From Brownfields to Low-Carbon Cities: A Methodological Framework for the Sustainable Renovation of Industrial Buildings and Their Envelopes

Abstract

The sustainable renovation of ageing industrial buildings presents both a challenge and an opportunity to enhance energy efficiency while preserving architectural and structural integrity. This study develops an integrated methodological framework for assessing and optimising multilayer wall systems in such conversions, combining thermal, environmental, and durability analyses. Six composite wall configurations were designed and numerically evaluated using steady-state 2D heat conduction and vapour-diffusion models. The results reveal substantial thermal improvement compared to the reference uninsulated brick wall (U = 1.41 W/m²·K). The proposed systems achieved U-values between 0.351 and 0.172 W/m²·K, meeting or surpassing European energy standards. The BP–EPS wall exhibited the lowest U-value (0.172 W/m²·K), while the FC–EPSR configuration achieved superior corner performance with a 2D surface temperature (Tsi) of 17.99 °C and the highest surface temperature factor (fRsi = 0.943), along with a reduced condensation risk, indicating more balanced overall performance. Weight and thickness reductions of up to 80.5% and 52%, respectively, were observed, enhancing retrofit feasibility and space efficiency. Life Cycle Assessment results indicated that optimised wall configurations reduced embodied carbon (A1–A3) by up to 78% and total life cycle emissions (A1–A3 + B6) by over 86% relative to the reference case. Vapour-diffusion analysis confirmed the FC–EPSR wall’s lowest condensation fraction, indicating excellent hygrothermal durability. Multi-criteria evaluation using the simple additive weighting method and Monte Carlo robustness analysis verified FC–EPSR as the most balanced and reliable system. Overall, the findings present a validated and replicable pathway for the sustainable renovation of industrial buildings, supporting the goals of European carbon neutrality and the circular economy.

1. Introduction

1.1. Background

The building and construction sector is a major contributor to global energy demand, accounting for approximately 30% of global final energy consumption and 37% of CO₂ emissions [1,2]. In this context, the European Union’s strategic goals for 2030 and 2050, which emphasise energy efficiency, CO₂ reduction, climate neutrality, and the principles of the Green Deal, impose a considerable burden on the construction industry to lead the shift toward a more sustainable built environment [3,4]. As a result, energy savings in buildings have become a key priority within the Sustainable Development Goals (SDGs) framework [5], highlighting the sector’s vital role in achieving global climate and sustainability targets. Given population growth and the rising demand for indoor comfort, energy consumption in buildings continues to increase, making it essential to adopt innovative strategies that minimise environmental impact while maintaining thermal comfort [6]. One effective strategy is building conversion, which significantly contributes to achieving the goals of the Paris Climate Agreement. This approach enhances energy efficiency and incorporates sustainable development principles by adaptively reusing existing structures, especially in former industrial areas. Such conversions help reduce environmental impact, support resilient urban development, and revitalise underutilised or abandoned buildings for new purposes [7,8,9,10,11,12].

Figure 1 presents a schematic overview of thermal energy loss and the benefits of energy-efficient wall systems in buildings suitable for conversion. Field observations at brownfield sites in the Czech Republic and other European countries, such as Poland and Germany, show that most existing buildings in these areas have uninsulated brick walls with poor thermal performance. However, many buildings maintain strong structural integrity due to their steel or concrete skeletons, making them highly suitable for adaptive reuse. Therefore, implementing energy-efficient wall systems is crucial for the successful and sustainable conversion of these buildings across the region. Overall, this methodological framework provides a structured and replicable approach for evaluating the energy retrofit potential of ageing industrial buildings and former industrial buildings in post-industrial areas, bridging the gap between thermal engineering analysis and practical implementation.

1.2. State of the Art

Energy rehabilitation serves as a critical strategy for restoring industrial buildings and bringing them back to full functionality [13]. To align with international sustainable development goals, the implementation of energy-saving solutions in existing buildings is essential [14]. A growing global emphasis on enhancing building energy efficiency is largely driven by the urgent need to mitigate climate change and reduce greenhouse gas emissions. In Europe, a significant portion of the building stock expected to remain in use by 2050 already exists today. As a result, improving the energy performance of these structures is vital to achieving long-term energy and climate objectives. Among the various factors influencing a building’s energy performance, the thermal quality of the building envelope is particularly significant. Walls, which typically comprise the largest surface area of the envelope, play a major role; their thermal transmittance (U-value) directly impacts heating and cooling energy demand [15,16,17,18,19]. In several European cities, such as Ostrava, Kraków, Wrocław, Bydgoszcz, London, and Poltava, the adaptive reuse of outdated industrial structures has resulted in successful conversions into cultural, residential, and other functional spaces [20,21,22,23,24]. These projects often prioritise preserving the architectural or historical legacy of the buildings. However, even though many of these structures have the physical and spatial capacity to be repurposed for residential use, the energy efficiency aspects of such conversions remain underexplored in current research. Many existing buildings in post-industrial areas lack adequate energy performance. While they are structurally suitable for conversion, greater emphasis must be placed on energy rehabilitation and achieving carbon neutrality to align with broader sustainability and climate goals. Many existing buildings in post-industrial areas lack adequate energy performance, as illustrated in Figure 2, which presents examples from the Karviná district.

This challenge is particularly relevant in post-industrial areas such as Karviná, where a substantial number of industrial buildings remain structurally viable or require only minimal reinforcement to support reuse. These structures represent a significant opportunity for sustainable conversion, especially for residential purposes. However, as with many other regions, energy rehabilitation remains an under-addressed aspect in the adaptive reuse process, highlighting the need for targeted strategies, such as energy-efficient wall system interventions, to meet both comfort and climate goals. By completely replacing the exterior walls with modern, energy-efficient envelope systems, these buildings can be successfully adapted for new uses. Given their scenic surroundings and spatial qualities, they are well-suited for conversion into senior residences, hotels, or residential housing. Among prominent examples are the former Jan Karel Mine complex, the Gabriela and Barbora Mine site [26,27,28,29,30,31,32], and the many decaying mining colonies [25] from the turn of the 19/20th centuries in the Karviná district—located in the Doly district of Karviná and in the Moravian-Silesian Region of the Czech Republic more generally, and include several unused buildings on various brownfield sites in Karviná. As shown in Figure 2, when considering renovating buildings in post-industrial locations, careful attention must be paid to their energy aspect, which has become increasingly important in recent decades. This is justified because the European Union (EU), including Eastern European countries, has committed to moving toward climate neutrality [33,34,35,36].

The issue of building conversions is a pressing topic in the sustainable development of buildings and the meaningful sustainability of cities and settlements. The prefabricated skeletal structures of the existing building stock (especially buildings for civic amenities and industrial administrative buildings), based on prefabrication (concrete precast units), have a long service life that has not yet been exhausted. Demolition would mean the high production of construction waste and CO₂ emissions. Conversions allow the preservation of the supporting structure and a reduction in environmental burden. This corresponds not only to the principles of sustainability but also to the principles of circularity in construction. Skeletal supporting structures have great potential for adaptive design, taking full advantage of the breadth of adaptability of the envelope. The practical benefits derived from social and cultural trends supporting the “re-use” of modernist skeletons, which are regarded as carrying the city’s cultural memory, cannot be overlooked. For example, adaptive reuse of prefabricated buildings reduces environmental impacts while supporting circular economy principles at the building scale, addressing the meso-level focus emphasised by Pomponi and Moncaster (2017) [37], who highlight the importance of considering individual buildings in circular economy research for the built environment. Similarly, Patil et al. (2022) [38] explored smart construction materials and advanced construction techniques, providing an overview of available materials and shedding light on innovative methods that can enhance building performance and sustainability. Similarly, Chandrasekaran et al. (2021) [39] emphasise that renovation of existing buildings is principally implemented through both structural and technical measures, primarily aimed at heat energy savings. Such renovations can be categorised based on the percentage of energy savings achieved, ensuring that retrofitted buildings benefit from an extended lifespan and improved quality of the original structure. This approach highlights that targeted interventions not only enhance thermal performance but also contribute to the sustainability and longevity of the building stock, reinforcing the broader principle of adaptive reuse and circularity in construction. In this context, the optimisation of wall systems plays a decisive role in improving overall energy performance. For instance, Abdeen et al. [40] demonstrated that enhancing wall insulation in existing buildings can reduce energy consumption by up to 38.8%. Likewise, Farhanieh and Sattari [41] found that applying only 2.5 cm of thermal insulation on external walls can lower energy use by 45% under Iranian climatic conditions. These findings confirm that wall retrofit strategies represent one of the most effective means of reducing energy demand and carbon emissions in building conversions.

Consequently, recent studies have increasingly applied multi-criteria decision-making (MCDM) frameworks as decision-support tools for evaluating retrofit and conversion strategies in the built environment. Daniel and Ghiaus (2023) demonstrated the use of the ELECTRE Tri method in residential building retrofits, integrating technical, economic, social, and environmental criteria in a structured decision process [42]. Expanding the scope, Caruso et al. (2023) introduced a framework that combines seismic vulnerability reduction with energy efficiency, based on climatic conditions and seismic hazard [43]. Comprehensive reviews, such as that of Villalba et al. (2024), further reveal the prevalence of methods like AHP, SAW, and TOPSIS in building assessment and retrofit studies [44], while Ongpeng et al. (2022) developed a hybrid AHP–VIKOR framework to adapt conflicting stakeholder priorities in sustainable retrofit projects [45]. Complementing these methodological advances, Jafari and Valentin (2023) proposed a decision matrix to guide the selection of optimisation objectives, showing how investor type can significantly influence retrofit outcomes [46]. Methodological contributions also include recent works on the SAW method, one of the most widely applied MCDM techniques, both in step-by-step guides (Taherdoost, 2023) and in sectoral applications, such as energy company performance assessment (Sigalingging et al., 2024) [47,48]. Finally, material-specific applications demonstrate the versatility of MCDM in retrofit-related contexts; for instance, SAW has been applied to wall brick selection, where autoclaved aerated concrete blocks emerged as the most energy-efficient and sustainable option (Nguyen, 2024) [49]. Collectively, these studies underscore the growing role of MCDM approaches in evaluating and optimising retrofit strategies, offering methodological rigour and practical insights that align with the urgent need for sustainable building transformation.

However, research still lacks comprehensive methodological frameworks that integrate the structural, energy, economic, and environmental aspects of skeleton conversions and reconstructions. Monitoring real implementations is also limited. Therefore, this study focuses on the design and evaluation of new envelope systems for real skeletal buildings, aiming to achieve long-term improvements in thermal and technical performance, energy savings, environmental benefits, and innovative integration into existing structures.

1.3. Research Objectives

The sustainable renovation and conversion of ageing buildings require innovative approaches to improve energy efficiency without compromising structural and architectural integrity. Existing wall systems in such structures often do not meet current energy standards, necessitating efficient, adaptable, and durable solutions. This research aims to address these challenges by focusing on the development and evaluation of advanced wall systems suitable for retrofit applications.

The innovation of the proposed research method lies in its holistic integration of thermal–technical requirements, energy intensity, and environmental impacts—three essential aspects that strongly support and advance the current trend toward “re-use” and post-industrial renovation. Although these factors are critical for sustainable conversions, they are still largely missing or only partially addressed in existing research. This study introduces a comprehensive and interdisciplinary methodological framework in which thermal performance, energy behaviour, and environmental impacts are evaluated together as integral components of the building conversion process. The approach moves beyond isolated assessments and provides a unified, systematic, and decision-oriented method tailored for sustainable building renovation. The framework simultaneously incorporates thermal modelling, a durability assessment, Life Cycle Assessment (A1–A3 and B6) covering the product stage (A1–A3: raw material supply, transport, and manufacturing) and the operational energy use stage (B6), and multi-criteria decision-making. Using this combined methodology, this study delivers the first unified comparison of six innovative multilayer wall systems used with a traditional post-industrial wall. The results identify FC–EPSR as the most thermally efficient and durable configuration. Furthermore, this study demonstrates up to 86% reduction in whole-life carbon emissions and shows how circular materials can significantly improve both thermal and environmental performance. The developed workflow provides a replicable decision-support tool for sustainable renovation, offering practical guidance for selecting envelope solutions in the adaptive reuse of post-industrial buildings. The specific objectives are thus designed as follows:

(1)

Evaluate suitable solutions for designing and optimising new perimeter casings using composite materials to enhance energy efficiency and adaptability.

(2)

Improve the thermal performance of the building envelope through innovative material selection and system design.

(3)

Carry out thermal simulations of exterior wall systems, comparing traditional materials to innovative composite-based alternatives.

(4)

Comparatively evaluate different wall configurations using multiple performance criteria, assessed through the simple additive weighting (SAW) method for multi-criteria decision-making.

(5)

Analyse the environmental impact of selected wall variants using a comprehensive Life Cycle Assessment (LCA).

(6)

Provide a comprehensive assessment of the findings, synthesising the results and offering recommendations for future research directions in the field of energy-efficient retrofitting.

The incorporation of energy-efficient wall systems in buildings selected for functional conversion is a consideration of substantial scientific significance, as it supports the development of sustainable construction practices, the application of circular economy principles, and the long-term optimisation of energy performance in the built environment. Accordingly, the stated objectives will be pursued within this defined scope.

2. Method

The scientific approach employed in this research integrates theoretical analysis to assess and compare the different material variants used in multilayered wall systems. The methodology begins by selecting representative ageing industrial buildings and former industrial buildings in post-industrial areas that are suitable for conversion and energy retrofitting. These structures are analysed regarding their thermal properties, structural integrity, and potential for compliance with current legislative requirements on energy efficiency and carbon neutrality. To evaluate thermal performance, detailed models of the building envelope are developed that account for critical factors such as thermal bridges, material properties, and insulation quality. Heat transfer through the walls is analysed based on conduction in multilayered wall systems. To conduct a comparative analysis of various wall systems based on multiple performance parameters, the simple additive weighting (SAW) method is employed. Additionally, the research incorporates a Life Cycle Assessment (LCA) methodology to evaluate the embodied carbon (EC) impact of different wall configurations. The following steps outline the methodology for achieving these goals (Figure 3).

3. Envelope Material Selection and Thermal Analysis Framework

The case study focuses on heat transfer through different types of external walls (perimeter walls) and comparative thermal analyses of walls made of traditional and alternative building materials. Figure 2 provides an overview of building structures in the Czech Republic during the last century, with an example of the post-industrial locality of Ostrava-Karviná. A field observation of post-industrial areas indicates that the structures of buildings and their envelopes of walls are generally similar, with solid brick walls of significant thickness. The wall configurations were selected to represent both traditional and modern retrofit solutions commonly used in Central and Eastern Europe. Selection criteria included material availability, compliance with national U-value standards, and structural feasibility for integration into prefabricated or skeletal systems. Each configuration was designed to achieve comparable mechanical stability while varying insulation type and thickness to assess thermal efficiency.

Table 1. Thermophysical properties of the reference and innovative composite wall layers.

Item	Description	Thickness, d (m)	Conductivity, $\mathit{\lambda }$ (W/m. K)	Density, ρ (kg/m3)	$\mathit{d}/\mathit{\lambda }$(m2K/w)
Ref-SLCR wall	Internal lime plaster	0.015	0.870	1600	0.0172
	Solid bricks	0.440	0.800	1700	0.5500
	External lime-cement plaster	0.025	0.990	2000	0.0253
PPB-EPS wall	Internal lime plaster	0.010	0.716	1600	0.0140
	Porotherm D 24 Profi brick	0.240	0.290	800	0.8276
	ETICS adhesive layer	0.003	0.700	1300	0.0043
	EPS	0.150	0.033	16.00	4.5455
	Reinforcement layer of ETICS	0.003	0.750	1000	0.0040
	Penetration + Mosaic plaster	0.003	0.700	1750	0.0043
BP-AFC wall	Internal lime plaster	0.010	0.7160	1600	0.0140
	Hollow brick filled with perlite	0.300	0.0900	654	3.3333
	AFC	0.020	0.0208	266.20	0.9615
	Penetration + Mosaic plaster	0.003	0.7000	1750	0.0043
BP-EPS wall	Interior gypsum plaster	0.010	0.528	1250	0.0189
	Hollow brick filled with perlite	0.300	0.0900	654	3.3333
	ETICS adhesive layer	0.003	0.700	1300	0.0043
	EPS	0.100	0.033	16.00	3.0303
	Reinforcement layer ETICS	0.003	0.750	1000	0.0040
	Penetration + Mosaic plaster	0.003	0.700	1750	0.0043
HPC-Cork wall	Interior gypsum plaster	0.010	0.528	1250	0.0189
	HPC (Inner)	0.075	0.707	2357	0.1061
	Cork	0.107	0.040	115	2.675
	HPC (Outer)	0.035	0.707	2357	0.495
	Penetration + Mosaic plaster	0.003	0.700	1750	0.0043
FC-EPSR wall	Interior gypsum plaster	0.010	0.528	1250	0.0189
	Foamed concrete	0.100	0.500	1300	0.2000
	ETICS adhesive layer	0.003	0.700	1300	0.0043
	EPS, recycled (2 layers 0.093)	0.186	0.035	30	5.310
	Reinforcement layer ETICS	0.003	0.750	1000	0.0040
	Penetration + Mosaic plaster	0.003	0.700	1750	0.0043

Ref-SLCR: (Reference case (solid bricks with cement rendering)), PPB-EPS: (Porotherm D 24 Profi Brick, with incorporated EPS), BP-AFC: (Brick filled with perlite, with incorporated AFC), BP-EPS: (Brick filled with perlite, with incorporated EPS), HPC-Cork: (High-performance concrete, with incorporated cork), and FC-EPSR: (Foamed concrete, with incorporated EPS, recycled). Sources: [3,50,51,52,53,54,55,56,57,58,59].

presents the physical characteristics of the six external walls, along with a description of their construction materials. The physical parameters of the selected building in the post-industrial area are identified as the Ref-SLCR wall. Additionally, a set of conventional external wall materials, commonly used in residential and civic amenity buildings in the Czech Republic, is identified and referred to as the PPB-EPS wall. Furthermore, four multilayer composite constructions of the perimeter wall, which are based on heat-insulating materials, are proposed. Each layer is characterised by its thickness, thermal conductivity, and density, and the term $d/\lambda$ represents the thermal resistance of each material layer, as detailed in the following Table 1.

The detailed process for evaluating the thermal performance of building envelopes is illustrated in Figure 4.

4. Model Validation

To verify the reliability of the numerical approach used in this study, a simplified validation was conducted before performing the final simulations of multilayer wall systems. The test case was constructed such that heat transfer occurred only in the x-direction, with uniform boundary conditions and no internal heat generation, resulting in a one-dimensional (1D) temperature profile across the domain.

A 1D steady-state heat conduction model was implemented in Python 3.12 using the finite difference method (FDM) under the assumptions of isotropic and homogeneous thermal conductivity and steady-state conditions. The model, formulated directly from Fourier’s law of heat conduction, was applied to three representative wall configurations (Ref-SLCR, PPB-EPS, and BP-AFC). This analytical/numerical reference provides a simplified benchmark for verifying the numerical results obtained from the 2D simulations. Under these conditions, the 2D domain behaves as a 1D system, allowing a direct comparison between the analytical/numerical 1D model and the 2D simulation results generated in AREA2017. The temperature distribution across the wall was calculated for each configuration, and the results from both models (see Appendix A, Figure A1) show a high level of agreement. The graphical plots illustrate a strong correlation, and the temperature difference at the midpoint of the wall was measured to be under 5%, indicating consistent and reliable results.

Furthermore, the reliability of the AREA2017 simulation was validated in accordance with EN ISO 10211 standards [60]. The numerical residuals were several orders of magnitude lower than the maximum allowable error (<10⁻⁴), confirming stable convergence. In addition, the software’s internal consistency checks for surface temperature and temperature factors were satisfied. These results demonstrate that the simulations are both numerically robust and physically consistent, providing a solid basis for analysing thermal bridges in the examined wall assemblies.

The 1D FDM model was used exclusively for verifying numerical accuracy, providing a benchmark for the 2D FEM simulations, while the 2D model validation relied on EN ISO 10211 standards. Potential three-dimensional or transient effects were not included, as their influence is expected to be minimal for planar wall sections; these could be considered in future studies.

5. Model Description

Under steady-state, two-dimensional (2D) conditions with no heat generation and constant thermal conductivity, the governing heat conduction equation is expressed as:

$${\displaystyle \frac{{\mathsf{\partial }}^{2}T}{\mathsf{\partial }{x}^2}}+{\displaystyle \frac{{\mathsf{\partial }}^{2}T}{\mathsf{\partial }{y}^2}} =0$$

(1)

with the corresponding heat flux components:

$$q_x=-k {\displaystyle \frac{\mathsf{\partial }T}{\mathsf{\partial }x}} , q_y=-k {\displaystyle \frac{\mathsf{\partial }T}{\mathsf{\partial }y}}$$

(2)

This equation represents the general mathematical form of steady-state heat conduction. The physical properties (thermal conductivity, boundary temperatures, and heat transfer coefficients) are fully incorporated in the numerical model, ensuring realistic and standards-compliant results.

Two complementary numerical approaches were used:

• 1D analytical solution and numerical verification (FDM): A one-dimensional steady-state heat conduction model was first formulated analytically using the separation of variables method, under the assumptions of isotropic and homogeneous thermal conductivity and no internal heat generation. This analytical model was subsequently discretised using the finite difference method (FDM) and implemented in Python. The domain was divided into a uniform grid, and central difference approximations were applied to obtain a system of algebraic equations. Solving this system yielded nodal temperatures and heat fluxes, providing a numerical verification of the analytical solution under simplified conditions.
• 2D simulations: The main 2D simulations were performed using AREA 2017, which solves the problem using the finite element method (FEM) with Galerkin formulation (triangular elements and Gauss elimination) according to the official programme manual [61]. AREA computes steady-state temperature fields, heat fluxes, and temperature factors following EN ISO 10211 and EN ISO 10211-2 (2002).
• Complementary calculations: TEPLO 2017 was used for the basic thermal parameters of building structures in accordance with EN ISO 6946 [62], EN ISO 13788 [63], ČSN 730540 [64], and STN 730540 [65].

Together, these approaches ensure a robust and consistent analysis framework, where the 1D analytical model verified using FDM serves as a verification tool for the 2D FEM simulations, while the FEM results provide the primary evaluation of heat transfer through multilayer wall systems.

Boundary Condition

In the thermal simulation of building envelopes, the choice of construction type significantly affects the modelling boundary conditions and internal heat transfer pathways. According to [66], double-skin facades are defined as two layers of facade separated by an air gap. In contrast, the wall configurations in this study consist of non-ventilated, bonded layers made up of rigid insulation directly attached to a load-bearing wall. The thermal performance of the walls in this investigation is modelled as a homogeneous multilayer system, with heat flow considered unidirectional and continuous. Moreover, as reported in [67], air layers with a thickness of up to 0.3 m can be considered thermally homogeneous for the purposes outlined in this document. Consequently, it does not create a ventilated cavity as anticipated in a true double-skin façade. Therefore, it should be registered as a single-skin wall, despite containing multiple physical layers, because it acts as a single thermally active entity with steady-state heat transfer simulations.

The most significant mistake, which is also recurrent, is to determine the U-value only from the assembly in the ideal section of a structure [68]. In this context, the correction in Teplo of the thermal transmittance coefficient due to systematic thermal bridges (ΔU) [W/(m²·K)] is an important parameter. It represents a correction value (in W/m²·K) added to the calculated thermal transmittance U-value of the wall to account for systematic (repetitive) thermal bridges. These include repeating geometric or material discontinuities (e.g., mortar joints in masonry or metal ties in insulation layers) that cannot be modelled in detail but significantly affect thermal performance. Also, it improves the real-world accuracy of the thermal transmittance calculation.

Equation (3) must always be used to correct thermal transmittance so as to account for the effect of systematic thermal bridges (ΔU_tbk), which represent repeating structural connections not visible in the ideal section [68]. The explanation of the corrected thermal transmittance parameters used in Equation (3) is provided in

Table 2. Explanation of the corrected thermal transmittance parameters.

Parameter (W/m2·K)	Definition of Parameters
U	Final U-value: The total thermal transmittance of the wall, including all corrections
Ui	Ideal (unadjusted) U-value: This is the calculated U-value based on the layer-by-layer thermal resistance of the wall
ΔUtbk	Correction for thermal bridges

.

$$\mathrm{U}={\mathrm{U}}_{\mathrm{i}}+{\Delta \mathrm{U}}_{\mathrm{t}\mathrm{b}\mathrm{k}}$$

(3)

The effect of thermal bridges within a building envelope, denoted as ΔU_tbk (in W/(m²·K)), is considered separately. This effect typically depends on the prevalence of thermal bridges within the structure. The values for ΔU_tbk are provided in

Table 3. Representative values for different thermal bridge modes.

Construction Mode	Solution	ΔUtbk (W/m2·K)
Minimal thermal bridging	Successfully optimised	0.02
Moderate thermal bridging	Typical or repeated	0.05
Standard thermal bridging	Standard	0.10
Significant thermal bridging	Neglected	0.15 and above

, based on [68]. The optimal value should be chosen through engineering judgement for accurate modelling.

In this study, a correction for thermal bridges (ΔU_tbk = 0.10 W/(m²·K)) is applied, which takes into account conventional thermal bridges, such as uninsulated brick walls. In comparison, alternative configurations achieve a ΔU_tbk of 0.02 W/(m²·K) by utilising continuous insulation and optimised modern design solutions.

The heat flow is assumed to be horizontal, typically from the interior to the exterior. Considering the typical weather conditions in Ostrava, Czech Republic, characterised by cold winters and moderate summers, a standard reference indoor temperature (T_i) of 20 °C is adopted, in line with commonly accepted thermal comfort criteria and national building standards; the outdoor temperature T_e is considered (−15 °C).

The interior and exterior surfaces are modelled via surface heat transfer resistances (Rsi = 0.13, Rse = 0.04 [m²·K/W]), representing Dirichlet-type boundary conditions with convective correction. The lateral edges of the 2D wall section are assumed adiabatic (no heat flux), reflecting negligible heat transfer in directions perpendicular to the main flow. Interfaces between layers are automatically handled by the software, ensuring continuity of heat flux across materials without additional resistance.

6. Results and Discussion

In this case study, key thermal and physical parameters, including U-value, temperature distribution, wall thickness, and weight, are analysed across various multilayered wall configurations according to the climate of the Czech Republic. The primary indicators of wall structures include resistance to heat transfer and the heat transfer coefficient, which determine the energy efficiency of buildings. To support retrofit design decisions, two key parameters—wall thickness and weight—are examined for wall typologies. These metrics are critical because existing structural elements (e.g., beams and columns) may have limited load-bearing capacity, restricting the use of heavy assemblies. These indicators are essential in evaluating the energy efficiency of building envelopes, particularly in retrofitting or conversion projects. The relationship between U-value and wall thickness was investigated to identify optimal designs that balance thermal resistance with material use and structural load. Such analysis supports the selection of thermally efficient wall systems suitable for adaptation in the chosen building typologies.

6.1. Physical and Structural Optimisation of Wall Systems

Figure 5 illustrates that the primary benefits of the material solutions for the perimeter walls of buildings in post-industrial settlements are the reduction in wall weight and thickness, which are crucial in terms of the architectural and structural aspects of buildings. The new design of multilayered walls significantly reduced the weight and thickness compared to the original walls in the post-industrial area.

The weight of HPC-Cork walls is reduced by 64.80% compared to the traditional wall (Ref-SLCR), and this value for the PPB-EPS, BP-AFC, and BP-EPS is approximately 73% lower than that of traditional walls. In conclusion, the FC-EPSR wall type demonstrates the lowest overall weight among all configurations studied. Compared to traditional walls, it shows a significant reduction of 80.50% in total weight. It is also lighter than the alternative options of PPB-EPS, BP-AFC, and BP-EPS (28%) and HPC-Cork (44.60%).

The thickness of the BP-EPS wall is 12.7% less than that of the traditional wall (Ref-SLCR). The PPB-EPS wall is 14.8% thinner compared to traditional walls. Additionally, the BP-AFC and FC-EPSR wall types are 30.6% and 36.5% thinner, respectively. In conclusion, the HPC-Cork wall type has the lowest overall thickness among all the configurations studied, showing a significant reduction of 52% in total weight compared to traditional walls. This significant reduction in weight and thickness enhances spatial efficiency and lowers structural weight, making it an attractive option for retrofitting or optimising existing buildings.

The relationship between wall thickness and weight for six different wall configurations shows that the Ref-SLCR configuration has both the greatest thickness and weight, indicating its dense and massive construction. In contrast, configurations like FC-EPSR and HPC-Cork exhibit significantly reduced thicknesses and lower weights, suggesting they are designed with lightweight composites. Most of the other wall types weigh a similar amount, even though their thicknesses vary.

The reduction in wall weight and thickness directly influences both the structural and thermal performance of the building envelope. From a structural standpoint, lighter assemblies minimise the load on existing beams and columns, which is essential in adaptive reuse projects where load-bearing capacity is often limited. Theoretically, this reduction also aligns with sustainability principles, as lower wall mass corresponds to lower embodied energy and reduced CO₂ emissions during material production and transport (Pomponi & Moncaster, 2017) [37]. From a thermal perspective, minimising the thickness of high-conductivity materials while maximising the insulation layer supports the principle of optimised thermal resistance based on Fourier’s law, where total resistance increases as the ratio of layer thickness to conductivity rises. These improvements enhance both energy efficiency and structural feasibility in retrofit applications.

6.2. Thermal Modelling Results

6.2.1. U-Values for Building Envelopes of Different Materials

Figure 6 illustrates the difference in calculated U-values for various wall types under two conditions: (1) excluding surface thermal resistances (Rsi and Rse) and any thermal bridge correction (ΔU), and (2) including both factors. The comparison shows that when Rsi, Rse, and the thermal bridge correction factor are taken into account, the overall U-value increases for all walls except the Ref-SLCR wall configuration. In the latter case, the added surface resistances outweigh the ΔU and so reduce the U-value, but for other wall configurations, the low base U-value makes ΔU more dominant and so slightly increases the U-value. This highlights the importance of considering surface effects and thermal bridging in realistic building energy simulations. Failing to account for these factors can result in an underestimation of heat losses through building envelopes.

According to the principles of steady-state heat transfer, total heat flow through a composite wall depends not only on the intrinsic conductivities of the materials but also on boundary resistances (Rsi and Rse) and geometric imperfections such as thermal bridges [69]. When ΔU (i.e., the correction due to thermal bridges) is included, it accounts for additional conductive pathways that locally increase heat flux, especially at joints, corners, and structural discontinuities. Recent investigations confirm this effect in contemporary façade systems, showing that deviations from ideal U-values due to thermal bridging can amount to significant percentage increases in transmittance [70]. Thus, the observed increase in U-values for most wall configurations aligns with theoretical expectations: assemblies with lower baseline U-values are more sensitive to thermal bridging effects because even small local conductivity increases lead to proportionally larger overall U-value changes.

The U-value of the BP-EPS wall, accounting for surface resistances and thermal bridge corrections, is 0.172 W/m²·K. This represents an 87.8% reduction compared to the U-value of the traditional wall (Ref-SLCR). The U-values for the FC-EPSR and PPB-EPS configuration walls are approximately 86% lower than those of Ref-SLCR. Finally, the U-values for the BP-AFC and HPC-Cork walls are respectively 82.8% and 75% lower than that for the traditional wall. Overall, optimised and modern wall assemblies can achieve structural and thermal goals with less mass and thickness, which is critical for retrofitting existing buildings, particularly abandoned buildings in post-industrial areas, and improving material efficiency in new construction trends.

Figure 7 illustrates the relationship between normalised wall thickness and normalised thermal transmittance (U-value) for various multilayered walls. This analysis is based on dimensionless methods to compare quantities independent of their absolute scales, and this plot reveals important insights into their relative thermal performance.

The plot shows that greater wall thickness does not necessarily result in better insulation. For example, the Ref-SLCR configuration, which is the thickest, shows the poorest thermal efficiency with the highest U-value. In contrast, other wall configurations demonstrate significantly lower U-values, indicating better insulating performance despite their reduced thickness. In conclusion, effective thermal design relies more on the intrinsic properties of materials rather than their dimensions alone. The dimensionless plot offers a scale-independent way to compare a material’s efficiency with its thickness and emphasises the importance of choosing materials with low U-values. This is particularly crucial for energy-efficient building design.

The dimensionless correlation between wall thickness and U-value supports the theoretical concept that thermal resistance does not scale linearly with thickness when multiple layers of different conductivities are present. This nonlinear behaviour occurs because adding low-conductivity insulation significantly increases the overall thermal resistance, whereas adding high-conductivity structural material adds very little. Hence, the data trend confirms the theoretical framework that optimising material conductivity yields more effective thermal performance than simply increasing wall thickness.

6.2.2. Temperature Distribution

Figure 8 illustrates the temperature distribution within six different multilayered walls. The temperature gradient varies within each layer because of changes in conductivity. Increasing conductivity results in a decreased temperature gradient. On the other hand, these six graphical plots present a comparative 2D temperature field (heat distribution) across different corner wall configurations under steady-state heat conduction. Each plot visualises how heat flows through an internal corner, typically a thermal bridge area where heat loss is more significant.

Heat distribution in the Ref-SLCR wall shows a significant temperature drop toward the internal corner. A large area is affected by low temperatures, which exhibits significant thermal bridging. These kinds of walls typically consist of materials with high thermal conductivity without an effective insulation layer. As a result, heat flows more freely through the wall corner, leading to a lower interior surface temperature, a larger cold zone, and a high risk of thermal bridging and surface condensation. The temperature gradient through the wall layers is relatively shallow because the high conductivity allows heat to transfer gradually over a larger distance.

Figure 8b–f illustrate a significantly smaller cold zone compared to the reference example. The gradient is steeper, but most of the internal wall maintains high temperatures. This solution improves thermal continuity, reducing heat loss at the corner. The temperature patterns follow Fourier’s law of heat conduction: for a given heat flux, higher thermal conductivity results in a lower temperature gradient. In multilayer walls, steady-state heat transfer ensures continuity of temperature and heat flux across interfaces. The curved isotherms near the corners illustrate a concentration of two-dimensional heat flow through materials with higher $k$, forming thermal bridges. Conversely, the steeper gradients and smaller cold zones in insulated walls (b–f) confirm, from a theoretical standpoint, that low-conductivity layers effectively reduce heat loss and improve thermal continuity.

6.2.3. Hygrothermal Performance, Surface Temperature, and Condensation Risk Assessment

Figure 9 and Figure 10 present a comparative analysis of six different wall configurations by evaluating three key parameters: interior surface temperature (Tsi), temperature factor (fRsi), and U-value. These parameters offer insight into the thermal performance of each wall and the risk of condensation.

The analysis of surface temperature factor (fR_si) and internal surface temperature (T_si) across different multilayered wall types in Figure 9 shows a clear correlation between thermal performance and condensation risk. The Ref-SLCR wall has the lowest performance, with an fRsi of 0.689 and a Tsi of 9.11 °C, showing a high potential for surface condensation and thermal discomfort. In contrast, the rest of the wall systems exceed the critical fR_si [63,71,72,73,74,75] threshold of 0.75, ensuring improved thermal behaviour and reduced risk of surface condensation.

To extend the assessment beyond heat conduction, a quantitative vapour-diffusion analysis was carried out to examine interstitial condensation in the wall assemblies. The results indicate that although many modern multilayer walls achieve sufficiently high surface temperatures, their internal condensation risks differ significantly. The FC–EPSR system demonstrates the minimal condensation fraction together with the highest fRsi value (0.943), confirming its superior hygrothermal behaviour. It successfully avoids both surface mould growth and moisture accumulation within the layers.

These high-performing walls maintain internal surface temperatures between 16.84 °C and 17.99 °C, closely aligning with indoor thermal comfort standards. Notably, while incremental increases in fRsi from 0.91 to 0.943 lead to marginal gains in Tsi, the sharp improvement from the reference wall to the others highlights the significance of selecting materials with adequate thermal resistance. Overall, the coupled heat-and-moisture results demonstrate that the FC–EPSR configuration provides the most balanced performance, ensuring high thermal comfort, minimal condensation risk, and long-term structural durability.

In the dataset provided in Figure 10, different wall systems are compared based on their Tsi values and corresponding U-values (W/m²·K). A general trend is observed where a decrease in U-value (indicating better insulation) correlates with an increase in Tsi, consistent with the principles of building physics. In fact, better-insulated walls reduce heat loss, keeping the internal surface warmer in cold exterior conditions. Moreover, BP-AFC, PPB-EPS, FC-EPSR, and BP-EPS walls demonstrate a consistent improvement since the U-value decreases from 0.243 to 0.172 W/m²·K and Tsi rises from 17.19 °C to 17.99 °C. This illustrates the expected enhancement in internal surface temperature due to improved thermal resistance.

However, a slight deviation is observed between the BP-EPS and the FC-EPSR walls. Although the FC-EPSR wall has a marginally higher U-value (0.195 W/m²·K) than the BP-EPS (0.172 W/m²·K), it exhibits a slightly higher T_si (17.99 °C vs. 17.86 °C). This discrepancy suggests that factors beyond the steady-state U-value—such as wall material configuration [76,77,78], thermal mass [79,80,81], and wall assembly layer placement [82,83,84]—may influence the internal surface temperature. Overall, the data prove that lower U-values normally result in a higher T_si, improving thermal comfort and reducing the risk of condensation. However, minor deviations confirm that, when evaluating thermal performance, it is important to consider the full wall system design, not just the U-value. In this context, a more thorough evaluation of the temperature distribution within these types of wall systems (Figure 11a,b), resulting from 2D numerical simulations, is conducted. Figure 11 clearly illustrates how 2D modelling can demonstrate the impact of material solutions for perimeter walls on surface temperature (T_si) and thermal comfort. Notably, the FC-EPSR wall, despite having a worse U-value than the BP-EPS wall, performs better thermally at the corner. This improved performance is due to reduced thermal bridging and more effective control of heat pathways.

The material layout of FC-EPSR reduces thermal bridging, producing the more symmetrical and concentrated isotherm pattern observed in Figure 11b; moreover, there are thinner cold zones and less distortion near the corner compared to the BP-EPS wall, which, in contrast, allows heat to spread out more freely. In addition, the FC-EPSR wall has a better 2D heat flow distribution, due to smoother thermal transitions between layers and better thermal contact between layers. Finally, all these factors lead to less temperature depression at the inner corner (higher Tsi), despite FC-EPSR’s slightly higher U-value.

6.3. Multi-Criteria Decision-Making Using SAW

To conduct a comparative analysis of various wall systems based on multiple performance parameters, the simple additive weighting (SAW) [85,86] method is employed. SAW is one of the most widely used techniques in multi-criteria decision-making (MCDM) [87,88,89] due to its simplicity, transparency, and effectiveness. The parameters considered all important factors, including internal surface temperature (Tsi), surface temperature factor (fRsi), thermal transmittance (U-value), weight per unit area, and total thickness. All criteria were normalised using ratio normalisation (dividing by the maximum value) to ensure comparability, and different weighting scenarios were applied to capture design priorities (see Appendix B,

Table A1. Weighting vectors and Monte Carlo first-rank frequencies for each decision scenario.

Scenario	Tsi	fRsi	U-Value	Weight	Thickness	Top-Ranked
Equal	0.20	0.20	0.20	0.20	0.20	FC-EPSR
Thermal priority	0.25	0.20	0.35	0.10	0.10	FC-EPSR
Comfort priority	0.35	0.25	0.20	0.10	0.10	FC-EPSR
Structural priority	0.15	0.10	0.15	0.35	0.25	FC-EPSR

): equal (all criteria equally weighted), thermal-priority (emphasising U-value and Tsi), comfort-priority (favouring Tsi and fRsi), and structural-priority (highlighting weight and thickness). The methodology was implemented in Python, where SAW scores were computed as weighted sums of normalised criteria to rank alternatives. To account for weighting uncertainty, a Monte Carlo sensitivity analysis [90] with 5000 Dirichlet-sampled trials was performed, tracking how often each wall ranked first to identify the most robust choice. This method provides a rational and quantifiable approach for selecting optimal wall configurations under multi-criteria considerations.

The bar chart in Figure 12 illustrates the ranking of wall types according to their performance scores across multiple criteria. The FC-EPSR wall, made with foamed concrete and recycled EPS as its main materials, has the highest score, indicating it offers the best overall performance based on thermal and structural parameters. In terms of thermal aspects, the FC-EPSR wall, based on its design and materials, minimised cold zones, indicating superior resistance to thermal bridging. This demonstrates that optimised geometry and material arrangement can outweigh U-value metrics (this wall had a U-value of 0.195—11% more than the lowest U-value) in ensuring indoor thermal comfort. Despite the BP-EPS wall having the lowest U-value (0.172), there is slightly more corner cooling compared to the FC-EPSR wall. In terms of structural aspects, the FC-EPSR wall weighs less (160.2 kg/m²) and enjoys a practicable thickness of 0.305 m, which can be highly suitable for abandoned post-industrial buildings since it reduces the dead load on structures. In fact, the HPC-Cork wall has the lowest thickness, but its weight is 44.6% higher than that of the FC-EPSR wall.

In evaluating wall system performance, the FC-EPSR wall consistently showed superior overall behaviour across all scenarios. When thermal performance was prioritised (Figure 13), the FC-EPSR wall retained its leading position, mainly due to its low U-value, which indicates superior insulation capability and enhanced energy efficiency. In the comfort-priority scenario (Figure 14), it continued to outperform other configurations owing to its high inner surface temperature and the highest surface temperature factor, both of which minimise the risk of surface condensation. Even under the structural-priority scenario (Figure 15), where parameters such as weight and thickness were emphasised, FC-EPSR remained the best-performing option due to its low mass and moderate thickness. Furthermore, the Monte Carlo sensitivity analysis confirmed the robustness of these results, showing that FC-EPSR ranked first in 81.4% of all trials. In contrast, BP-EPS (9.6%) and HPC-Cork (9.0%) achieved top ranking only occasionally, suggesting that they may be preferable in specific, weighting situations.

6.4. Life Cycle Assessment (LCA) of Different Wall Configurations

The construction industry is one of the least sustainable activities on the planet, accounting for 40% of the total energy demand and nearly 44% of total material use, while generating 40–50% of the global production of greenhouse gases. The largest environmental impact from buildings occurs during their operational phase due to energy consumption for thermal conditioning [91].

Table 4. Embodied carbon (kg CO₂e/kg) of various thermal insulation materials based on different categories.

Case	Each Wall Layer	EF	Case	Each Wall Layer	EF
Ref-SLCR wall	Internal lime plaster	0.780	BP-AFC wall	Internal lime plaster	0.780
	Solid bricks	0.330		Hollow brick filled with perlite	0.290
	External lime-cement plaster	0.890		AFC	4.200
PPB-EPS wall	Internal lime plaster	0.780		Penetration + Mosaic plaster	0.238
	Porotherm D 24 Profi brick	0.200	HPC-Cork wall	Interior gypsum plaster	0.164
	ETICS adhesive layer	0.357		HPC (Inner)	0.190
	EPS	5.800		Cork	1.156
	Reinforcement layer of ETICS	0.412		HPC (Outer)	0.190
	Penetration + Mosaic plaster	0.238		Penetration + Mosaic plaster	0.238
BP-EPS wall	Interior gypsum plaster	0.164	FC-EPSR wall	Interior gypsum plaster	0.164
	Hollow brick filled with perlite	0.290		Foamed concrete	0.467
	ETICS adhesive layer	0.357		ETICS adhesive layer	0.357
	EPS	5.800		EPS, recycled (2 layers 0.093)	0.321
	Reinforcement layer ETICS	0.412		Reinforcement layer ETICS	0.412
	Penetration + Mosaic plaster	0.238		Penetration + Mosaic plaster	0.238

EF = Emission factor (per kg, kg CO₂-eq/kg material), Sources: created based on [92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109].

presents the embodied carbon (kg CO₂e/kg) of various thermal insulation materials based on different categories.

Life Cycle Assessment (LCA) is a tool used to estimate the potential environmental impacts and resources consumed throughout a product’s life cycle (cradle-to-grave or cradle-to-gate), i.e., from raw material acquisition, through production, use phases, and waste management, to the recycling phase [110]. The method for doing this assessment is defined in the ISO standards 14040 [111] and 14044 [112]. The life cycle centre is required to assist decision-making when choosing the best technology to handle and reduce the environmental impact of buildings during their design or refurbishment [113]. This research applies a cradle-to-gate (A1–A3) Life Cycle Assessment (LCA) methodology [114,115,116] to assess the embodied carbon (EC) impact of different wall configurations, following ISO 14040 and ISO 14044 standards. Since the primary aim of the retrofitting is to improve energy performance, the scope of this study is extended to include the operational energy use phase (B6). Considering both embodied and operational impacts ensures that the assessment captures the full life cycle carbon footprint and provides a solid basis for environmentally responsible design decisions.

The B6 evaluation is based on the boundary conditions defined in Section Boundary Condition and the thermal performance results presented in Section 6. These results are used to calculate the annual heating energy demand according to the steady-state heat transfer approach described in EN 832 [117]. The calculated energy demand is then converted into operational carbon emissions by applying the emission factor of the heating fuel, which in this study is EF_energy = 0.202 kg CO₂-eq/kWh for natural gas [118] in the Czech Republic. To obtain the lifetime operational carbon footprint [119,120], annual emissions are multiplied by the reference service life of the building [121,122], which is assumed to be 50 years. For the climatic context of Ostrava, located in Central/Eastern Europe, heating is needed for approximately half the year. This duration equates to about 4380 h per year, which is adopted as the length of the annual heating season in the operational carbon calculations.

Table 4 shows the embodied carbon (kg CO₂e/kg) for each material of the wall. In addition, the output of the cradle-to-gate (A1–A3) LCA approach to analysing the walls’ environmental impact is shown in Figure 16. It shows the comparative analysis of global warming potential (kg CO₂eq/m²) or GWP for various wall configurations in the building envelope per unit area (m²).

The graph visualises GWP (kg CO₂eq/m²) values, clearly indicating that walls with the HPC-Cork, FC-EPSR, PPB-EPS, and BP-EPS configuration have lower carbon, with a GWP of less than 75 (kg CO₂eq/m²), which is approximately 4.5 times lower than that of the Ref-SLCR (traditional wall in post-industrial areas), and also that BP-AFC has a 3.3 times lower carbon footprint compared to the Ref-SLCR. The Ref-SLCR wall, mainly comprising solid bricks and lime-cement plaster, produces over 310 kg CO₂-eq/m², placing it in the high-impact category. It reflects the carbon-intensive nature of conventional masonry and cementitious materials. We know that low-density insulation materials, natural components like cork, recycled materials, and effective cement alternatives play a major role in reducing GWP because both the HPC-Cork wall (composed of high-performance concrete + cork) and the FC-EPSR wall (composed of foamed concrete + recycled EPS) perform best, each emitting under 70 kg CO₂-eq/m².

Operational carbon emissions (B6) over 50 years, along with the total carbon footprint of the wall configurations, are presented in Figure 17 and Figure 18. For all wall configurations, the B6 contribution greatly surpasses embodied emissions (A1–A3). In the uninsulated wall (Ref-SLCR), operational carbon represents about 88% of the total life cycle impact, highlighting that neglecting the use phase would greatly underestimate environmental performance. Additionally, adding insulation significantly decreases operational carbon emissions. The best-performing configuration (BP-EPS) reduces total carbon from 2494.75 kg CO₂-eq/m² (Ref-SLCR) to 338.42 kg CO₂-eq/m², achieving an 86% reduction. The small increase in A1–A3 is quickly balanced out, resulting in significant long-term savings.

A key part of sustainable renovation and reconstruction within the EU policy framework is promoting circularity and responsible material sourcing in construction. The Life Cycle Assessment (LCA) in this study included both embodied (A1–A3) and operational (B6) carbon emissions for the six wall configurations. While operational emissions make up most of the total carbon footprint, the embodied impacts of the proposed systems are greatly affected by material choices and recyclability.

The alternative wall assemblies developed in this study incorporate materials with a high potential for circular use. These materials include hollow and solid ceramic bricks, foamed concrete, high-performance concrete (HPC), cork insulation, and expanded polystyrene (EPS)—both virgin and recycled. Many of these materials align with a circular economy approach: concrete and foamed concrete can be recycled into crushed aggregates for road sub-bases or reused in new mixes; ceramics can be repurposed as lightweight aggregate or backfill material; recycled EPS helps to reduce plastic waste; and cork offers a renewable, biodegradable insulation option. These characteristics improve end-of-life recovery options and decrease the overall embodied carbon footprint.

6.5. Integrated Discussion

European building stock, primarily constructed in the 20th century, is characterised by high energy demands, as highlighted in the new study [123]. Integrated modernisation interventions, eco-design, and sustainability are therefore urgently needed and desirable [124]. This timely study proposes renovation interventions for buildings to enhance their energy efficiency and structural performance (e.g., seismic performance). Reconstruction efforts take an integrated approach, focusing on structural, architectural, and energy improvements. In particular, replacing or upgrading the external envelope through multifunctional, prefabricated, and modular façade systems has been shown to significantly reduce heat loss across various climate zones. This study presents the proposed integrated solution as a promising alternative to demolition and reconstruction, offering deep energy renovation and structural modernisation while minimising waste production, additional energy consumption, and CO₂ emissions, thus facilitating the sustainable renovation of the existing building stock. For a society that is increasingly concerned with the environmental impact of buildings, our results make for necessary reading.

The comparative analysis of U-values for different wall systems and national building standards [125,126,127,128,129,130], as shown in Figure 19, reveals a clear improvement in thermal performance with the proposed retrofitted wall designs.

The reference wall (Ref-SLCR), which represents existing post-industrial buildings in the Czech Republic, has a notably high U-value of 1.41 W/(m²·K), indicating significant heat loss and poor thermal efficiency. In contrast, all the designed wall systems have much lower U-values, ranging from 0.351 W/(m²·K) to 0.17 W/(m²·K), effectively meeting or surpassing current regulatory requirements in several European countries. Specifically, the BP-EPS wall, with the lowest U-value of 0.172 W/(m²·K), surpasses even the most stringent national standard, i.e., England’s standard for new buildings (0.18 W/(m²·K)). Most other proposed systems, such as FC-EPSR and PPB-EPS walls, also meet the standards of countries with cold and temperate climates, including Poland, Slovakia, Austria, and the Czech Republic.

These results verify that the retrofitted wall systems are suitable not only for use in the Czech Republic but also for similar climate zones across Central and Northern Europe. Their effectiveness, however, depends on maintaining consistent indoor–outdoor boundary conditions, addressing thermal bridges (ΔU), and adhering to comparable construction practices. In warmer climates, such as Spain or Turkey, where regulatory U-value limits are more lenient, the proposed solutions would still perform well but might be considered thermally over-engineered in light of local economic and energy priorities. Overall, the proposed wall systems demonstrate a high potential for upgrading thermal performance in existing buildings, supporting energy efficiency goals, and for compliance with modern European building codes.

To further validate these findings, the obtained U-values were compared with previously published results for various concrete and brick envelopes. In recent research on concrete-based envelopes, measured U-values were reported between 0.795 and 1.23 W/(m²·K) for autoclaved concrete block walls (Calis, 2024 [131]), 0.314 and 0.323 W/(m²·K) for lightweight concrete walls (Tejedor et al., 2020 [132]), and 1.22 and 1.26 W/(m²·K) for traditional concrete block envelopes (Meng et al., 2015 [133]). Similarly, Mandilaras et al. (2014 [134]) reported U-values of 0.20–0.26 W/(m²·K) for cavity walls incorporating vacuum insulation panels. For brick-based envelopes, thermal transmittance is typically higher and more variable: Litti et al. (2015 [135]) measured 0.428–1.933 W/(m²·K) for solid brick masonry, while Calis (2024 [131]) found values between 0.718 and 2.07 W/(m²·K) for pumice and clay block systems under different thermal conditions. Compared to these datasets, the U-values achieved in the present study (0.351–0.17 W/(m²·K)) represent a notable enhancement in thermal efficiency, particularly when considering the balance between thermal resistance, material mass, and retrofit feasibility. This indicates that the optimised multilayer assemblies proposed here perform significantly better than traditional concrete or brick walls and align closely with or exceed the performance of advanced insulated wall systems reported in the recent literature.

Moreover, multi-objective optimisation approaches have been widely employed to integrate multiple performance objectives, such as thermal efficiency, energy savings, and visual comfort, in building envelope design. For example, Ibrahim et al. (2025 [136]) applied such methods to automated shading systems, optimising daylighting and energy performance to balance comfort and efficiency. While their focus was on shading, this illustrates the broader applicability of multi-criteria approaches in building design. In the present study, a similar principle underpins the use of SAW combined with Monte Carlo analysis to evaluate wall configurations across thermal and structural criteria.

6.5.1. Integrated Prefabricated Facade Systems for Energy-Efficient Building Renovation in the EU

Current trends in constructing external facades for both new buildings and renovations are highly conducive to integrated designs. For instance, the authors of [137] describe ways to enhance the energy performance of buildings by incorporating energy systems into the building structure, thereby promoting efficient prefabrication. This approach aligns with the principles of sustainable construction. A new methodology is proposed for the design, implementation, and operation of buildings and urban areas, aiming to fulfil a diverse range of functional, economic, environmental, social, and cultural requirements. However, our argumentation—based on simulation results—suggests that the envelopes of frame structures can be effectively reconstructed with multilayer walls in an appropriate composition, and the envelope can be made energy-efficient. This aligns with legislative documents in the EU. The Building Act [138] states, among other things, that energy reconstruction should be performed in a suitable, economically and technologically acceptable manner. The proposed solution may serve as a promising alternative to demolishing the entire structure, which would result in unnecessary waste production, increased energy consumption, and CO₂ emissions. This would run counter to the EU’s strategic documents, which emphasise achieving climate neutrality by 2050.

Looking at the EU’s current growth strategy, climate change and energy sustainability priorities establish, for instance, the need for a 32.5% energy efficiency improvement by 2030. The improvement in energy efficiency has key impacts on the efforts that the EU is undertaking to reduce energy consumption and greenhouse gas (GHG) emissions [139]. Each Member State must employ the available instruments to effectively design a national regulatory and policy framework for building energy performance. The most important regulatory and policy instruments to promote energy efficiency in buildings are identified in the general literature on policy instruments for energy efficiency [140,141,142,143] and on environmental policy instruments [144,145,146,147]. Regulation of design requirements can take various forms, encompassing both specific standards and explicit directives. Within individual Member States, policy makers are expected to make choices regarding the mix of instruments to increase adaptive flexibility and to reduce risk in pursuing sustainability [148]—in particular, regarding energy efficiency in buildings [149,150,151]. Over the years, different laws related to energy usage in buildings have been ratified by the European Parliament, the main being the Energy Performance of Buildings Directive (EPBD, 2002/91/EC). It was first adopted in 2002 and later revised in 2010 (EPBD recast, 2010/31/EU). The legislation articulates two main mechanisms for enabling energy assessment in the building sector: energy regulation (e.g., building codes) and energy certification [152]. Under the EPBD, the energy certification of buildings becomes compulsory in Member States and thus has a vital role in saving energy [153].

Increasing the energy performance of buildings is key to securing the transition to a low-carbon economy and achieving the EU’s climate and energy objectives. Several studies have looked into its implementation in different European countries, showing the key features and potential in increasing the energy efficiency, e.g., the study performed by Dascalacki et al. [154] in Greece, by Tronchin and Fabbri [155] in Italy, and by Ekins and Lees [156] in the UK. The presented methodology is particularly suitable for buildings in the temperate climate zone, which includes much of Central Europe. However, given the very low U-values achieved by the designed wall systems (as low as 0.17 W/(m²·K)), the methodology also offers strong potential for application in colder climate regions, where higher insulation standards are required.

EU Member States implement European regulations on buildings and the built environment in very different ways. This is recognised by the European Commission itself, which has set up a range of programmes to help Member States with implementation issues and to share experiences. They include the Concerted Action initiatives to address specific issues, the EPBD Buildings Platform, and SAVE demonstration projects, all introduced as part of the Commission’s ‘Intelligent Energy-Europe’ programme. Despite teething difficulties, a European survey of national experts in October 2007 found that around 90% of respondents expected the EPBD to contribute to national climate change objectives and the modernisation of the housing stock (ENR, 2008). A study modelling the housing stock in the EU-15 and the likely impact of the EPBD found that it might be expected to reduce CO₂ emissions by 34 million tonnes per year, and that this could be doubled if the EPBD were extended to cover the whole housing stock [157].

EU data indicate that around 35% of buildings in the EU are older than 50 years, and almost 75% of the building stock is energy-inefficient according to current standards; a realistic assumption is that around 80–90% of these buildings will still be standing around 2050. Prefabricated construction from the second half of the last century, due to the structural arrangement of its load-bearing system and assembly technology (especially of the skeletons), appears to be a very good technology for new thermal and energy renovation, involving partial deconstruction rather than completely new construction. Appropriately selected technological operations and new materials are contributing to the transformation of these buildings, which have the potential to become efficient urban structures, sustainable and environmentally friendly, while reducing the carbon footprint. This kind of renovation on these types of buildings can be characterised as sustainable, as contributing to increasing the adaptability of existing buildings. Hence, the proposed wall systems not only meet energy performance targets but also align with EU circular economy objectives by promoting material reuse, waste reduction, and lower life cycle emissions.

6.5.2. Practical Implications

The presented results have direct implications for retrofit decision-making. First, the marked reduction in U-values (from 1.41 to 0.17 W/m²·K) demonstrates that replacing external walls with modern multilayer assemblies can rapidly bring legacy façades into compliance with current energy regulations, reducing operational heating demand and associated emissions. Second, the large reductions in wall weight (up to 80.5%) and thickness (up to 52%) indicate that many prefabricated, lightweight systems can be integrated into existing frame skeletons without extensive strengthening works, reducing conversion costs and site disruption. Third, the superior corner performance and lower condensation fraction of the FC–EPSR system show that assembly layout and layer sequencing (not only U-value) crucially affect hygrothermal risk and long-term durability. Taken together, these findings support a pragmatic retrofit strategy: prioritise systems that balance low U-value, reduced mass, and robust 2D thermal continuity (to minimise thermal bridges), and complement the technical selection with LCA and economic appraisal to ensure whole-life benefits.

6.5.3. Limitations and Uncertainties

Although this study provides a comprehensive assessment of multilayer wall systems using both analytical and numerical models, several limitations and uncertainties should be acknowledged.

First, the simulations were conducted under steady-state, two-dimensional conditions, neglecting transient and three-dimensional effects that may occur in real building envelopes due to dynamic outdoor temperatures, solar radiation, or air infiltration. These effects are expected to be moderate for planar sections but could be significant near junctions and interfaces, suggesting the need for future 3D dynamic validation.

Second, while boundary resistances (Rsi = 0.13, Rse = 0.04 m²·K/W) were applied to represent convective surface exchanges, these standardised values may not fully capture the variability of real indoor and outdoor conditions, especially under conditions of fluctuating wind or humidity.

Third, the model validation, based on 1D analytical solution and numerical verification (FDM) and compliance with EN ISO 10211 convergence criteria, confirmed numerical reliability (mid-wall temperature difference < 5%, residuals < 10⁻⁴). Nevertheless, the absence of experimental or in situ measurements introduces some uncertainty regarding real-world heat losses through construction joints, workmanship imperfections, or air gaps.

Despite these constraints, the consistency between 1D and 2D results, the rigorous standard-based validation, and the use of multiple modelling tools (TEPLO, AREA, and Python-FDM) ensure that the findings are numerically robust and scientifically credible. Future research should integrate transient coupled heat-and-moisture simulations, laboratory hot-box tests, and probabilistic sensitivity analyses to refine model accuracy and capture real operating variability in retrofit scenarios.

Finally, it should be emphasised that no in situ measurements, such as thermal imaging, wall moisture content assessment, or long-term energy consumption monitoring, were performed in this study. Furthermore, the hygrothermal assessment was limited to steady-state diffusion analysis in accordance with EN ISO 13788. Therefore, future research should incorporate in situ experimental validation and transient modelling approaches to improve the reliability and real-world applicability of the results.

7. Conclusions and Recommendations

7.1. Conclusions

This study evaluated the thermal, structural, and hygrothermal performance of traditional and innovative multilayer wall systems for the adaptive reuse of industrial buildings. The key findings are summarised below:

• Innovative wall systems significantly outperform the traditional Ref-SLCR wall, achieving up to 87.8% lower U-values and 80.5% lower weight, thereby supporting energy-efficient retrofit applications.
• HPC-Cork, PPB-EPS, BP-AFC, BP-EPS, and FC-EPSR walls show major reductions in thickness (up to 52%) and density, minimising structural loads in conversion projects.
• Wall thickness does not correlate directly with insulation performance; material conductivity and layer configuration are the dominant factors in thermal efficiency.
• The BP-EPS wall provides the lowest corrected U-value (0.172 W/m²·K), while FC-EPSR delivers the most balanced overall thermal behaviour.
• 2D simulations show that traditional walls experience strong thermal bridging, producing low corner temperatures and extended cold zones.
• Innovative systems maintain higher isotherm continuity, reducing corner heat losses and improving envelope uniformity.
• Despite having a slightly higher U-value, FC-EPSR achieves better corner temperature (Tsi) than BP-EPS. This is due to FC-EPSR’s smoother heat-flow transitions and reduced thermal-bridge distortion.
• The reference wall shows low thermal comfort (fRsi = 0.689, Tsi = 9.11 °C), representing high condensation and mould risk.
• All innovative walls exceed the critical fRsi = 0.75 threshold, achieving a Tsi of 16.84–17.99 °C, consistent with indoor comfort standards.
• Lower U-values generally result in higher Tsi, though minor deviations show the importance of a complete layer-configuration analysis.
• FC-EPSR has the lowest internal condensation fraction (~35.7%) and the highest fRsi (0.943), proving superior moisture resilience and long-term durability.
• Across all MCDM–SAW scenarios, FC-EPSR consistently ranks as the best overall performer.
• Monte-Carlo uncertainty analysis confirms FC-EPSR’s robustness, maintaining top ranking under variable weighting conditions.
• Optimised multilayer composite walls can substantially improve energy efficiency, structural feasibility, thermal comfort, and moisture safety in building retrofits.
• The FC-EPSR configuration provides the most balanced performance across all criteria, representing a highly effective solution for the adaptive reuse of industrial buildings and for achieving long-term energy and climate objectives.
• Although the BP–EPS wall system achieves the lowest corrected U-value, the results indicate that it may not represent the optimal solution when real building conditions are considered. The FC–EPSR configuration shows improved overall hygrothermal performance, with relatively higher Tsi and fRsi values and a lower condensation risk. Its enhanced behaviour at the internal corner, as observed in the 2D numerical simulations, suggests a reduction in thermal bridging effects and a potential improvement in local thermal comfort. Therefore, these findings suggest that wall performance should not be evaluated based on U-value alone, but should also consider thermal bridge effects and moisture-related behaviour.
• This study evaluated six wall configurations by integrating embodied (A1–A3) and operational (B6) carbon emissions to determine their whole-life environmental performance. The results show that, for the (A1–A3) stage, innovative assemblies such as HPC-Cork, FC-EPSR, PPB-EPS, and BP-EPS achieve the lowest global warming potential, each remaining below 75 kg CO₂-eq/m², which is approximately 4.5 times lower than the conventional Ref-SLCR wall. In contrast, the reference wall exceeded 310 kg CO₂-eq/m², underscoring the carbon-intensive nature of traditional masonry and cementitious materials.
• The Ref-SLCR wall, with an embodied carbon footprint of 310.06 kg CO₂-eq/m², is comparable to some insulated alternatives, such as BP-EPS, which has a footprint of 72.11 kg CO₂-eq/m². However, when considering operational carbon emissions, the Ref-SLCR wall performs significantly worse. The results emphasise that improving thermal performance is the most effective method for reducing life cycle emissions. Even though high-performance insulated walls may have slightly higher embodied carbon, they achieve much lower carbon emissions throughout the building’s service life.
• Material selection also played a critical role in circularity. Recycled EPS, foamed concrete, high-performance concrete, ceramic bricks, and cork demonstrated notable end-of-life recovery potential, supporting EU objectives for resource efficiency and low-carbon renovation. Overall, the findings highlight that combining low-impact materials with effective insulation provides a practical and scalable pathway for reducing whole-life carbon in existing building envelopes, particularly in post-industrial European contexts.
• This study is based on steady-state 2D simulations with standardised surface resistances and no experimental validation; results should be interpreted as a comparative assessment, with caution when applied to real retrofit projects.

7.2. Recommendations

In future research, a set of recommendations will be developed for the adaptive reuse of buildings with frame construction types in post-industrial locations:

• The proposed approach will be extended to include other components of the building envelope, such as roofs, floors, ceilings, and window openings, as well as their interaction with technical building systems. This broader scope will enable a more comprehensive assessment of the overall thermal behaviour and energy performance of renovated structures. Furthermore, coupling the wall-based heat transfer model with dynamic simulations of complete building envelopes will provide deeper insights into seasonal variations, airtightness, and integrated retrofit strategies. Such developments will enhance the applicability of the presented methodology for whole-building energy optimisation in sustainable renovation projects.
• The integration of photovoltaic (PV) modules into the exterior building shell, particularly façades, will be considered as part of multifunctional envelope systems. Building-integrated photovoltaics (BIPV) can enhance on-site renewable energy generation while contributing to the thermal performance and shading of façades. The results of this study, which highlight the role of external layers and surface temperatures in reducing heat losses, demonstrate that similar principles can be applied to BIPV façades to achieve both energy efficiency and architectural integration in renovated industrial buildings.
• Future studies should also evaluate proposed systems for their economic efficiency, including life cycle cost analysis (LCCA), payback period, and net present value (NPV), to ensure long-term cost-effectiveness. This aspect is particularly relevant for post-industrial buildings, where investment decisions depend on both energy performance and financial viability. In addition, the environmental assessment should be extended to a full cradle-to-grave life cycle analysis, including end-of-life stages (C1–C4) and Beyond-Life (D), to provide a more comprehensive evaluation of long-term environmental impacts.