Nearly Zero-Energy Buildings (NZEBs): A Systematic Review of the Current Status of Single-Family Houses in the EU

Abstract

The building sector, responsible for approximately 40% of global energy consumption, is increasingly embracing nearly zero-energy buildings (NZEBs) to promote environmental sustainability. Focusing specifically on single-family houses, this review systematically examines current NZEB practices across Europe, aiming to identify regional adaptation strategies and highlight performance disparities. The primary research question explored is as follows: how do design strategies, renewable energy integration, and climate adaptation measures for single-family NZEBs vary across Northern, Eastern, Southern, and Western European countries? A key gap in the literature is the lack of cross-comparative analysis of regional NZEB approaches for single-family houses, despite their significant share in Europe’s housing sector. Effective NZEB implementation depends on interdisciplinary collaboration among architects, engineers, and energy experts to optimize building design elements, including orientation, envelope insulation, and HVAC systems, tailored to regional climatic conditions. A systematic analysis of case studies was conducted, synthesizing data on primary energy consumption, CO₂ emissions, and building envelope performance. The findings reveal regional differences: Northern Europe exhibits primary energy consumption at 27–68 kWh/(m²·y) (mean: 48.2), Eastern Europe at 29–68 (mean: 42.5), Southern Europe at 35–42 (mean: 39.1), and Western Europe at 27–85 (mean: 51.5), with higher emissions in Eastern Europe compared to Denmark, for instance. These patterns underscore the role of climatic conditions and regulatory frameworks of the regions in shaping NZEB strategies. Despite shared goals of decarbonization and occupant comfort, significant knowledge gaps remain, particularly regarding long-term operational performance and regional comparison of other building types.

1. Introduction

The building sector, encompassing both commercial and residential structures, is a major contributor to global energy consumption, accounting for approximately 40% of primary energy (PE) use in regions such as the United States (US) and Europe, and around 30% in China [1,2]. Furthermore, electricity consumption accounts for a substantial portion, frequently translating to 40% or more of primary energy use and energy-related CO₂ emissions [3]. Given the construction industry’s considerable role in CO₂ emissions, analyzing the energy use of buildings has become increasingly critical to address environmental sustainability challenges [4]. This significant energy demand and greenhouse gas emission underscores the urgency of adopting sustainable energy practices. The World Health Organization [5] reports that around 7 million premature deaths occur annually as a result of the combined effects of ambient and indoor air pollution. The US and Europe have committed to becoming carbon neutral by 2050, and China has pledged to reach “carbon neutrality” by 2060 [6]. To address these challenges and achieve these targets, extensive research has been conducted worldwide on energy-efficient technologies that reduce dependence on non-renewable primary energy sources.

Renewable energy integration has emerged as a vital strategy to combat global warming, reduce air pollution, and ensure energy security [7]. Zero-energy buildings (ZEBs), and their subset, nearly zero-energy buildings (NZEBs), represent a transformative approach in this context. These buildings aim to balance their energy needs through on-site or nearby renewable energy production, effectively minimizing their environmental footprint [8]. By methodically reducing CO₂ emissions into the atmosphere, nearly zero-energy buildings play a pivotal role in decarbonizing the economy and fostering sustainable development [9]. Beyond energy efficiency, NZEBs also reflect advancements in architectural design, smart technologies, and energy systems. They not only reduce greenhouse gas emissions but also improve occupant comfort and lower operational costs, aligning with the goals of sustainable urban development and global climate action frameworks.

Nearly zero-energy buildings are defined in literal, graphical, and mathematical forms by various researchers and organizations, reflecting the multifaceted nature of this concept [8,10,11]. The EU Directive [12] defines NZEBs as highly energy-efficient buildings with minimal energy demand, primarily met through on-site or nearby renewable energy sources. Similarly, the International Energy Agency describes NZEBs as buildings with very high energy performance, relying mainly on renewable sources for their limited energy needs. The Federation of European Heating, Ventilation and Air-conditioning Associations (REHVA) [11] further specifies NZEBs as grid-connected buildings that maintain a balance between PE consumption and the energy fed back into the grid. In the US, the Department of Energy [13] adopts a performance-based approach, defining NZEBs as energy-efficient buildings where the total annual delivered energy does not exceed the energy exported from on-site renewables.

For a building to be classified as a nearly zero-energy building (NZEB), its total on-site renewable energy generation should offset or exceed its imported energy over a given period. When the building’s boundary is fixed, the energy balance remains either zero or positive over a defined period [1]. The balance between the energy a building generates on-site and the energy it consumes from external sources is referred to as net energy. It is determined by comparing the total renewable energy produced within the buildings such as from solar panels or wind turbines to the energy imported from the grid or other external supplies. Additionally, different energy sources are assigned specific weight factors to reflect their environmental impact or efficiency, giving higher priority to renewable energy over conventional sources [11]. By combining these definitions and representations, the concept of NZEBs becomes more comprehensive, catering to both theoretical frameworks and practical applications.

Europe is at the forefront of adopting the NZEB concept, demonstrating a strong commitment to sustainable development and energy efficiency. The European Union (EU) has established comprehensive guidelines to achieve NZEB targets, most notably through the Energy Performance of Buildings Directive (EPBD) 2010/31/EU [14], which mandates that all new buildings meet NZEB standards by 2021. Additionally, new buildings occupied and owned by public authorities were required to comply by 2019. The Renewable Energy Directive (RED) 2018/2001/EU [15] further complements these efforts by ensuring that at least 32% of energy consumption in buildings comes from renewable sources by 2030. However, a recent study by Maduta et al. [16] indicates that many European countries are still falling short of the recommended benchmarks, particularly in reducing non-renewable energy demand. Therefore, member states are also required to develop tailored strategies, considering local climate conditions, economic factors, and technological advancements to meet these ambitious goals. This research is significant as it addresses the urgent need to reduce energy consumption and associate emissions in buildings, which directly contribute to climate change mitigation and improved human health. The application value lies in providing guidance for policymakers, engineers, and architects to develop strategies that balance energy efficiency, occupant comfort, and economic viability while transitioning towards NZEB targets with sustainable and low-carbon futures.

To achieve NZEB targets, design practices prioritize thermal insulation, efficient HVAC systems, smart technologies, and the integration of renewable energy systems, coupled with life-cycle cost analysis to ensure economic viability. Countries like Denmark, Germany, and Sweden have made notable progress, with Denmark enforcing strict energy efficiency in building codes and Germany offering financial incentives for NZEB projects [17,18,19,20]. Despite these advancements, challenges such as high initial costs, technical barriers, and varying readiness levels among EU nations persist. However, Europe continues to focus on smart technologies, enhanced policy frameworks, and financial mechanisms to accelerate NZEB adoption, contributing significantly to global sustainability goals. The problem statement of this study is that despite the ambitious NZEB goals and existing policy frameworks in Europe, there is a persistent gap in implementing across regions, due to varying local challenges, climatic differences, and economic disparities. Figure 1 illustrates the evolution of EPBD requirements toward NZEBs, showcasing the progressive transition from past regulations to future aspirations, underscoring Europe’s commitment to sustainable urban development.

The scope of this review focuses on the evaluation of the NZEB concept and its implementation across Europe, including performance indicators, challenges, and regional differences in adopting practices. It also covers technical and policy frameworks that support or hinder the implementation of NZEBs, providing insights that can guide future research and practical application. This holistic approach to building design and operation ensures that NZEBs contribute meaningfully to the transition toward a low-carbon future, showcasing their critical role in sustainable architecture. By incorporating innovative solutions such as energy-saving technologies, smart systems, and renewable energy sources (RES), NZEBs highlight the path toward a more sustainable environment.

2. Materials and Methods

2.1. Subject of Study

This paper aims to examine existing nearly zero-energy buildings (NZEBs) across different geographical subregions of Europe, conducting a comparative analysis focused on architectural design, smart technologies, and renewable energy systems. The study considers European Union (EU) member states, aligning with EU regulations and recommendations, ensuring a consistent framework for evaluation. The selected regions include Northern Europe (Denmark, Estonia, Finland, Ireland, Latvia, Lithuania, Sweden), Eastern Europe (Bulgaria, Czech Republic, Hungary, Poland, Romania, Slovakia), Southern Europe (Croatia, Cyprus, Greece, Italy, Malta, Portugal, Slovenia, Spain), and Western Europe (Austria, Belgium, France, Germany, Luxembourg, The Netherlands), as illustrated in Figure 2 [23]. It is important to note that division into regions is only a simplified assumption for statistical purposes, as climatic conditions can vary significantly within countries due to factors such as mountainous terrain and coastal influences.

2.2. Research Framework and Methodology

The method adopted in this review study was divided into three steps. Firstly, an integrative literature review was carried out to find and select articles related to the main subject of this study. It was developed through systematic research on scientific publication databases, including Scopus, ScienceDirect, IEEE Xplore, SpringerLink, ASCE Library, and Google Scholar. For further analysis, the authors chose papers covering the topic of NZEB in the residential sector in Europe, including energy efficiency, best practices, and sustainable solutions. A comprehensive search strategy was implemented using keywords such as “Nearly-Zero Energy Buildings”, “NZEB”, “energy efficiency in buildings”, renewable energy integration”, “smart building technologies”, “climate conditions in Europe”, and “sustainable building design in Europe”. Only documents in English, published between 2010 and 2025, were considered.

Secondly, the process of selecting and screening of relevant studies followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines to ensure transparency and reproducibility. A total of 242 studies were initially identified. After removing duplicates and screening titles and abstracts, potentially relevant studies were kept for further processing. A thorough full-text analysis was conducted, resulting in the inclusion of 89 articles and reports that met the criteria for this review. The inclusion criteria focused on case studies addressing NZEB implementations in Europe’s residential sector. Also, articles discussing energy efficiency strategies, renewable energy integration, smart technologies, NZEB performances, and policy framework were included. Non-European case studies and articles focusing exclusively on commercial or industrial buildings were excluded. A flowchart illustrating the selection process is included in Figure 3 [24].

The selected studies were divided into regional categories to reflect geographic and climatic diversity. Finally, strategies, technical solutions, and the main outcomes were analyzed. The article presents the foremost approaches applied in EU member countries. The main limitations, challenges, and primary considerations for future studies are discussed. Recognizing the diverse temperature and climate conditions across these subregions, the study evaluates how the key performance metrics—namely architectural design strategies, integration of smart technologies, primary energy consumption, CO₂ emissions, building envelope performance, and renewable energy systems—are applied and adapted in each context. This comprehensive evaluation aims to identify region-specific solutions that optimize energy usage while maintaining sustainability.

To achieve this goal, the paper focuses on the following objectives:

• Evaluate Design Strategies: examine the special design features of selected NZEBs across Northern, Eastern, Southern, and Western Europe, highlighting adaptations for varying temperature and climate conditions.
• Assess Renewable Energy Systems: examine the adoption and efficiency of renewable energy sources (solar, wind, geothermal, etc.) in NZEBs across diverse European climates to identify region-specific strategies.
• Compare Climate Adaptation Strategies: explore how NZEBs in various European subregions address local climatic challenges, such as extreme cold in Northern Europe or high temperatures in Southern Europe, to achieve energy efficiency and sustainability.
• Evaluate Energy Performance: investigate energy efficiency of selected buildings by analyzing the energy consumption and production patterns of NZEBs in different regions.

This review followed the PRISMA 2020 guidelines [24]. The review protocol was not registered in any public registry, such as PROSPERO. The PRISMA 2020 checklist is presented in Table S1 (Supplementary Materials).

3. Comparative Overview of Conditions in Europe

3.1. Public Policies and Financial Mechanisms

The adoption of nearly zero-energy buildings (NZEBs) across Europe varies significantly due to differences in public policies, financial programs, and regional economic conditions [16]. According to Ohene et al. [25], interventions and policies of several governments are focused mainly on residential NZEBs. Northern Europe leads in NZEB implementation, with Denmark and Sweden setting stringent energy performance standards and offering substantial financial incentives [26]. Denmark was among the first EU member states to establish a national definition and roadmap for NZEBs, supported by its building regulations (BR18) that emphasize energy efficiency and renewable energy use [27]. These regulations require strict compliance with new constructions and renovations, with a focus on high insulation standards and efficient technical systems. Similarly, Sweden enforces strict energy efficiency standards, promotes renewable energy, and encourages converting existing buildings into NZEBs through legislative and financial measures [28]. Estonia has also emerged as a leading country in NZEB implementation by establishing minimum energy performance requirements in 2019, aligning with cost-optimal calculations, resulting in highly energy-efficient buildings [29]. Finland has adopted NZEB standards, focusing on integrating renewable energy sources and enhancing building insulation. However, economic challenges have led to a more gradual implementation compared to Estonia [30]. While Ireland has defined NZEB requirements, focusing on PE consumption of new buildings, Latvia and Lithuania have transposed the EPBD into national legislation, setting minimum energy performance requirements and promoting the use of renewable energy in buildings [31]. These robust frameworks have positioned Northern Europe as a leader in energy-efficient construction.

Eastern Europe’s progress in implementing NZEB policies varies, with challenges in policy frameworks, financing, and market readiness. Romania has faced delays in adopting EPBD initiatives, with implementation still incomplete. A significant issue in the country is unauthorized construction, driven by a complex permitting process that requires around 24 bureaucratic procedures and takes approximately 260 days to complete. This lengthy and costly process, combined with limited investment capacity, has led many to consider building without a permit [32]. In contrast, Poland has steadily tightened thermal protection regulations under the NZEB standard, emphasizing improved thermal insulation and reduced non-renewable primary energy demand [33,34]. Beyond Romania and Poland, Bulgaria has implemented NZEB requirements through its National Energy and Climate Plan, focusing on improving building energy performance and increasing the use of renewable energy sources. The Czech Republic has taken significant steps towards NZEB implementation, including plans to diversify its energy mix by constructing new nuclear reactors to reduce reliance on fossil fuels [35,36]. Hungary and Slovakia have also adopted measures to enhance building energy efficiency, aligning with EU directives and focusing on modernizing their building stock to meet NZEB standards, demonstrating a commitment to strengthening public policies in the region [37].

Southern Europe has made advancements with phased plans and tax incentives. Spain’s national NZEB plan includes intermediate targets from 2015 to 2020 and regulatory definitions to promote nearly zero-energy buildings [38]. Slovenia emphasizes reducing energy needs for heating, cooling, and air-conditioning, while expanding the contribution of renewable sources to overall energy consumption [39,40]. Italy has been offering tax incentives since 2007 for energy-efficient retrofits, including thermal insulation, solar panels, and advanced heating systems, which have supported the adoption of NZEBs [19]. Croatia has mandated that all new buildings adhere to NZEB standards since 2020. The country has also established a roadmap for increasing the number of NZEBs and has launched training centers to promote energy efficiency in the building sector [41]. Cyprus has initiated projects to transform public school buildings into NZEBs, integrating renewable energy sources and improving energy performance [42]. Greece has enacted the National Climate Law (Law 4936/2022), aiming to reduce greenhouse gas emissions by at least 55% by 2030 and 80% by 2040 [43]. The law includes measures to improve energy efficiency and promote NZEB standards. Malta and Portugal have also implemented NZEB requirements, focusing on integrating renewable energy systems and enhancing building insulation to meet EU targets.

Western Europe demonstrates steady progress in NZEB implementation due to well-aligned policies and financial mechanisms. Germany has integrated NZEB requirements into its Energy Conservation Regulation [44], supported by financial incentives and alignment with KfW Efficiency House standards [19]. France enforces low energy requirements through the EPBD and its thermal regulation RT 2012, applicable to both residential and commercial buildings since 2013 [26]. Austria, through the “Haus der Zukunft” (House of the Future) program, supports innovative energy efficiency projects that contribute to NZEB development [45]. Belgium has defined regional NZEB standards by setting different PE consumption values for Flemish regions, Walloon region, and Brussels-capital region separately. Luxembourg and the Netherlands have also established NZEB definitions and are actively promoting energy-efficient building practices to meet EU directives [46].

In summary, the pace of NZEB implementation across Europe is shaped by the strength of public policies, availability of financing programs, and regional economic conditions. Countries with comprehensive policies and robust financial support achieve faster adoption, while those with economic challenges or less stringent regulations experience slower progress.

3.2. Meteorological Conditions

Nowadays, thermal conditions pose significant challenges to human well-being, affecting quality of life, health, and overall comfort [47]. These concerns are closely linked to sustainable development, emphasizing the need for climate-responsive designs and strategies. With increasing global populations and rising energy demands, coupled with the growing users’ expectations and climate change, there is a heightened focus on creating energy-efficient and adaptive indoor environments [48]. As a significant portion of the global and European population resides in energy-intensive spaces [49], the shift towards sustainable construction practices is essential. Implementing the NZEB concept can contribute to this transition by promoting energy-efficient designs across various sectors, including residential, educational, commercial, and healthcare buildings.

Conducting a comparative analysis of thermal and climatic conditions across European regions is essential for understanding the diverse challenges and opportunities faced by Europe. Recent research utilizing climate observation and reanalysis data has revealed a degradation in optimal thermal conditions across European cities over recent years. According to Antonescu et al. [50], southern European countries are experiencing a growing frequency of heat-related stress, whereas northern European countries are seeing a noticeable reduction in the occurrence and duration of cold stress. According to Ruosteenoja et al. [51], the thermal summer in Northern Europe—defined as periods with a daily mean air temperature above 10 °C—is projected to lengthen by approximately 30 days on average by mid-century (2040–2069). In contrast, the thermal winter—characterized by daily mean temperatures below 0 °C—is expected to shorten by up to 60 days, indicating a significant shift in seasonal patterns due to climate change. Therefore, it is crucial to carefully consider these anticipated climate changes when designing new NZEBs, ensuring they are tailored to the specific conditions of their locations.

Based on studies by Katavoutas et al. [47] and Błażejczyk et al. [52], a few selected areas in the European regions were considered—Helsinki (Northern Europe), Warsaw (Eastern Europe), Athens (Southern Europe), and Rotterdam, Vienna (Western Europe)—to assess climate change impacts on thermal stress by the end of the 21st century. According to the updated Köppen–Geiger climate classification [53], Helsinki experiences a boreal (Dfb) climate, characterized as fully humid with warm summers. In contrast, Rotterdam, Warsaw, and Vienna could be classified into the category of a warm temperate (Cfb) climate, also characterized as fully humid with warm summers. However, despite having the same climate type, Rotterdam’s coastal location contrasts with Vienna’s position as a continental city located at the base of the Alps, resulting in distinct climatic nuances influenced by their geographical settings. Warsaw’s inland climate exhibits less maritime influence than Rotterdam, resulting in reduced sea breeze effects and greater day-to-night and seasonal thermal amplitudes. Athens is situated within the Mediterranean climate zone, featuring a warm temperate climate with dry, hot summers (Csa) [47].

Coastal areas experience unique thermal dynamics due to the interplay between urban structures and maritime influences. The proximity to large water bodies introduces the sea breeze phenomenon, which significantly impacts urban thermal conditions. During the daytime, especially in warmer periods, sea breezes transport cooler air from the sea into the city, mitigating urban overheating and reducing the intensity of the Urban Heat Island (UHI) effect. However, the interaction between sea breezes and UHIs is complex. In some instances, the UHI can create a stagnation region over the area, delaying the inland penetration of the sea breeze. This delay can prolong higher temperatures in urban areas before the cooling effects of the sea breeze set in [54].

Studies in various European coastal areas have highlighted these dynamics. For example, research in Bari, a Mediterranean coastal area in Southern Italy, demonstrated that the UHI reaches its maximum intensity during summer nights. The study emphasized that urban characteristics significantly influence how the UHI phenomenon manifests, with sea breezes playing a crucial role in modulating thermal conditions [55]. Similarly, an investigation in Bilbao, a coastal area in northern Spain, examined the characteristics of the UHI using climate data and specific measurement campaigns. The findings revealed that the complex topography and sea/land breeze dynamics significantly influence the urban climate, affecting both spatial characteristics and the temporal evolution of the UHI [56]. Incorporating these coastal climatic influences is also essential when designing and implementing sustainable urban development strategies, particularly in the context of NZEBs. Understanding the mitigating effects of sea breezes can inform climate-responsive designs that enhance urban resilience and livability.

4. Performance of NZEBs Across European Regions

For a detailed comparison of the performance of nearly zero-energy buildings (NZEBs) across different regions of Europe, specific performance indicators can be referenced. CO₂ emissions for buildings in Northern Europe (e.g., Denmark) range from approximately 5 to 10 kg CO₂/m² annually, attributed to advanced insulation systems and renewable energy sources, as highlighted by [29]. In Southern Europe, where cooling demands are higher, these emissions can increase to 15–20 kg CO₂/m² annually [57]. Primary Energy (PE) consumption in Northern Europe is about 40–55 kWh/m² annually, resulting from optimized heating technologies, as noted by [11]. Meanwhile, in Southern Europe, energy demand may reach 50–70 kWh/m² due to the significant need for cooling, as recorded in [58]. Operational costs in Western Europe (e.g., Germany, The Netherlands) average between 8 and 12 EUR/m² annually, thanks to efficient energy systems [59]. In Eastern Europe (e.g., Poland), operational costs can be as high as 15 EUR/m², reflecting regional challenges in energy efficiency, as noted by [60]. These variations highlight the diverse climatic, technological, and economic factors influencing NZEB performance across Europe. Understanding these regional differences is crucial for developing tailored strategies that enhance energy efficiency, reduce emissions, and optimize operational costs, ultimately supporting the broader goal of sustainable building practices.

The European Commission provided benchmark values for energy performance of NZEBs for the four European climatic zones [61]. The Nordic climatic zone extends over Northern European countries, the Continental climatic zone extends over Eastern European countries, the Mediterranean climatic zone extends over Southern European countries, and the Oceanic climatic zone extends over Western European countries. For single-family houses, the PE requirements range from 50–90 kWh/(m²·y), while for office buildings, the range is 80–100 kWh/(m²·y). The values of PE and RES for each of the climatic zones are presented in

Table 1. EC recommendation on energy performance of single family houses and renewable benchmarks (Source: [61]).

Energy Use	Nordic	Continental	Mediterranean	Oceanic
Net PE, kWh/(m2·y)	40–65	20–40	0–15	15–30
PE threshold including RES, kWh/(m2·y)	65–90	50–70	50–65	50–65
RES, kWh/(m2·y)	25	30	50	35
RES as % of Total PE, %	32%	50%	87%	61%

.

According to Table 1, countries with milder climates (Mediterranean and Oceanic) typically have the lowest net primary energy requirements and the highest share of renewables. However, when assessing the PE requirements of buildings, regardless of whether the energy is sourced from renewables or not, the variation across the four climatic zones is relatively narrow. Figure 4 highlights significant discrepancies in energy performance requirements across EU member states, showing how national NZEB benchmarks vary depending on regional contexts. Despite these climatic zone-specific differences, most EU member states have set ambitious goals to limit primary energy use for residential buildings to no more than 50 kWh/(m²·y), reflecting a strong commitment to energy efficiency and sustainability [29]. European Union member states in Northern, Eastern, Southern, and Western regions have established their NZEB requirements for single-family houses in their national plans, which include numerical indicators of primary energy use expressed in kWh/m² per year.

A comparison of national NZEB standards across the EU member states reveal notable discrepancies in energy performance requirements. Countries such as Bulgaria, Cyprus, Czechia, Finland, Hungary, Latvia, and Romania have adopted NZEB standards with permissible PE values exceeding the European Commission’s recommended benchmarks, indicating underutilized energy efficiency potential. In contrast, Croatia, Denmark, and Ireland have implemented stricter NZEB standards with PE values lower than the recommended levels. It is important to note that Germany and Luxembourg define NZEB requirements based on reference buildings and performance benchmarks rather than specifying fixed PE values or ranges [63].

This notable discrepancy in energy performance requirements warrants further investigation to uncover the underlying factors contributing to these variations among member states. While these values provide a broad representation of the energy performance of single-family houses across countries, it is essential to review recent case studies and assess the latest developments to evaluate the accuracy and relevance of these energy performance values in the current context. To deliver a thorough analysis, this study categorizes Europe into four key regions—Northern, Eastern, Southern, and Western. The energy performance of single-family houses within these regions is systematically examined in the following sections, aiming to highlight regional trends, identify persistent challenges, and explore opportunities for enhancing energy efficiency across the European continent.

4.1. Case Studies from Northern Europe

Northern European countries (Figure 5), characterized by their heating-dominated climate patterns, require a strong focus on ventilation and heating systems in design practices [64]. This ensures a comfortable indoor climate while maintaining low energy consumption.

Many researchers have conducted studies about achieving NZEB targets in northern European countries [65,66,67,68]. A recent study by Simson et al. [29] analyzed performance criteria and calculation approaches for residential NZEBs in Oceanic and Nordic climate zone countries, focusing on Finland, Denmark, and Estonia, by considering both real-world and simulation data according to the European Commission (EC) recommendations. Among them, a single-family house in Denmark characterized by modern designs and advanced technical solutions, served as an exemplary model of contemporary NZEB implementations. The house reflected traditional Danish architectural design and construction practices. Particular attention was given to construction materials, renewable energy generation, and energy usage to assess and compare the building’s energy performance.

Building construction included advanced insulation systems, using materials and construction methods with low thermal transmittance to maintain low U-values. This construction represents a typical Danish approach, where masonry is commonly used for external walls. Both the roof and wall assemblies are insulated with mineral wool, enhancing thermal performance. The roof achieves a U-value of 0.09 W/(m²·K), while the external walls, composed of a masonry outer leaf and lightweight concrete inner leaf, include cavity insulation with an average U-value of 0.29 W/(m²·K). The ground slab, constructed with concrete and EPS insulation, provides a U-value of 0.12 W/(m²·K). The house has a net heated area of 138 m². The heating and domestic hot water (DHW) system is powered by an efficient ground source heat pump, with space heating delivered through a low-temperature underfloor heating system. A balanced mechanical ventilation system with heat recovery efficiency of 88% is installed, providing a ventilation rate of approximately 0.5 air changes per hour. The house has an annual primary energy consumption of 27 kWh/(m²·y), precisely meeting the NZEB energy requirement for single-family houses as outlined in their national plans. This value is also significantly below the European Union (EU) threshold of 50 kWh/(m²·y) for such buildings. Furthermore, the initial design incorporates a 24 m² photovoltaic (PV) system, which generates 73% more energy than the amount required to fulfill the NZEB criteria, demonstrating exceptional energy efficiency and sustainability [29].

Overheating in Danish low-energy homes is also a growing concern, particularly in summer, due to factors like climate change, overly simplified design, strict energy codes, and increased use of electrical devices. It adversely affects occupant health, comfort, and productivity, while reducing system efficiency, raising maintenance costs, and contributing to CO₂ emissions [69]. A study by Tozan et al. [70] examined CO₂ emissions from 51 Danish buildings, reporting an average emission of 7.96 kg CO₂/m² annually. To address overheating, [71] analyzed passive and natural cooling technologies in a Danish NZEB single-family house built in 2017. The study evaluated PCM panels, automated external shading, and automated natural ventilation using computational modeling. Results showed that automated natural ventilation offered the best thermal comfort, with no overheating observed in any scenario, highlighting the resilience of these cooling methods.

Erhorn & Erhorn-Kluttig [72] conducted an analysis of NZEB performance for single-family houses in different European climates. One such study examined a house in Dublin, Ireland, with a net conditioned floor area of 160 m². The building features concrete block external walls with gypsum hard-rock plaster and EPS insulation, achieving a U-value of 0.14 W/(m²·K). The roof is insulated with bio-based spray foam (U-value: 0.13 W/(m²·K)), while the ground slab, constructed with concrete and EPS insulation, provides a U-value of 0.11 W/(m²·K). The house is equipped with a weather-compensated gas boiler, a mechanical ventilation system with 91% heat recovery, and a thermal storage tank integrated with a solar hot water heating system. The total construction cost is 1063 EUR/m². The annual primary energy consumption is 47.1 kWh/(m²·y), with 30% of this energy sourced from renewables, making it below the NZEB energy requirement for single-family houses as specified in Ireland’s national plans.

Similarly, the study evaluated a 197 m² single-family house in Lithuania, emphasizing superior insulation with U-values of 0.1 W/(m²·K) for the walls, 0.08 W/(m²·K) for the roof, and 0.1 W/(m²·K) for the ground slab. This house utilizes district heating for space heating and hot water, along with a mechanical ventilation system featuring 85% heat recovery. It records an even lower primary energy consumption of 42.7 kWh/(m²·y), with 60% of its energy coming from renewable sources, placing it within Lithuania’s NZEB standards. These cases highlight the diverse strategies employed across the Northern European region to achieve NZEB performance while emphasizing energy efficiency and renewable integration. Figure 6 presents a comparison of thermal transmittance values and primary energy (PE) consumption for single-family houses in several Northern European countries, including those previously discussed.

The data reveals that while the single-family house in Denmark exhibits a relatively high thermal transmittance value for walls compared to other countries in the region, its PE consumption remains notably lower. Furthermore, Denmark’s PE consumption value is within the recommended limits set out in its national plans. Similarly, for the other countries analyzed, the PE consumption values also fall significantly below the thresholds outlined in their respective national plans. This comparison underscores the feasibility of meeting NZEB targets for single-family houses in Northern Europe. The results emphasize the effectiveness of comprehensive energy efficiency measures and renewable energy integration in achieving the NZEB goals for this region.

4.2. Case Studies from Eastern Europe

Eastern Europe (Figure 7) is emerging as a promising destination for renewable energy investments, driven by its untapped potential and increasing focus on sustainability [73]. The International Renewable Energy Agency (IRENA) highlights the cost-competitive and vast renewable energy potential in this region, further encouraging the transition toward cleaner energy solutions [74]. This shift has also influenced the adoption of energy-efficient building practices, including the NZEB concept, in Eastern Europe. Countries in this region are gradually aligning with European Union directives, setting ambitious energy efficiency targets and implementing supportive policies to encourage the construction of NZEBs [21].

A study conducted by Fedorczak-Cisak et al. [33] has analyzed and discussed the energy performance of a single-family house located in Poland. The simulation model of the house has developed adapting prefabricated wood-based technology following Polish definitions of NZEBs, ensuring higher energy efficiency. The building has been constructed with an advanced insulation system and materials such as glued sandwich wood, using methods that allow for lower thermal transmittance. The external walls are made using a BSO façade system with wood wool filling, achieving a U-value of 0.19 W/(m²·K), while the roof was constructed with a wool layer between rafters achieving a U-value of 0.15 W/(m²·K). Additionally, the ground slab provides a U-value of 0.29 W/(m²·K).

The house has a net heated area of 120 m². The heating and domestic hot water (DHW) system is powered by an efficient air/water heat pump, with space heating delivered using centrally regulated radiators. The building is equipped with a mechanical supply and exhaust ventilation system with 85% recovery efficiency. The house has an annual primary energy consumption of 68 kWh/(m²·y), which is higher when comparing with the PE consumption values in Northern Europe but lower than the NZEB energy requirement for single-family houses as outlined in Polish national plans. Furthermore, the author has conducted a cost estimation of the building, which determined a construction cost of 625 EUR/m².

D’Agostino & Parker [75] developed a single-family house simulation model following NZEB guidelines to examine energy usage and CO₂ emissions in a few selected countries across Europe. The building has a net heated area of 120 m², and U-values of walls, floor, and roof are defined as per the selected location. According to the simulation results, the building has an average emission of 24 kg CO₂/m² annually in the case of Poland, which is nearly three times higher than in Denmark.

Another study was conducted in Romania to assess the building performance of a single-family house with a heating area of 133 m² [28]. The external walls are made of 250 mm thick bricks with 200 mm of polystyrene insulation achieving a U-value of 0.145 W/(m²·K), while the ground floor is insulated with 100 mm of extruded polystyrene below the concrete slab and an additional 50 mm layer over the concrete slab, achieving a U-value of 0.215 W/(m²·K). Also, the roof is insulated with a 300 mm basaltic wool layer achieving a U-value of 0.148 W/(m²·K). The ventilation system works through five decentralized units with heat recovery at 90%. The study has mainly focused on evaluating the annual primary energy demand before and after adapting the NZEB requirements. Following the renovations, the Primary Energy (PE) consumption was significantly reduced from 89 kWh/(m²·y) to 29 kWh/(m²·y), representing a substantial improvement in energy efficiency. This 67% reduction not only demonstrates the effectiveness of the applied measures but also underscores the potential for upgrading the energy performance of buildings in this region. The post-renovation value of 29 kWh/(m²·y) aligns closely with, or even surpasses, typical benchmarks observed in Northern European countries, where energy-efficient building practices are more commonly implemented. This comparison highlights that, with adequate funding and appropriate renovation strategies, buildings in this region can achieve energy performance levels comparable to—if not better than—those in leading European contexts.

The Eastern European region demonstrates significant potential for renewable energy generation through solar photovoltaic (PV) systems. A study by Atsu et al. [76] highlighted this potential by analyzing a building in Hungary equipped with 152 m² (9.6 kWp) of photovoltaic panels. The system achieved an impressive annual energy generation underscoring the substantial capability of solar PV systems in this region. These advancements highlight the region’s leadership in sustainable energy practices. Figure 8 illustrates the thermal transmittance values and PE consumption of selected single-family houses in the Eastern European region.

The thermal transmittance values for all the analyzed houses are below 0.3 W/(m²·K), reflecting high levels of insulation. In the case of Poland, three buildings were evaluated, each displaying different PE consumption values. However, all the values remain within the national recommendations, highlighting how technical advancements and energy efficiency practices can significantly influence PE consumption, even within the same country. The figure also demonstrates that the PE consumption values for a selected case in Romania are within their respective national targets. The considerable renewable energy potential in the region may have played a significant role in achieving these energy efficiency levels. These findings underscore the importance of leveraging renewable energy resources and implementing advanced energy efficiency measures to meet and exceed NZEB requirements in Eastern Europe.

4.3. Case Studies from Southern Europe

Southern European countries (Figure 9) are actively developing NZEB definitions and implementation strategies tailored to address the unique climatic challenges of the region, particularly the increasing risk of summer overheating [58]. In Mediterranean cities, the energy demand for summer cooling often rivals or even surpasses the energy required for space heating during the colder months [77].

Southern European countries face significant challenges in NZEB implementation, as highlighted by Attia et al. [58]. A critical observation is that most existing NZEB practices and studies focus on cold climates, not on approaches specifically tailored for Mediterranean regions—characterized by cooling-dominated climates—either by roughly estimating or omitting cooling demands altogether [78]. Designers often prioritize reducing energy needs for space heating, an approach suitable for colder climates but less effective for Southern Europe. For instance, while reducing U-values can lower heating demands, excessive insulation may inadvertently increase cooling energy consumption and exacerbate summer indoor overheating [79]. Unlike in Northern and Eastern European regions, where reducing heating energy demand is the primary focus, Southern Europe—characterized by Mediterranean climates—shifts its emphasis towards minimizing cooling energy demand while maintaining optimal thermal comfort. This climatic distinction fundamentally influences sustainable building design strategies in the region. As a result, architectural features such as large overhangs, external shading devices, and enhanced natural ventilation are commonly integrated into building designs to effectively mitigate summer overheating. These passive design approaches are central to achieving energy efficiency and occupant comfort in Southern European climates, reflecting a regional adaptation to climate-specific sustainability priorities.

Barthelmes et al. [80] analyzed the energy performance of a single-family house located in Piedmont, Italy, designed to adapt NZEB solutions. The design aimed to strike a balance between minimizing winter heating and summer cooling loads while ensuring indoor comfort. The house is a single-story structure with a net conditioned floor area of 135 m², carefully planned based on bioclimatic principles, including room and window orientation and the use of sunscreens. The external vertical envelope, composed of reinforced concrete bearing walls and infill masonry walls, provides high thermal inertia. The insulation layer of the walls (U-value = 0.15 W/(m²·K)), floor slab (U-value = 0.19 W/(m²·K)), and roof (U-value = 0.15 W/m²·K) is made of rock wool panels. To further enhance energy efficiency, the envelope design minimizes thermal bridges and air infiltration while incorporating effective barriers against rising damp.

The space heating and domestic hot water (DHW) system is powered by an efficient water-to-water heat pump (COP = 4.78), with space heating delivered through radiant floors. The building is equipped with a controlled mechanical ventilation (CMV) system with 85% recovery efficiency. Additionally, the building’s electricity needs are fully met by a 7 kW peak grid-connected photovoltaic (PV) system, covering a surface area of 56 m². The energy demand of the house for heating is 17.2 kWh/(m²·y), while the cooling requirement is 19.7 kWh/(m²·y), highlighting the cooling-dominated climate of the region. Furthermore, the author has conducted a cost estimation of the building and determined a construction cost of 557 EUR/m², which is lower when compared to the cases in Ireland and Poland.

Another analysis conducted for a single-family house with a net conditioned floor area of 144 m² in Sicily, Italy, by Causone et al. [81] showed that the total primary energy demand of 41.1 kWh/(m²·y) can be easily managed via a 8.14 kW peak grid-connected photovoltaic (PV) system which generates 67.5 kWh/(m²·y). This house also shares similar characteristics with the previously discussed house in Italy, with U-values of 0.13 W/(m²·K) for the walls, 0.13 W/(m²·K) for the roof, and 0.23 W/(m²·K) for the ground slab. Additionally, the energy systems incorporated into the building include an earth-to-air heat exchanger, a mechanical ventilation system with heat recovery, an electrical air-to-water heat pump, and a thermal storage tank with solar thermal system. According to the D’Agostino & Parker’s [75] simulation model, the building has an average emission of 20 kg CO₂/m² annually in the case of Italy, which is in the same range as in Poland.

Ascione et al. [57] analyzed a single-family house in Benevento, Italy, which utilizes geothermal boreholes to pre-cool ventilation air in summer and preheat it in winter before processing through an aerothermal heat pump. The building also incorporates advanced smart technologies, including tools, sensors, and actuators for efficient energy management. A tablet app enables occupants to remotely control heating, cooling, artificial lighting, and monitor energy parameters. These innovative approaches to renewable energy integration and smart building management highlight the potential for achieving sustainable and energy-efficient housing solutions.

Furthermore, the details are available for a single-family house in Malta, built following NZEB guidelines [72], with a net heating and cooling area of 209 m². The walls are constructed using 0.5 m thick stone masonry (two limestone walls with a 0.05 m air cavity), resulting in a U-value of 1.57 W/m². The roof comprises reinforced concrete slabs with 125 mm expanded polystyrene insulation, stone chippings, and a concrete screed, achieving a U-value of 0.25 W/m². The ground slab, with a U-value of 1.97 W/m², completes the thermal envelope of the building. Heating and cooling systems are powered using an inverter split-type air-conditioning system, while hot water is supplied by a flat-plate solar water collector. This system fulfills all annual hot water requirements, making the energy requirement for hot water effectively zero. The house achieves an annual primary energy consumption of 39.47 kWh/(m²·y), well below the NZEB energy requirement for single-family houses as defined in Malta’s national plans. Figure 10 presents a comparison of PE consumption and U-values for example buildings in Southern Europe.

Similarly, cases from different cities in Italy show comparable PE consumption values, all of which fall within the recommended range. Figure 10 shows that the selected single-family house in Malta has high thermal transmittance values for walls and the ground slab compared to examples from Italy. Despite the variations in insulation levels, the PE consumption values meet the national requirements in both Malta and Italy.

4.4. Case Studies from Western Europe

Western Europe (Figure 11) has emerged as a leader in implementing the NZEB concept, spurred by stringent European Union (EU) directives, such as the Energy Performance of Buildings Directive (EPBD) [82]. These regulations aim to reduce greenhouse gas emissions and energy consumption across the built environment, with a particular focus on achieving net-zero targets by 2050 [83]. The region’s diverse climatic conditions and advanced infrastructure have enabled the development of tailored NZEB strategies that prioritize energy efficiency, renewable energy integration, and occupant comfort. Countries like Germany and the Netherlands have demonstrated significant progress in both retrofitting existing buildings and constructing new NZEB-compliant structures [58].

A study conducted by Erhorn-Kluttig et al. [84] analyzed and discussed the energy performance of a single-family house located in Berlin, Germany. A key feature of the house is its modular design, allowing elements to be relocated or sustainably disposed of after the project’s lifespan, enhancing resource efficiency. The house has developed adapting box type architecture following the definitions of NZEBs ensuring higher energy efficiency. The building was constructed with advanced insulation systems, and materials such as timber. The external walls, roof, and ground floor are made using timber panels achieving U-values of 0.11 W/(m²·K).

The house features a net heated area of 130 m² and utilizes an efficient air-to-water heat pump for heating and domestic hot water (DHW). Space heating is delivered through a central heating system, while a mechanical supply and exhaust ventilation system with 80% recovery efficiency ensures optimal air quality. The roof and façade-mounted PV systems generate 65.6 kWh/(m²·y), which is used by the building, stored in a 40-kWh battery, or fed into the grid. This level of PV generation aligns well with the figures observed in the previously discussed case study in Sicily, Italy, indicating comparable solar energy potential. With an annual primary energy consumption of 61.1 kWh/(m²·y), the house achieves a net positive energy balance. However, the construction and building service system costs are relatively high at 12,600 EUR/m², reflecting the ambitious plus-energy design and the modular construction approach, which allows for material separation and reuse upon deconstruction. According to D’Agostino & Parker’s [75] simulation model building, a single-family house has an average emission of 17.5 kg CO₂/m² annually in the case of Germany. This value falls within a similar range to emission levels observed in typical buildings across Eastern and Southern European countries. However, it also highlights a notable contrast with Northern European regions, where more advanced and widespread sustainable building practices have led to significantly lower emission profiles.

Another housing project conducted in the Netherlands focused on ecological and biological building techniques [72]. This development includes 27 single-family houses, each with a net heated area of 98 m², operating as separate units. These positive energy houses are designed with low investment costs while achieving superior energy performance. The buildings feature a highly insulated envelope with triple glazing, resulting in consistent U-values of 0.8 W/(m²·K) for the walls, roof, and ground slab. To mitigate high indoor temperatures during summer, the roof is partially covered with sedum plants, and the structure incorporates high thermal mass. Burglar-proof features have been integrated to enable high ventilation rates at night and during the absence of occupants.

The house is equipped with a mechanical ventilation system with heat recovery, and hot water is provided in combination with solar thermal panels. Additionally, photovoltaic panels on the roof generate 95.3 kWh/(m²·y), meeting the electricity demands of building service systems and household equipment. The house’s annual primary energy consumption is 44.2 kWh/(m²·y), well below the NZEB energy requirements for single-family houses as defined by national standards. The construction and building service system costs for these houses are 1800 EUR/m², which is notably lower compared to similar projects, such as those in Germany, yet higher when compared with the cases discussed in the other European regions. However, this demonstrates the feasibility of achieving high energy efficiency while maintaining relatively low costs in the same region.

The Western European region demonstrates significant potential for renewable energy generation through PV systems. A study by Höfler et al. [85] highlighted this potential by analyzing a residential building in Austria equipped with 630 m² (92 kWp) of photovoltaic panels. The system achieved an impressive annual energy generation of approximately 80 MWh/year, equivalent to 146 kWh/(m²·y), underscoring the substantial capability of solar PV systems in this region.

Further emphasizing Austria’s progress in the clean energy transition, Wargocki [86] reported a 32% reduction in greenhouse gas emissions from buildings over a decade, driven by significant improvements in energy efficiency and renewable energy adoption. Currently, 75% of electricity, 60% of space heating, and 31% of primary energy in Austria are sourced from renewables, supported by annual investments exceeding 2 billion euros in the energy transition. These advancements highlight the region’s leadership in sustainable energy practices. Figure 12 summarizes the U-values and PE consumption for selected example buildings in Western Europe.

The figure indicates that the selected single-family house in the Netherlands exhibits higher thermal transmittance values for walls, roof, and ground slab compared to other regions. In contrast, the cases from Germany, Belgium, France, and Luxembourg demonstrate lower and relatively consistent U-values across these components. Regarding primary energy consumption, the highest value is observed in the selected case from Austria, while Luxembourg records the lowest. These variations highlight regional differences in energy efficiency practices and building standards, emphasizing the importance of tailored approaches to achieving NZEB targets.

5. Discussion

5.1. Average Primary Energy Consumption in European Regions

Figure 13 presents a comparative analysis of the average primary energy consumption (kWh/m²·y) of single-family houses across four European regions: Northern, Eastern, Southern, and Western Europe. The evaluated energy performance values—aligned with Objective 4 and derived from the case studies discussed in Section 4 of this manuscript—have been analyzed collectively and visualized through a box plot, offering a detailed comparison of energy performance across regions.

In Northern Europe, primary energy consumption ranges from 27 kWh/(m²·y) to 68 kWh/(m²·y), with a median of 47.1 kWh/(m²·y) and a mean of 48.2 kWh/(m²·y). The moderate energy consumption, with minimal outliers, suggests consistent performance likely driven by well-established energy standards. This moderate consumption and narrow spread suggest the effectiveness of well-established energy standards and consistent application of climate-adapted design strategies, supporting Objectives 1 and 3 of this study to ‘evaluate the design strategies’ and to ‘compare climate adaptation strategies’. Northern Europe’s buildings typically incorporate advanced insulation, compact forms, and robust heat recovery systems to manage the colder climate, ensuring a reliable level of energy efficiency across diverse projects.

Eastern Europe exhibits a similar range, with values between 29 kWh/(m²·y) and 68 kWh/(m²·y), a median of 36.6 kWh/(m²·y), and a mean of 42.5 kWh/(m²·y). The moderate spread indicates a relatively uniform application of energy-efficient practices across the region. While slightly lower than in Northern Europe, this region’s consistent values underscore the growing adoption of energy-efficient practices, though there remain notable gaps in renewable energy integration (Objective 2) and thermal insulation compared to Western Europe. The convergence in energy consumption hints at region-wide policy shifts towards more stringent efficiency measures, even if climatic extremes are less pronounced than in the North.

Southern Europe demonstrates the most consistent energy consumption, tightly clustered between 35 kWh/(m²·y) and 42 kWh/(m²·y), with a median of 39.4 kWh/(m²·y) and a mean of 39.1 kWh/(m²·y). This small range reflects how mild climates (Objective 3) and passive design strategies—like shading devices, natural ventilation, and thermal mass—lead to inherently low cooling demands. However, Southern Europe’s data also highlights a strong reliance on building design and passive measures rather than significant deployment of renewable energy systems (Objective 2). This underscores an opportunity to complement robust design features with enhanced renewable energy adoption to fully meet NZEB goals.

Western Europe, however, exhibits the largest variability, with consumption spanning from 27 kWh/(m²·y) to 85 kWh/(m²·y), a median of 46.6 kWh/(m²·y), and a mean of 51.5 kWh/(m²·y). This wide range suggests significant differences in building practices, energy systems, and climatic conditions across the region. While some buildings showcase exemplary performance—integrating renewable energy systems (Objective 2) and advanced designs—others lag, likely due to variations in renovation rates or policy enforcement. Western Europe’s data highlights a clear opportunity to harmonize best practices and close performance gaps, aligning more fully with the NZEB paradigm.

Overall, the data reveal regional disparities in primary energy consumption, influenced by factors such as climate, building technologies, and renewable energy integration. Northern, Eastern, and Southern Europe demonstrate the most efficient performances, while Western Europe shows potential for improvement despite advancements in some areas. These insights highlight the need for targeted strategies to further optimize energy consumption and achieve broader compliance with NZEB standards across Europe.

5.2. Challenges in Adaptation

A deeper analysis of barriers to technology adaptation in the context of nearly zero-energy buildings reveals several critical challenges that hinder widespread implementation. High initial costs associated with advanced energy-efficient materials, renewable energy systems, and smart technologies remain one of the primary obstacles, particularly in regions with limited financial resources or low public funding [21]. For example, the integration of ground source heat pumps, high-performance insulation materials, and photovoltaic systems significantly increases upfront investment, which can deter both private investors and public sector stakeholders.

Additionally, it is important to have technical expertise and skilled labor in the design, installation, and maintenance of these sophisticated systems. The complexity of technologies such as Building Management Systems (BMS) and advanced HVAC systems often requires specialized knowledge, which can be challenging to find in all regions, especially in economically constrained areas [87].

Regulatory and administrative hurdles also play a pivotal role in slowing the adoption of NZEB standards. Inconsistent or insufficient policy frameworks, lengthy approval processes, and fragmented building codes create uncertainty for developers and investors [61]. These challenges are particularly pronounced in countries with underdeveloped regulatory support for NZEBs, where there is a lack of alignment with EU directives or insufficient enforcement mechanisms.

Furthermore, user-related barriers, including resistance to change, lack of awareness, and limited access to affordable financing options, also significantly affect adoption [59]. Many potential users are unfamiliar with the long-term benefits of NZEBs, such as reduced operational costs and improved indoor comfort, which limits demand. Cultural factors, such as differing attitudes toward energy-saving behaviors and reluctance to adopt new technologies, exacerbate these challenges.

Addressing these challenges requires a multifaceted and integrated approach. Financial incentives, such as subsidies, tax credits, and low-interest loans, can help alleviate high upfront costs. Streamlined regulations and harmonized building codes can reduce administrative complexity and encourage compliance. Targeted training programs for architects, engineers, and construction workers are essential to build the technical expertise needed to design and maintain NZEBs. Finally, raising public awareness through education campaigns and showcasing successful NZEB projects can help overcome resistance to change and build societal acceptance of sustainable building practices [6].

In addition to these systemic barriers, the role of occupant behavior must also be addressed, as it significantly influences the real-world performance of NZEBs. Variations in thermostat settings, ventilation practices, and energy usage patterns can lead to deviations from predicted energy efficiency outcomes [59]. Incorporating intuitive smart systems and offering user education on energy-saving practices can bridge the gap between theoretical and actual performance. By tackling these challenges holistically, stakeholders can accelerate the adoption of NZEBs, contributing to the global goals of reducing carbon emissions, enhancing energy efficiency, and promoting sustainable urban development.

5.3. Policy Recommendations and Practical Steps

To effectively implement the principles of nearly zero-energy buildings (NZEBs), it is essential to develop comprehensive and regionally tailored strategies that address policy, financial, technical, and societal challenges. Government policies must establish clear and ambitious regulatory frameworks that align with the EU Energy Performance of Buildings Directive (EPBD), including stricter energy efficiency standards in building codes and mandatory integration of renewable energy systems [61]. These regulations should be complemented by robust enforcement mechanisms to ensure compliance, particularly in countries where alignment with EU directives remains a challenge.

A critical element of successful implementation is the introduction of financial support mechanisms. These include grants, tax incentives, and preferential loans designed to reduce the financial burden on developers, homeowners, and businesses investing in NZEB technologies [21]. For instance, programs such as Germany’s KfW financial incentives for energy-efficient retrofitting and construction have proven effective in driving adoption [88,89]. Expanding access to similar funding models across Europe could significantly accelerate progress, particularly in economically constrained regions.

Local-level initiatives are equally vital. Educational programs and awareness campaigns can enhance public understanding of NZEB benefits, such as lower operational costs, improved indoor comfort, and contributions to climate goals [59]. These efforts should target diverse stakeholders, from policymakers and developers to end-users, ensuring widespread support and engagement.

Practical steps involve piloting NZEB projects to showcase feasibility and success, serving as benchmarks for further development. Building a skilled workforce is another priority, which requires establishing a network of certified professionals trained in NZEB design, construction, and maintenance. Digital tools, such as Building Information Modelling (BIM) and Digital Twins, should be promoted to streamline the design process and optimize building performance [6]. Technological support should also emphasize the development of local supply chains to ensure the availability of high-quality materials and equipment, fostering regional self-reliance.

Finally, a robust monitoring and evaluation system is critical for tracking progress. Regular reporting and benchmarking among European Union member states can facilitate the exchange of best practices and foster collaborative efforts to address shared challenges. These measures should include transparent metrics, such as reductions in energy consumption, carbon emissions, and operational costs, to assess the real-world impact of NZEB policies.

By integrating these policy recommendations and practical steps into a coherent strategy, Europe can overcome existing barriers to NZEB adoption. This approach not only accelerates the transition to sustainable building practices but also ensures equitable progress across all regions, contributing significantly to achieving energy neutrality and long-term climate goals.

5.4. Practical Technological Parameters in Different European Climates

A deeper comparison of the impact of specific technological parameters of NZEBs on energy efficiency under varying climatic conditions highlights key elements that drive their performance. In colder climates, such as Northern Europe, the focus is on advanced insulation systems (U-value < 0.15 W/m²K), high-efficiency heat recovery ventilation (efficiency > 90%), and ground source heat pumps with a coefficient of performance (COP) > 4, as demonstrated by Simson et al. [29]. These technologies significantly reduce heat loss and improve energy performance in regions with long heating seasons. In contrast, Southern Europe, characterized by cooling-dominated climates, prioritizes solar shading systems that can reduce solar heat gains by over 50%, natural ventilation capable of achieving two air changes per hour, and rooftop photovoltaic (PV) systems with a capacity exceeding 0.2 kW/m², as highlighted by [57]. In Eastern Europe, where economic constraints often dictate technological choices, there is a focus on cost-effective solutions, such as moderately advanced insulation with U-values below 0.25 W/m²K, alongside the gradual development of PV infrastructure with installations ranging from 0.1 to 0.15 kW/m², as noted by Borowski [60]. These practical parameters emphasize the importance of tailoring technologies to local climatic and economic conditions, allowing NZEBs to achieve optimal energy efficiency while minimizing costs.

6. Conclusions

The implementation of nearly zero-energy building (NZEB) practices across Europe exemplifies a unified yet adaptable approach to achieving energy efficiency, sustainability, and carbon neutrality in the built environment. Each region—Northern, Eastern, Southern, and Western Europe—leverages its unique climatic, economic, and regulatory conditions to tailor NZEB strategies, resulting in a mosaic of innovative solutions that reflect both regional diversity and shared goals.

Northern Europe’s focus on combating cold climates through advanced insulation, airtight construction, and geothermal heating systems showcases the effectiveness of leveraging technological maturity and strong regulatory support. Eastern Europe demonstrates significant progress through cost-effective retrofitting strategies and the gradual integration of renewable energy sources, such as solar and biomass. Southern Europe emphasizes passive cooling, shading, and climate-adaptive design to mitigate summer overheating, reflecting the region’s response to its cooling-dominated climate. Meanwhile, Western Europe stands as a leader with its comprehensive policy frameworks, and balanced emphasis on new constructions and retrofitting, supported by a well-funded research ecosystem.

Despite the regional differences, all approaches align with overarching goals: reducing greenhouse gas emissions, enhancing energy performance, and improving occupant comfort. The harmonization of NZEB practices, driven by the Energy Performance Buildings Directive (EPBD), fosters collaboration and knowledge exchange among member states, enabling Europe to leverage shared innovations and experiences.

This paper contributes new insights by systematically comparing primary energy consumption, CO₂ emissions, and design strategies across different European climates. By synthesizing these patterns, we highlight how regional climatic and policy contexts shape distinct NZEB solutions, thus offering a comparative understanding that bridges gaps in the current literature. Furthermore, the cross-regional comparisons underscore the value of context-sensitive approaches in advancing Europe’s collective climate objectives.

However, several limitations should be acknowledged. The reviewed case studies focus primarily on single-family residential buildings, with limited focus on other building types such as apartments, offices, educational, and industrial facilities. Additionally, regional data gaps and differences in data quality may introduce biases or uncertainties in the comparative analysis. Variability in climate modeling, simulation approaches, and regulatory baselines further complicates direct comparisons across studies.

Future research should address these limitations by adopting the following approaches:

• Expanding the analysis to include diverse building types (e.g., commercial, educational, industrial) to offer a more holistic understanding of NZEB performances across the built environment.
• Conducting longitudinal studies to evaluate the operational performance of NZEBs over time, including occupant behavior as well.
• Investigating embodied carbon and lifecycle emissions of NZEBs, which are increasingly critical for truly net-zero carbon strategies.
• Exploring the integration of emerging renewable energy technologies—such as advanced storage systems, demand response, and district-scale solutions—tailored to region-specific climates and energy systems.
• Developing a roadmap for harmonizing NZEB policies and certification schemes across Europe to ensure consistent performance standards and equitable progress towards the EU’s 2030 and 2050 decarbonization targets.

In conclusion, achieving NZEB targets is not a one-size-fits-all process; it requires nuanced, context-sensitive strategies tailored to specific regional needs, while advancing collective climate objectives and shared decarbonization goals. This paper advances the existing knowledge by providing a comparative lens on regional NZEB performance, design strategies, and climate adaptation measures. As Europe progresses toward its 2030 net-zero goals, the evolution of NZEB practices, along with future research guided by these insights and dedicated to bridging existing gaps, will be pivotal in delivering a sustainable, resilient, and energy-efficient built environment for all.