the selected materials and the modular construction method could significantly reduce carbon emissions compared to the existing wet construction method. The findings of this study provide a crucial direction for the expanded application and use of modular construction methods in future building projects.
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1. Introduction
Efforts have been made to address the intensifying climate crisis caused by global warming resulting from greenhouse gas (GHG) emissions. The construction sector accounts for approximately 37% of the total GHG emissions from all industries; therefore, urgent intervention is required to reduce GHG emissions in this sector [1]. The Republic of Korea is among the countries contributing to reducing GHG emissions. Korea has declared carbon neutrality by 2050 and expects 40% of its total GHG emissions to be reduced, as outlined in the Nationally Determined Contributions by 2030. Approximately 32.8% of these emissions are expected to be reduced by the building sector [2,3].
Considering the carbon neutrality goal in the building sector, EN 15804 [4] was enacted by the European Committee for Standardization for objectively evaluating the life cycle environmental impacts of building materials. This is a key strategy for reducing carbon emissions and has been adopted as an international standard.
EN 15804 is the most widely used global reporting standard specializing ISO 14025 [5] in the field of Environmental Product Declarations (EPDs Type III) in the construction industry. As shown in Figure 1, it stipulates GHG emissions from the life cycle, which involves the production of materials (A1–A3), transport and construction (A4 and A5), use (B1–B7), dismantling and disposal (C1–C4), and outside the system boundary (Module D), and various environmental impacts by module . Based on this, architects and designers can select materials with low carbon emissions via EPDs [6].
Life-cycle assessment (LCA) examination for GHG reduction from an architectural perspective shows that, compared with the typical wet method, the modular method can reduce environmental impacts by 2–5% via the partial application of prefabs alone and by 20–50% via the application of advanced modularization. Particularly, the modular method can achieve carbon emission reduction in material production via optimization and standardized mass production [8,9].
A comparison between the typical wet and modular methods based on small residential buildings in Korea showed that buildings that applied the latter can reduce carbon emissions in the material production stage by approximately 35% [10]. The material production stage represents a considerable proportion of the life-cycle carbon emissions of buildings. In new buildings where high-efficiency and low-energy operations are generalized, the proportion of A1–A3 in life-cycle carbon emissions tends to increase to approximately 50%. The international roadmap presents initial embodied carbon reduction as a key task [11,12]. Marsh et al. [13] emphasized the importance of the A1–A3 stages while quantifying uncertainty in product-stage embodied carbon calculations for buildings. The LCA model of prefabricated buildings showed that the material production stage accounts for the largest proportion of total carbon emissions. Similarly, carbon footprint analysis results for residential and commercial buildings in the United States confirmed that the material production stage exhibits the largest proportion of carbon emissions from the entire supply chain [14,15]. Li and Masera [16] examined the methodology for building embodied carbon assessment from a circular economy perspective and emphasized that the A1–A3 stages are essential in the life cycle carbon emissions of buildings. As discussed above, embodied carbon emissions in the A1–A3 material production stages play an important role in the total building life-cycle emissions. Therefore, in this study, carbon emissions were firstly quantified during the material production stage as a foundation for embodied-carbon assessment, while future work will extend the boundary to the A4 and A5 (transport and installation) and B–C–D (use and end-of-life) stages. Carbon emissions in the building operation stage are expected to decrease owing to the shift in building operation to high-efficiency and low-energy systems, while the proportion of emissions from the material production stage is expected to increase relatively.
Compared with other industries, the construction industry is considered a low-productivity industry. Labor-intensive construction based on on-site production increases dependency on skilled workers, and non-standardized processes have been highlighted as the main causes [17]. The indicator of the report on labor productivity in the Korean construction industry by Seong and Yoo [18] has declined over recent decades from 104.1 in 2011 to 94.5. This shows that the productivity of the Korean construction industry is lower than that of advanced countries in contrast with the increase in average productivity across industries during the same period. Particularly, the shortage of skilled workers because of the aging of construction workers emerges as a significant issue [18]. Because of limited workforce supply, foreign workers have been filling this gap. This workforce structure problem causes defects because of degradation in construction quality and increases risks associated with delays during the construction period. These factors could decrease the sustainability of the construction industry in the future [19,20]. Based on a defect analysis in Korean apartment buildings, defects in bathrooms, as a representative factor, rank third among all types of defects, and major defects include non-compliance with waterproofing membrane thickness, defective backfilling for tile adhesion, tile detachment, and leakage [19,20]. These problems can be observed overseas, and large-scale overseas surveys reveal waterproofing and leakage as the most frequent defects, indicating that water-related bathroom processes involve high defect risks [21].
Bathroom construction involves complex processes, as many construction tasks must be performed sequentially in a small space, thereby causing frequent interference between processes and significant challenges in construction management [22,23]. Particularly, the conventional wet construction method, a representative bathroom construction method, requires the participation of multiple skilled workers and high skill levels, causing a high defect occurrence rate because of limitations in quality control for each task and unclear liabilities for defects [24]. Waterproofing exhibits a higher difficulty because the waterproof layer needs to be reconstructed after removing the finishing materials, requiring considerable cost, materials, energy, and time. Therefore, a novel approach is required to address the problems of low productivity and workforce aging at construction sites and improve construction quality in spaces that integrate multiple tasks, such as bathrooms.
Modular construction, which is among the methods for addressing such problems, has attracted attention as an alternative for construction productivity innovation and quality improvement. Modular bathrooms are a representative modular construction element technology for addressing inefficiency. Modular bathrooms refer to bathrooms for which bathroom components (e.g., floor panels, walls, ceilings, plumbing, and finishing materials) are manufactured and assembled in a factory.
As for modular bathroom types, the 3D module method installs the pre-assembled one-bathroom unit on site by lifting it using a crane. It is mainly used in Europe and Singapore and is referred to as a prefabrication bathroom unit or Bathroom Pods. The 2D method assembles the materials produced in factories at the site. It is mainly used in Japan and Korea and is referred to as the unit bathroom and system bath.
Unlike bathrooms that apply conventional wet construction, these modular bathrooms can be rapidly constructed through minimal assembly and connection work on site, ensuring uniform quality through factory production while significantly reducing on-site manpower and construction period. Bathroom modularization has been introduced for decades in Europe and Japan, and modular bathrooms have been applied in apartment buildings since the mid-1990s in Singapore [25,26]. In Japan, the modular bathroom known as unit bath has been applied since the 1960s, and modular bathrooms are constructed in almost every bathroom. In Korea, modular bathrooms have been applied to construct large-scale accommodation facilities for hosting international sporting events and new towns in the 1980s and 1990s.
In Korea, however, modular bathrooms were not widely distributed because low-cost bathroom images were fixed because of the finishing material discoloration caused by the use of low-cost plastic materials as wall panels at that time and the resonance caused by poor floor construction. These problems have been addressed, and performance has been improved through increased wall panel strength and tile finishing identical to wet bathrooms. Because previous images have not been addressed, however, modular bathrooms have been applied only in state-rented housing.
Table 1 compares the wet and modular construction methods commonly applied in bathrooms in Korea. Compared with the wet construction method, the performance of modular bathrooms in Korea has been improved through research and development to shorten the construction period, ensure quality, improve constructability, and secure the same level of finishing performance. No approach, however, has addressed the impact of environmental loads for GHG reduction.
Korea requires a paradigm shift in construction production from manpower- to system-oriented methods. As an element technology representing the transformation of the construction industry into the manufacturing industry, modularization is expected to be expanded for bathrooms, which are among the areas exhibiting the highest proportion of defects. Therefore, modular bathrooms that can reduce environmental loads while being applicable at conventional wet construction sites are necessary. Accordingly, this study aimed to develop an environmentally friendly modular bathroom system applicable to residential buildings in Korea, produce a prototype, and verify its performance. Additionally, the carbon reduction potential of the modular bathroom system was analyzed via LCA. Unlike previous building-level studies, the present study focused on a functional space unit—the bathroom level—conducting a component-based LCA within the prefabrication framework. The analysis scope is aligned with international practices and limited to the A1–A3 stages (Cradle-to-Gate), focusing on embodied carbon assessment. The analysis methodology follows international frameworks such as ISO 14040, ISO 14044 [27], EN 15804, and the World Green Building Council (WGBC) Embodied Carbon Roadmap.
The findings of this study are expected to provide a scientific basis for evaluating the carbon reduction potential of partial modularization in the construction sector and contribute as a foundational reference for advancing its industrialization. Although this study focuses on the A1–A3 (Cradle-to-Gate) stages, future research will incorporate the A4–A5 (transport and installation) and B–C–D (use and end-of-life) phases. This expansion will allow for a comprehensive full LCA of modular systems.
2. Methodology
In this study, a bathroom was constructed using the modular construction method and its performance was verified through the mock-up test. Finishing materials, as well as floor waterproof, wall, and ceiling panel materials, were selected to development an environmental load-reducing modular bathroom and its basic performance verification. The details for each stage are described next.
2.1. Material Selection and Performance Evaluation Overview for Modular Bathrooms
2.1.1. Material Selection Overview for Modular Bathrooms
-. Floor waterproof panel materials
The three floor waterproof panels applicable in Korea, namely, sheet molding compound (SMC), fiberglass-reinforced plastics (FRPs), and thermo-plastic resin (TPR), were analyzed based on the production method, productivity, quality stability, manpower dependence, and construction characteristics based on suitability for the modular construction method.
-. Wall panel materials
Finishing materials applicable in Korea were examined to select wall panel finishing materials through performance verification. As presented in Table 2, performance verification was performed for recyclable steel materials and large panel-finishing materials available in Korea.
2.1.2. Material Performance Evaluation Overview
The test method used in this study was based on the KS F 2223 [28] (test for decorative metal plates in complex sanitary units for housing), a Korean standard. An additional performance test was conducted based on KS L 1001 [29] (test of the chemical resistance of ceramic tiles) to ensure performance at the tile level, which are finishing materials for bathrooms in Korea. The performance test criteria are presented in Table 3.
2.2. Mock-Up Test Overview
2.2.1. Basic Performance Test Overview
The basic performance of the bathroom system was verified by designing a prototype for mock-up testing, as shown in Figure 2. The prototype had an area of approximately 3.2 m2 (1.5 × 2.4 m) with a ceiling height of 2.2 m, which corresponds to the typical size range of residential bathrooms in Korean apartment buildings (approximately 3.0–4.0 m2 depending on the unit size). In this study, a 3.2 m2 bathroom was selected to represent small residential housing. The wall panel width was designed to range from 750 to 1050 mm, and one bathroom comprised 10 panels.
KS F 2223 (test of decorative metal plates in complex sanitary units for housing), which serves as the performance criteria of modular bathrooms in Korea, was applied. The criteria are presented in Table 4.
2.2.2. Field Applicability Evaluation
To evaluate the field applicability of the developed bathroom, the wet and developed bathrooms were compared through mock-up construction in the same environment as the actual wet construction. The analysis unit was one bathroom while the scope of analysis included the construction tasks for the bathroom unit, construction period, and amount of each material used.
2.3. LCA
The carbon emission factors were compiled from the verified Environmental Product Declarations (Korea Environmental Industry and Technology Institute, KEITI) database and the national Life-Cycle Inventory (LCI, KEITI) database, as listed in Table 5. Each dataset complies with EN 15804 and ISO 14025 standards. However, the UK AAC block EPD (H + H UK Ltd. (Chester, UK), 2024) reports a variation in GWP-fossil for A1–A3 of −1.8%, indicating low data variability, while the Danish ABS panel EPD (Kima Accessories Aps, (Hvidovre, Denmark), 2023) qualitatively notes that uncertainties in certain impact indicators are relatively high. In this study, these inherent uncertainties were acknowledged; however, they were not explicitly quantified because the applied datasets already represent verified manufacturer-level or national average values that are regarded as reliable within EN15804-compliant programs. Future research will include more detailed uncertainty and sensitivity analyses. Carbon emissions were calculated by multiplying the input quantity by the carbon emission factor (carbon emissions = quantity × carbon emission factor).
Regarding the analysis scope, the quantities of each material used to construct the wet and environmental load-reducing modular bathrooms were calculated to compare and analyze the carbon emissions (kgCO2eq) during the material production stage.
The analysis unit is one bathroom, and major materials were classified into floor waterproofing materials, cement mortar, wall materials, ceiling materials, and finishing materials. In the wet construction method, masonry was applied with bathroom partition wall and AP/PD wall materials for floor waterproofing and wall tiling. In the modular construction method, a lightweight steel-framed wall was applied because floor waterproofing and wall tiling were not necessary, and ALC blocks were applied to the AD/PD side to minimize wet construction. Table 6 presents the diagrams and major materials of the wet and modular bathrooms.
3. Results
3.1. Material Selection and Performance Evaluation Results for the Modular Bathroom
Performance evaluation was performed following the KS test standards to select materials for constructing the modular bathroom. The results are presented next.
3.1.1. Floor Waterproof Panel Material Evaluation Results
Three waterproof panel materials were evaluated for constructing the modular bathroom. The results are presented in Table 7.
In the evaluation results, the SMC waterproof panel can form a desired shape because the sheet flows inside the metal mold when the unsaturated polyester resin reinforced with fillers, catalysts, release agents, and glass fibers is inserted into the heated metal mold and compressed [32]. This increases the waterproof panel strength by enabling rib formation and facilitates the integrated production of wall panel joints. This method is highly productive by producing approximately 100 units for one press per day and can ensure uniform quality. The production of a metal mold, however, requires considerable initial cost.
The FRP waterproof panel is fabricated using the manual deposition method (hand-lay-up). Because it is fabricated by repeatedly depositing glass fibers and resin, the fabrication time is longer than that for SMC and the quality significantly varies depending on the skill level of workers [33]. This method involves low productivity because two to four units can be produced by two to three workers per day depending on the worker’s skill level, which determines the quality. Furthermore, this method is favorable for customized low-volume production but not for mass production and quality assurance.
The TPR waterproof panel is fabricated by heating and bending a thermoplastic sheet and applying thermal fusion welding to joints to prevent leakage. This method can be used to produce 50 to 70 units per day, and performance depends on the worker skill level during the welding process .
In terms of constructability, the SMC and FRP waterproof panels can reflect the floor gradient and produce geometry for wall panel assembly, thereby ensuring uniform quality and convenient construction. However, compared with the SMC and FRP waterproof panels, the TPR waterproof panel requires separate mortar work for finishing after waterproof panel installation. Thus, the poor floor gradient caused by the worker skill level increases the likelihood of stagnant water defects.
3.1.2. Wall Panel Material Test Results
Figure 3 presents the wall panel test. Table 8 presents the results of testing four wall panel materials following the KS test standards for wall panel material selection.
In the wall panel material test results, the porcelain enamel exhibited rust in the corrosion-resistance test and surface discoloration in the chemical-resistance test. The plastic composite board exhibited bending in the boiling water-, detergent-, and corrosion-resistance tests, as well as bending/surface deformation in the heat-resistance test. The inorganic composite board could not meet performance because surface discoloration occurred in the chemical-resistance test. Only the color steel plate met the performance criteria as no problem was observed for all test parameters.
3.2. Mock-Up Test Results
3.2.1. Basic Performance Test Results
After the assembly of the aforementioned materials, the overall performance of the modular bathroom system was evaluated, as presented in Figure 4 and Table 9.
In the moisture-resistance test, no problem was observed from the floor waterproof panel, wall panel, ceiling panel, and each joint. In the deformation test results, 2 mm was measured from the ceiling panel for a tolerance of 10 mm or less and 1 mm from the waterproof panel for a tolerance of 5 mm or less, representing approximately 20% of the tolerance for both panels. For the wall panel, 3 mm was measured for a tolerance of 7 mm or less, representing approximately 40% of the tolerance.
In the impact strength test, no problem was observed from the wall panel, waterproof panel connected to the wall panel, and ceiling panel joints, thereby ensuring impact resistance. In the surface hardness test, the Barcol hardness value was 54, which met the criterion of 30 or higher. The basic performance test results confirmed that the developed modular bathroom meets the performance requirements for the construction and use of modular bathrooms.
3.2.2. Field Applicability Evaluation Results
An evaluation was performed to verify the field applicability of the developed systems. The construction sequences of the wet and modular bathrooms are shown in Figure 4 and Figure 5, respectively. The wet bathroom requires water supply, equipment for electrical outlet construction, and temporary work for electricity before bathroom construction. These are followed by masonry wall construction for waterproofing and tile finishing, wooden door temporary frame construction, liquid and membrane waterproofing after surface arrangement, freshwater tests for leakage check, heating pipe installation for bathroom floor heating, plastering for wall tiling, wall tiling, flood BED mortar construction for floor tiling, floor tiling, wall and floor tile grouting, masonry shelf top plate attachment, wooden door main frame and ceiling panel installation, sanitary ware/furniture/accessory installation, and cauking. The wet bathroom construction sequence is shown in Figure 5.
The modular bathroom was constructed as follows: floor mortar pouring for waterproof panel installation, waterproof panel installation, wall panel assembly, lightweight steel-framed partition wall construction, wooden door temporary frame construction, ceiling panel assembly, wooden door main frame installation, masonry shelf top plate attachment, floor tiling and grouting, sanitary ware/furniture/accessory installation, and cauking, as shown in Figure 6. The exterior and interior views of the assembled modular bathroom are shown in Figure 7.
The wet bathroom was constructed through 11 tasks and 22 processes, as presented in Table 10; and 15 days were required for bathroom unit construction, as shown in Figure 8. The modular bathroom was constructed through three tasks and 15 processes, as presented in Table 11; and five days were required for bathroom unit construction, as shown in Figure 8. When a wet bathroom is changed into a modular bathroom, tasks can be reduced by approximately 70%, processes reduced by 30%, and the bathroom unit construction period reduced.
Table 12 compares the amount of materials used for the wet and developed modular bathrooms, stating 4355 and 1220 kg of materials used, respectively. This indicates that the weight could be reduced by 28%.
For the floor, 867 kg was used in the wet bathroom as mortar (800 kg) and tiles, and subsidiary materials (58 kg) were mainly used. For the modular bathroom, the weight was reduced by approximately 37% (550 kg), as the waterproof panel (28 kg) and mortar (482 kg) were mainly applied. For the walls, 3479 kg was calculated for the wet bathroom as masonry (2247 kg), Remitar for plastering (660 kg), and tiles and subsidiary materials (543 kg) were used. For the modular bathroom, the weight decreased by 19% (662 kg) compared with that of the wet bathroom due to the use of a lightweight steel-framed wall and subsidiary materials (413 kg), as well as the wall panel (189 kg).
3.3. LCA Results
LCA was conducted by applying the carbon emission factors in the material production stage for the major building materials used in the wet and modular bathrooms. The carbon emission analysis results are presented in Table 13. The total carbon emissions from the wet bathroom were 860 kgCO2eq/unit. Masonry (32%), mortar (21%), tile (19%), and plastering (14%) were identified as key carbon emission factors, as shown in Figure 9. In particular, masonry, mortar, and plastering represented 67%, indicating that cement-based materials were the main causes of carbon emissions.
The total emissions from the modular bathroom were 730 kgCO2eq/unit, which is 15% lower than that of the wet bathroom. The key carbon emission factors were found to be the wall panel (52%), lightweight steel frame (20%), and mortar (16%), as shown in Figure 10. Among them, the wall panel and lightweight steel frame represented 72%, indicating that steel-based and subsidiary materials for modularization were the main causes of carbon emissions.
For the floor, carbon emissions decreased by 20% from 229 kgCO2eq for the wet bathroom to 182 kgCO2eq for the modular bathroom. For the walls, the emissions decreased by 13%, from 617 (wet) to 535 kgCO2eq (modular).
4. Discussion
In this study, carbon emissions in the A1–A3 material production stages were compared and analyzed after applying wet and modular bathrooms to the functional space known as bathroom, unlike the building unit LCA. Accordingly, a modular bathroom was constructed by evaluating and selecting adequate materials, and its basic performance and field applicability were verified following Korea’s KS standards through mock-up testing.
First, materials for the modular bathroom were selected. After evaluating three floor waterproof panels, the SMC waterproof panel was determined as suitable in terms of quality deviation minimization by the metal mold, mass production, and manpower-influence minimization. Based on the wall panel selection results, the porcelain-enameled panel exhibited limitations in terms of corrosion resistance, the plastic composite board exhibited limitations in terms of durability, and the inorganic composite board exhibited limitations in terms of chemical resistance. This implies that they can be vulnerable to real-use conditions in actual residential environments, such as humidity, detergents, and temperature changes, over time. The color steel plate, however, exhibited stable performance for all test parameters, including corrosion resistance, durability, and chemical resistance. Particularly, it is expected to exhibit stable performance over time in actual bathroom environments because corrosion, surface deformation, and bending were not observed. Consequently, these findings confirm that color steel plates are the most suitable materials for modular bathroom wall panels.
Second, the mock-up test results showed that the modular bathroom system met the basic performance criteria for all parameters, including moisture resistance, deformation resistance, impact strength, and surface hardness. Particularly, the ceiling and waterproof panels exhibited approximately 20% of the tolerance and the wall panel approximately 40% of the tolerance in the deformation test, indicating that structural stability were ensured. In the impact strength test, impact resistance was also proven as no abnormality was observed from the members and joints. Additionally, the surface hardness value exceeded the criterion, ensuring sufficient resistance to walking, scratching, and material loading, which frequently occur during field construction. Consequently, the developed modular bathroom is deemed suitable in terms of field applicability and usability.
The field applicability evaluation results revealed that, compared with the wet bathroom, the modular bathroom can significantly reduce tasks and processes. As the wet bathroom depends on a number of wet processes (e.g., waterproofing, plastering, and tiling), it involves a long construction period, high possibility of quality deviation, and difficulty in site management due to process interference. However, the modular bathroom simplified processes and shortened the construction period through the on-site assembly of prefabricated members, including the waterproof, wall, and ceiling panels.
The wet bathroom required 11 tasks, 22 processes, and a construction period of 15 days; by contrast, the modular bathroom required only three tasks, 15 processes, and five days, resulting in an approximately 70% reduction in the number of tasks, 30% reduction in the number of processes, and 65% reduction in construction period. This shows the possibility of reducing manpower dependence and shifting to standardized construction methods in the domestic construction industry with intensified instability in the supply of skilled workers. A reduction in construction period may also lead to overhead cost savings and process interference minimization at construction sites, which is expected to improve the stability of the overall project schedule. Additionally, a reduction in the proportion of wet processes is favorable from an environmental perspective as it can reduce noise, dust, and waste during construction. These findings indicate that, compared with the conventional wet bathroom, the modular bathroom has additional benefits in terms of ease of quality control, stability in manpower supply, and a reduction in environmental loads as well as construction efficiency.
A comparison of the amount of materials used showed that the modular bathroom can reduce weight by approximately 28% compared with that of the wet bathroom. In the wall sector, in particular, the modular method applying a lightweight steel-framed wall and wall panel could reduce weight by 19% compared with that of the wet method applying masonry, plastering, and tiles. This is because, compared with the conventional wet method, the modular bathroom facilitates both structural weight reduction and process simplification through a reduction in wet processes. Furthermore, a reduction in the amount of materials used may lead to a reduction in environmental loads during the production and transportation of materials from an LCA perspective as well as an improvement in construction efficiency.
Third, the LCA analysis results revealed high carbon emissions from the wet bathroom because of the use of large quantities of cement-based materials (e.g., masonry, mortar, and plastering). However, the modular bathroom could reduce total emissions by approximately 15% by replacing wet processes and applying lightweight materials. Previous modularization studies have primarily evaluated embodied-carbon performance at the building level. Jang et al. [10] analyzed a Korean residential building and reported approximately 36% lower embodied carbon for modular construction in the A1–A3 stages, while Arslan et al. [8] synthesized 27 prefabrication cases and found an average reduction of approximately 16% compared with conventional wet construction. Although these studies differ in functional unit and system boundary from the current study—which focused solely on a bathroom module—the reduction observed here (15%) exhibits a comparable trend, suggesting that modularization consistently contributes to embodied-carbon reduction across different scales.
In future work, this analysis will be expanded from the bathroom level to an entire residential building in which the proposed modular system is applied, allowing a system-wide comparison under consistent boundaries. To validate the robustness of the 15% reduction result, a sensitivity analysis will be performed in future work to examine how variations in key emission factors (e.g., approximately ±10%) may influence the comparative performance between the wet and modular methods.
It is worth noting that the modular bathroom exhibited environmental load concentration by certain materials, as the proportions of the wall panel and lightweight steel frames were 52% and 20%, respectively. This shows that future carbon-reduction strategies for the modular system should lead to follow-up studies on the eco-friendliness of alternative materials and improvements in production processes for carbon emission reduction rather than being confined to simple replacement of wet processes.
Additionally, the wet bathroom is highly likely to cause additional environmental loads during processes (e.g., an increase in construction period and waste generation), whereas the modular bathroom is more favorable for on-site carbon reduction and ensuring quality stability through process simplification and reduced material inputs. Therefore, the modular bathroom can be considered an alternative for improving both environmental performance and productivity throughout the construction project in addition to initial embodied-carbon reduction.
In this study, various carbon-reduction analyses were conducted for the modular bathroom, in which the following limitations were identified. First, the LCA scope was confined to A1–A3 (material production stages) and could not include the A4 and A5 (transportation and construction), B (use and maintenance), and C/D (dismantling and recycling) stages. Second, because of limitations in securing carbon emissions from the applied products, indicators for products similar to those certified by the Korean EPD were used for some materials, and the national LCI DB and overseas product EPD were applied. Finally, an LCA linking the structural member reduction effect caused by the weight reduction in the modular bathroom with the structural design could not be conducted, requiring further research.
Despite these limitations, the main contributions of this study can be summarized as follows: It quantitatively presents the possibility of reducing embodied carbon in the construction sector through a detailed functional space unit approach as a step toward building-level LCA. Materials and processes were simplified compared with the case of tile-oriented wet finishing, and a direction to reduce A1–A3 carbon emissions was presented by applying large metal panels that are not generally used in Korean residential bathrooms. Additionally, performance verification through mock-up testing proved that metal panel design from a DfMA perspective is practically applicable at Korean construction sites. This study also has originality and industrial implications in that it confirmed that the one-piece molding and standardization characteristics of the SMC waterproof panel are suitable for modular construction from a DfMA perspective. This study exhibited originality in that it improved applicability in Korea by applying an LCA methodology that combined the EPD of actual products, domestic and overseas LCI DBs, and overseas product EPD considering the Korean construction environment.
The originality of this study lies in the integration of methodological and practical aspects that have not been jointly addressed in previous studies. Specifically, a component-level (LU) LCA framework focusing on a functional space unit—the bathroom—rather than the building scale is introduced, a DfMA-based modular system is applied and verified through mock-up testing, and a hybrid LCA methodology that integrates actual product EPDs with Korean and international LCI databases is proposed. These elements collectively represent a novel analytical framework for the component-level embodied-carbon assessment of modular bathroom systems applicable to the Korean construction sector.
The findings of this study are expected to be used as basic data for establishing building material low-carbonization strategies at the national level in the future.
5. Conclusions
This study aimed to develop an environmental load-reducing modular bathroom system for residential buildings in Korea and compare carbon emissions in the material production stage with the conventional wet construction method. Accordingly, material testing following the KS standards, performance evaluation through mock-up testing, material input calculation, and LCA were conducted. Based on the results, the main conclusions of this study can be summarized as follows.
First, the material evaluation results revealed that the SMC waterproof panel, which can minimize quality deviation and facilitate mass production, is suitable as a floor waterproof panel, and that the color steel plate that met all of the performance criteria of the KS test standards is the optimal material for wall panels. This material selection for the modular bathroom is expected to ensure uniform quality and long-term durability.
Second, the mock-up test results showed that the developed modular bathroom met the KS standards in the moisture resistance, deformation, impact strength, and surface hardness tests. Particularly, compared with the wet bathroom, the modular bathroom could ensure both construction efficiency and ease of quality control through an approximately 70% reduction in construction tasks, a 30% reduction in processes, and a 65% reduction in construction period for the unit bathroom. Its weight was approximately 28% of that of the wet bathroom, confirming the possibility of weight reduction.
Third, in the LCA results, carbon emissions from the wet bathroom in the material production stage were 860 kgCO2eq/unit, whereas those from the modular bathroom were 730 kgCO2eq/unit, indicating an approximately 15% reduction in carbon emissions. Cement-based materials (e.g., masonry and mortar) were key emission factors for the wet construction method as their proportion was 67%, whereas steel-based materials (e.g., wall panels and lightweight steel-framed walls) exhibited a large proportion for the modular method. This shows that the modular bathroom can ensure the initial embodied carbon reduction effect by reducing the use of cement-based materials.
In summary, this study shows that, compared with the conventional wet bathroom, the developed modular bathroom is an alternative construction method that can ensure construction efficiency, uniform quality, and the initial embodied carbon reduction effect. It is expected to contribute to addressing the challenges of solving the shortage of skilled workers in the Korean construction industry, improving construction quality, and achieving carbon neutrality.
The findings of this study can be used as foundational data for establishing design guidelines and embodied-carbon reduction policies that promote the adoption of modular bathroom systems in the Korean construction sector. Beyond embodied carbon reduction, modular bathroom systems offer operational lifetime advantages. The standardized structure and replaceable components enable component-level maintenance and upgrades, extending service life and minimizing material waste over time. Additionally, as the system uses recyclable steel panels and detachable joints, it supports circular construction by facilitating material reuse and recycling at the end-of-life stage. Such circularity-oriented design aligns with current sustainability goals in modular architecture. Although this study focused on the A1–A3 (cradle-to-gate) stages to maintain methodological consistency with EN 15804 and ISO 14044, future work will expand the scope to include the A4 and A5 (transport and installation) and B–C–D (use and end-of-life) stages. Through this cradle-to-grave expansion, the proposed modular bathroom system would be scientifically validated by integrating dynamic parameters such as transport energy, maintenance, and material recyclability.
The proposed A1–A3 component-level LCA framework could be used during the planning and feasibility stages to compare the embodied-carbon performance between modular and conventional wet bathroom systems. During the planning and detailed design stages, the proposed framework could be applied to select and optimize material combinations for floor, wall, and ceiling assemblies using EN-15804- and ISO-14025-compliant EPD and LCI datasets. Additionally, as its system boundary and indicators are consistent with the international standards referenced herein (i.e., ISO 14040/44, ISO 14025, EN 15804, and EN 15978), the proposed assessment framework could be directly connected with building LCA schemes and emerging upfront embodied-carbon requirements that adopt these standards.
Conceptualization, S.-H.L.; methodology, S.-H.L., J.-H.J. and J.-C.P.; software, S.-H.L.; validation, J.-H.J.; formal analysis, Y.-W.S. and S.-H.L.; investigation, J.-H.J. and S.-H.L.; resources, J.-H.J. and S.-H.L.; data curation, Y.-W.S. and S.-H.L.; writing—original draft preparation, Y.-W.S. and S.-H.L.; writing—review and editing, Y.-W.S., S.-H.L. and J.-C.P.; visualization, Y.-W.S.; supervision, Y.-W.S.; project administration, J.-H.J. and S.-H.L.; funding acquisition, J.-C.P. and Y.-W.S. All authors have read and agreed to the published version of the manuscript.
Data are contained within the article.
Authors Sang-Heon Lee and Joo-Hyeun Jung were employed by the HDC Hyundai Development Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Footnotes
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Figure 1 System boundaries according to EN 15804/EN 15978 [
Figure 2 Drawing of the prototype bathroom for mock-up testing.
Figure 3 Wall panel material test.
Figure 4 Basic performance test for the modular bathroom.
Figure 5 Wet bathroom construction sequence.
Figure 6 Modular bathroom construction sequence.
Figure 7 Exterior and interior views of the assembled modular bathroom.
Figure 8 Comparison of bathroom unit construction periods (arrows indicate process flow).
Figure 9 Carbon emissions from each material for the wet bathroom.
Figure 10 Carbon emissions from each material for the modular bathroom.
Comparison of bathroom construction methods.
| Category | Wet Construction | Modular Construction |
|---|---|---|
| Overview | Quality variations by skilled workers | System-oriented construction completed by simple on-site assembly of factory-fabricated bathroom components |
| Conceptual diagram | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] |
| Benefits | Verified construction method | Quality assurance and construction period shortening |
| Shortcomings | Difficulty in site management by complex tasks/processes | Insufficient construction experience |
Comparison of materials applicable as wall panels.
| Category | Porcelain Enamel | Color Steel Plate | Plastic Composite Board | Inorganic Composite Board |
|---|---|---|---|---|
| Image | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] |
| Overview | Enhances corrosion resistance and functionality by coating the metal surface with a glassy glaze | Applies printing and special coating on the corrosion-resistant steel plate surface | Attaches a pattern sheet to the board composed of PVC, PET, and calcium carbonate | Attaches a pattern sheet to the board made by mixing stone powder and resin |
| Features | Recyclable/high cleanness/expensive | Recyclable/high cleanness/mid-price | Lightweight/intermediate cleanness/finishing material available in Korea/low cost | Heavy/intermediate cleanness/finishing material available in Korea/low cost |
Performance test criteria for wall panel materials.
| Target Material | Test Standard | Test Item | Test Method | Criteria | |
|---|---|---|---|---|---|
| Four wall panel materials | KS F 2223 | Impact resistance | Observe the coating peeling after dropping a 12.7 mm steel ball (mass: 500 g) on the test surface center of the specimen from a height of 300 mm | No coating peel-off | |
| Adhesion | Presence or absence of the loss caused by tape adhesion after making 11 cuts at 1 mm intervals | Three or less coating peel-offs | |||
| Boiling water resistance | Immerse the specimen in boiling water (95% or higher) for 3 h | No wrinkling, cracking, swelling, and peel-off | |||
| Corrosion resistance | (1) 240 h of testing for galvanized steel plates and | No red rust | |||
| Detergent resistance | Immerse the specimen in a 0.5% solution of neutral laundry detergent at 75 °C for 6 h | No problem | |||
| Pencil hardness | Use an HB pencil (to apply 7.5 N) | No problem with HB | |||
| Moisture resistance | Maintain the specimen in a 40-°C and 90% humidity atmosphere for 240 h | No severe wrinkling, cracking, swelling, and peel-off | |||
| Heat resistance | Observe changes in appearance after leaving the specimen in a constant-temperature chamber at 130 ± 10 °C for 3 h | No problem | |||
| Hot water resistance | Repeat the process of immersing the specimen in hot water at 60 ± 2 °C for 7 h and then removing and leaving it indoors at room temperature for 17 h | No rust or swelling on the surface | |||
| KS L 1001 | Chemical resistance | Acid resistance | Check the surface after dropping the HCI 3% solution on the surface and washing it with water after 8 h | No problem | |
| Alkali resistance | Check the surface after dropping the NaOH 3% solution on the surface and washing it with water after 8 h | No problem | |||
Performance test criteria for modular bathrooms.
| Category | Test Method | Criteria | |
|---|---|---|---|
| Moisture resistance | All | Exposure to boiling for 1 h after sealing the opening | There should be no deformation or problem that inhibits usage |
| Deformation | Wall panel | Attach a 150 mm-diameter, 5 mm-thick rubber plate to the center of the wall panel and apply a load of 98 N (10 kgf) | A maximum deformation of 7 mm or less |
| Ceiling panel | Apply a 4 kg weight to the center of the ceiling and check the maximum deformation after 1 h | A maximum deformation of 10 mm or less | |
| Waterproof panel | Fill 80% of the bathtub with water and place a 100 kg weight at its center. Then, check the maximum load at the bottom center after 1 h | A maximum deformation of 5 mm or less | |
| Impact strength | Wall panel | After raising a 200 mm-diameter fabric bag (15 kg) to a length of 1 m and an angle of 30°, repeat an impact five times | There should be no defect (e.g., deformation, damage, and crack) that inhibits usage |
| Waterproof panel | Drop a sandbag (7 kg) from a height of 1 m five times | There should be no defect (e.g., deformation, damage, and crack) that inhibits usage | |
| Surface hardness | Waterproof panel | Calculate the average value by checking at least 10 positions | Barcol hardness of 30 or higher |
Carbon emission factors for major bathroom materials.
| Category | Material | Carbon Emission Factor (kgCO2eq) | Basis |
|---|---|---|---|
| Floor | Mortar | 0.23 | Korea EPD (2024), KEITI |
| Tile | 0.52 | Korea EPD (2025), KEITI | |
| Tile adhesive (for pressing) | 0.24 | Korea EPD (2024), KEITI | |
| Wall | Tile adhesive (for direct setting) | 0.18 | Korea EPD (2024), KEITI |
| Brick | 0.12 | Korea LCI DB, KEITI | |
| Tile | 0.52 | Korea EPD (2025), KEITI | |
| Concrete block | 0.15 | Korea LCI DB, KEITI | |
| Remitar for interior wall plastering | 0.18 | Korea EPD (2024), KEITI | |
| ALC block | 0.21 | Overseas EPD (2024), H + H UK Ltd., EPD HUB, UK [ | |
| Gypsum board | 0.14 | Korea LCI DB, KEITI | |
| Glass wool insulation | 1.71 | Korea EPD (2023), KEITI | |
| High corrosion-resistant steel plate | 2.00 | Korea EPD (2024), KEITI | |
| Ceiling | ABS panel | 1.47 | Overseas EPD (2023), |
Diagrams and major materials of the wet and modular bathrooms.
| Category | Wet Bathroom | Modular Bathroom | |
|---|---|---|---|
| Bathroom diagram | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | |
| Material | Floor | Wet waterproof | Waterproof panel |
| Mortar for floor finishing | Mortar for waterproof panel installation | ||
| Tile finishing (including adhesive) | Tile finishing (including adhesive) | ||
| Wall | Masonry (interior wall) | Lightweight wall (interior wall) | |
| Wet waterproof | Gypsum board | ||
| Wall plaster mortar | Glass wool insulation | ||
| Tile finishing | Wall panel | ||
| Ceiling | Ceiling panel | Ceiling panel | |
Comparison of floor waterproof panel materials.
| Category | SMC Waterproof Panel | FRP Waterproof Panel | TPR Waterproof Panel |
|---|---|---|---|
| Image | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] | [Image omitted. Please see PDF.] |
| Production method | The unsaturated polyester resin sheet reinforced with fillers, catalysts, release agents, and glass fibers is formed using a metal mold. | Glass fibers and resin are deposited using the manual deposition (Hand-Lay-Up) method. | The thermoplastic resin sheet is heated, bent using a bending machine, and subjected to joint thermal fusion welding. |
| Productivity | High productivity | Low productivity | Intermediate productivity |
| Quality stability | High | Low | Intermediate |
| Manpower dependence | Low | High | Intermediate |
| Economic feasibility | High initial mold cost | Low initial investment | No metal mold required |
| Product characteristics | High geometric freedom (e.g., ribs) | High geometric freedom (e.g., ribs) | Low geometric freedom |
| Construction process | Three processes | Three processes | Three processes |
KS-standard test results for four wall panel materials.
| Test Item | KS | Criterion | Test Results | ||||
|---|---|---|---|---|---|---|---|
| Porcelain Enamel | Color Steel Plate | Plastic Composite Board | Inorganic Composite Board | ||||
| Impact resistance | KS F 2223 | No surface problem | No problem | No problem | No problem | No problem | |
| Adhesion | 3 or less coating peel-offs | 0 | 0 | 0 | 0 | ||
| Boiling water resistance | No surface problem | No problem | No problem | Bending | No problem | ||
| Corrosion resistance | No rust | Rust | No problem | Bending | No problem | ||
| Detergent resistance | No problem | No problem | No problem | Bending | No problem | ||
| Pencil hardness | No problem | No problem | No problem | Marks on the surface | No problem | ||
| Moisture resistance | No surface problem | No problem | No problem | No problem | No problem | ||
| Heat resistance | No surface problem | No problem | No problem | Bending/surface deformation | No problem | ||
| Hot water resistance | No surface problem | No problem | No problem | No problem | No problem | ||
| Chemical resistance | Acid resistance | KS L | No surface problem | Surface discoloration | No problem | No problem | Surface discoloration |
| Alkali resistance | No surface problem | No problem | No problem | No problem | No problem | ||
| Performance level | Not satisfactory | Satisfactory | Not satisfactory | Not satisfactory | |||
Basic performance test results for the modular bathroom.
| Category | Criterion | Test Result | Performance Level | |
|---|---|---|---|---|
| Moisture resistance | All | No problem | No problem | Satisfactory |
| Deformation | Wall panel | 7 mm or less | 3 mm | Satisfactory |
| Ceiling panel | 10 mm or less | 2 mm | Satisfactory | |
| Waterproof panel | 5 mm or less | 1 mm | Satisfactory | |
| Impact strength | Wall panel | No problem | No problem | Satisfactory |
| Waterproof panel | No problem | No problem | Satisfactory | |
| Surface hardness | Waterproof panel | Barcol hardness of 30 or higher | 54 | Satisfactory |
Wet bathroom tasks.
| Wet | ||
|---|---|---|
| No. | Task | Content |
| 1 | Equipment | Faucet box installation |
| 2 | Electricity | Electric box installation |
| 3 | Masonry | Masonry (wall) |
| 4 | Masonry (shelf) | |
| 5 | Wooden door | Wooden door temporary frame |
| 6 | Filling | |
| 7 | Waterproofing | Liquid waterproofing |
| 8 | Membrane waterproofing | |
| 9 | Job | Freshwater |
| 10 | Plastering | Plastering |
| 11 | Tile | Wall tiling |
| 12 | Floor mortar | |
| 13 | Floor tiling | |
| 14 | Wall tile grouting | |
| 15 | Floor tile grouting | |
| 16 | Job | Masonry shelf top plate |
| 17 | Wooden door | Main frame |
| 18 | Interior | Ceiling panel |
| 19 | Furniture | Furniture |
| 20 | Equipment | Sanitary ware and accessories |
| 21 | Job | Shower booth |
| 22 | Cauking | Cauking |
Modular bathroom tasks.
| Modular | ||
|---|---|---|
| No. | Task | Content |
| 1 | Modular bathroom | Floor mortar |
| 2 | Waterproof panel | |
| 3 | Wall panel | |
| 4 | Lightweight wall | Lightweight wall frame |
| 5 | Lightweight wall gypsum | |
| 6 | Wooden door | Wooden door temporary frame |
| 7 | Modular bathroom | Ceiling panel |
| 8 | Main frame | |
| 9 | Masonry shelf top plate | |
| 10 | Floor tiling | |
| 11 | Floor tile grouting | |
| 12 | Furniture | |
| 13 | Sanitary ware and accessories | |
| 14 | Show booth | |
| 15 | Cauking | |
Calculated amount of each bathroom material used.
| 1 Bathroom | Wet Bathroom | Modular Bathroom | ||
|---|---|---|---|---|
| Material | Weight (kg) | Material | Weight (kg) | |
| Floor | Waterproof | 9 | Waterproof panel | 28 |
| Tile (adhesive included) | 58 | Tile (adhesive included) | 39 | |
| Mortar | 800 | Mortar | 482 | |
| Sub total | 867 | Sub total | 550 | |
| Wall | Masonry | 2247 | ALC | 60 |
| Waterproof | 28 | Lightweight wall | 413 | |
| Plastering | 660 | Wall panel (filler included) | 189 | |
| Tile | 543 | |||
| Sub total | 3479 | Sub total | 662 | |
| Ceiling | Ceiling panel | 9 | Ceiling panel | 9 |
| Total | 4355 | Total | 1220 | |
Carbon emission calculation results.
| Part | Wet | Modular | ||
|---|---|---|---|---|
| Material | Carbon Emissions (kgCO2eq) | Material | Carbon Emissions (kgCO2eq) | |
| Floor | Waterproof | 25 | Waterproof panel | 44 |
| Tile (adhesive included) | 21 | Tile (adhesive included) | 23 | |
| Mortar | 182 | Mortar | 116 | |
| Sub total | 229 | Sub total | 182 | |
| Wall | Masonry | 277 | ALC | 13 |
| Waterproof | 70 | Lightweight wall | 144 | |
| Plastering | 120 | Wall panel | 377 | |
| Tile | 150 | |||
| Sub total | 617 | Sub total | 535 | |
| Ceiling | Ceiling panel | 14 | Ceiling panel | 14 |
| Total | 860 | Total | 730 | |
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