A Roadmap for the Certification of Polyurethane Flexible Connectors Used as Envelope Products in the Next Generation of Healthy, Nearly Zero-Energy Buildings

Abstract

Currently, in the European Economic Area (EEA), producers of building materials are implementing innovative solutions that provide a chance for the widespread construction of zero-emission and zero-energy buildings. However, they encounter legal barriers related to the lack of standardization procedures enabling the rapid placement of innovative construction products on the market. The European project Horizon 2020: Measuring Envelope Products and Systems Contributing to the Next Generation of Healthy, Nearly Zero-Energy Buildings (MEZeroE) aims to support producers of innovative envelope products used in zero-energy buildings, including in the field of certification, CE (European Conformity), marking and placing them relatively quickly on the market. This article presents one of the research procedures developed and tested by Pilot Measurement & Verification Lines (PM&VL7) as part of the MEZeroE project for Flex&Robust polyurethane flexible connectors. This procedure considers the applicable legal requirements regarding CE marking and also indicates a certification path for this type of product.

1. Introduction

The Intergovernmental Panel on Climate Change (IPCC) predicts that if CO₂ and other greenhouse gas emissions are not significantly reduced soon, global temperatures will increase by more than 1.5–2 °C this century [1]. Reducing greenhouse gas (GHG) emissions is key to combating global warming. In 2023, the total CO₂ emissions from energy combustion and industrial processes amounted to 37.4 Gt [2]. In Europe, buildings are responsible for about 36% of energy-related greenhouse gas (GHG) emissions and 40% of final energy consumption [3].

One of the European Union’s (EU’s) actions in this area is the Green Deal, which aims to reduce greenhouse gas emissions in the EU by 55% by 2030 and achieve carbon neutrality by 2050 [4]. Achieving such ambitious goals requires significant changes in the construction sector [5]. The REPowerEU plan has set a decarbonization path and raised the renewable energy target to 45% [6]. Plans to achieve climate neutrality in Europe by 2050 are included in the revised version of the Energy Performance of Buildings Directive (EPBD) [7] and in the Energy Efficiency Directive (EED) [8,9]. The European Commission proposes to change the nearly zero-energy requirements into zero-emission requirements for all new buildings from 1 January 2030 and for all new buildings occupied or owned by public authorities from 1 January 2027. These targets will be achieved thanks to the concept of nearly zero-energy buildings (nZEB) [3,10,11,12,13,14]. nZEB buildings are characterized by high energy efficiency, with very low demand for energy from renewable energy sources [15,16,17,18,19,20,21,22]. Emission reduction is required throughout the life cycle of buildings, during construction, operation, and maintenance. Currently, more and more nZEB buildings are being built in EU Member States [23,24,25]. However, the gap in the energy efficiency of new buildings from the beginning of operation [8,26,27,28], resulting from climate change [23,29,30], makes it necessary to improve the energy efficiency of existing nZEB buildings as well. This energy efficiency gap is due to the fact that throughout Europe, most buildings were constructed before the introduction of thermal standards and energy efficiency regulations [31,32,33]. Therefore, in order to reduce energy consumption and greenhouse gas emissions, it is necessary not only to construct nZEB buildings but also to retrofit existing buildings. The key element to achieving acceptable thermal comfort inside buildings is their envelope [34].

Therefore, the challenge for the construction industry is to meet the growing demand for nZEB buildings and the circular economy [15,35,36]. Manufacturers of building materials are implementing innovative solutions that have the potential to enable the widespread construction of nZEB buildings. However, they encounter legal barriers related to the lack of standardization procedures enabling the rapid placement of innovative construction products on the market [37,38,39].

The Construction Product Regulation (CPR) evaluation [40] and feedback from Member States and stakeholders have clearly identified barriers to the functioning of the single market for construction products and thus the failure to achieve CPR objectives [37]. The conditions for circulating construction products on the European market will be amended. On 10 April 2024, the European Parliament adopted a legislative resolution at the first reading on the proposal for regulating the European Parliament and the Council, laying down harmonized conditions for the marketing of construction products [41]. However, it should be noted that the amended CPR [41] will not enter into force until a decade from now. The key changes to the CPR [41] include the following:

• Ensuring the smooth functioning of the single market and the free movement of construction products;

• Increasing the role of the sustainable use of natural resources in the life cycle of construction products;

• Enabling the construction sector to achieve climate and sustainability objectives and participate in the digital transformation of the economy;

• Ensuring competitiveness and eliminating barriers to harmonized assessment documents.

Project Horizon 2020: Measuring Envelope Products and Systems Contributing to the Next Generation of Healthy, Nearly Zero-Energy Buildings (MEZeroE), implemented in January 2021, aims to create an ecosystem combining the infrastructure resources and knowledge of academic and research centers with innovative solutions proposed by the industry in the form of a multi-sided virtual market open to the exchange of knowledge and experience between interested parties from the construction industry. The ecosystem will provide ready-made services in the fields of modeling, testing, and monitoring nZEB Enabler Envelope Technology Solutions (nEES) while creating a comprehensive knowledge management and training environment [42].

The project aims to consider the trends of Industry 4.0 in terms of fast decision making and customer orientation open to innovation, with particular emphasis on manufacturers of carbon-neutral construction products and healthy indoor environments [42].

The project tests standard, modified, and non-standard products using state-of-the-art equipment. Support for manufacturers in certifying the product and introducing it to the market is also provided. Standard tests have been extended to tests in live laboratories located throughout Europe, in which a new parameter of a non-engineering variable will be introduced, expressed through user opinions resulting from the comfort of use [42].

MEZeroE will be accessible online “https://mezeroe-platform.eu/ (accessed on 1 September 2024)” as a multi-sided virtual marketplace with a single entry point, including nine Pilot Measurement & Verification Lines (PM&VL) and three open innovation services (OIS) covering training, business model development, systematic intellectual property, and knowledge management. MEZeroE will accelerate the introduction of prototypes to the market as certified construction products. The final objective of the project is to develop information paths for the CE marking of different groups of construction products used in the building envelope. The paths will include procedures in accordance with Regulation 305/2011 (CPR) [40] or its amendment [41] and ways to quickly standardize and introduce innovative products to the market [42].

This article presents one of the research procedures developed and tested by Pilot Measurement & Verification Lines (PM&VL7) as part of the MEZeroE project for the polyurethane flexible connectors (PUFCs) offered by the MEZeroE project partner Flex&Robust. This procedure considers the applicable legal requirements regarding CE marking and indicates the certification path for this product type.

2. Legal Requirements for CE Marking

The path to the CE marking of construction products is specified in the still-applicable Regulation (EC) No. 305/2011 of the European Parliament and the Council of 9 March 2011, Construction Product Regulation (CPR) [40]. This regulation also specifies harmonized conditions for placing construction products on the market or making them available by establishing harmonized rules for expressing the performance of construction products in relation to their essential characteristics.

In accordance with the applicable law, this regulation requires implementation in European Union countries. In Poland, it was introduced by the Act of 16 April 2004 on construction products [43], which specifies the rules for placing construction products on the market. It approximates the implementing acts and laws of all EU Member States concerning construction products. According to Art. 5, paragraph 1 of this Act [43], a construction product that has been covered by a harmonized technical specification, i.e., a harmonized standard (hEN) or a European Technical Assessment (ETA), may be placed on the market or made available on the internal market only and exclusively in accordance with the provisions of the CPR [40].

The procedure for CE marking and then the placing of a construction product on the market in accordance with the CPR is conducted according to the scheme in Figure 1. According to the scheme, the European system [40] allows for the placement of a construction product on the market based on a harmonized technical specification, which is an hEN or a European Assessment Document (EAD).

In the case of the hEN (“harmonised standard” means a standard adopted by one of the European standardisation bodies listed in Annex I of Directive 98/34/EC on the basis of a request issued by the Commission in accordance with Article 6 of that Directive) for a construction product, CE marking and the placing of innovative products on the European market has a procedure established in Annex ZA of the hEN. For a construction product not covered or not fully covered by the hEN, an alternative harmonized technical specification, constituting the basis for CE marking, is the European Technical Assessment (ETA). ETAs are issued by Technical Assessment Bodies (TABs) based on EADs developed by the European Organisation for Technical Assessment (EOTA) and cited by the Commission in the Official Journal of the European Union [40]. An ETA entails a documented Assessment and Verification of Constancy of Performance (AVCP) of a construction product in relation to its essential characteristics in accordance with the relevant EAD. After conducting the AVCP, the manufacturer issues a Declaration of Performance (DoP) and marks the product with the CE mark on its basis.

3. Characterization of the Flex&Robust Polyurethane Flexible Connectors

Components of envelopes offered by Flex&Robust are structural and non-structural bonding connectors, constructing joints between various envelope components and between envelopes and structural elements.

The Flex&Robust line of products is dedicated to the innovative structural and non-structural bonding of elements constructing civil engineering structures made of various materials (concrete, masonry, wood, and metal). The specially designed products can carry static, dynamic, and cyclic loads and simultaneously transfer high deformations (Figure 2). They resist elevated temperatures and reduce stress concentrations by redistributing them over a large bonding area. Flex&Robust products can be used in seismic areas and strong wind areas as solutions that are resistant to repeatable loads and thus do not need to be replaced like other connectors after catastrophic events. They protect connected envelope components against damage.

A wide range of Flex&Robust products’ innovative properties can be adjusted to various requirements using special components. The Flex&Robust line of products can construct polyurethane flexible joints (PUFJs) and fiber-reinforced polyurethanes (FRPUs) that manifest a significant load and deformation capacity, fulfilling vibroacoustic and thermal comfort requirements, as well as manifesting durability, waterproofing, and non-conductive electric properties.

PUFJs reduce stress concentrations and distribute them across a larger surface, which allows large loads to be transferred. The innovative PUFJ is an effective structural connection in timber construction, an alternative to using dowel-type connections. Polyurethane construction PUFJs are stable at elevated temperatures up to 200 °C; thus, it is expected that PUFJs will work stably in fire conditions when they are protected by a timber layer.

3.1. Flex&Robust Layer (PUFJ)

One possible PUFC can be prefabricated or constructed on site: the flexible layer (PUFJ), which connects structural and non-structural envelop components. The Flex&Robust layer can be adjusted to any bonding surface shape and thickness and used in designing mechanical properties. As an example, a flexible layer (PUFJ) connecting a structural element (RC frame) and a non-structural envelope panel (infill masonry wall) is presented in Figure 3.

3.2. Flex&Robust Composite (FRPU)

Another possible PUFC is prefabricated or constructed on site: composite material (FRPU), which also connects structural and non-structural envelop components. The Flex&Robust composite can be adjusted to any bonding surface shape and thickness as well as used in mechanical property design. As before, a flexible composite (FRPU) connecting a structural element (RC frame) and a non-structural envelope panel (infill masonry wall) is presented in Figure 4.

3.3. Flex&Robust Injection (PUFJ)

The last possible PUFC solution is liquid injectable material with a fast curing time, which is suitable for filling gaps and bonding structural and non-structural envelop components, as well as for repairing damaged elements.

The Flex&Robust injection can be adjusted to any bonding surface shape and thickness (cracks/spaces) and used in mechanical property design. A flexible injection (PUFJ) connecting a structural element (RC frame) and a non-structural envelope panel (infill masonry wall) is presented in Figure 5.

4. PM&VL7 Test Line

The PM&VL7 research line Mechanical and durability tests of connectors and their impact on vibroacoustic, thermal and microclimate comfort was established at the Cracow University of Technology (CUT) as part of the MEZeroE project. An interdisciplinary team of 30 researchers proposed 69 types of varied tests offered to industrial partners within four sub-lines:

• (7.1) Mechanical;

• (7.2) Durability;

• (7.3) Vibroacoustic;

• (7.4) Thermal and microclimatic comfort.

Mechanical, vibroacoustic, and thermal tests were conducted as independent tests or constituted diagnostic features before and after aging. Within sub-line 7.2, durability tests cover aging and diagnostics before, after, and during aging, including microscopic observations and chemical tests (Figure 6), in addition to the tests mentioned above.

The main objective of PM&VL7 is the development of specific testing procedures for the evaluation of connector safety, with connectors considered as joints between envelope components and connections between envelope panels and structural elements. The proposed pilot line allows users to conduct typical tests on connections and examine their safety in use, with reference to vibroacoustic and thermo-humidity comfort (defined in European standards and not defined in the pilot line), e.g., for innovative solutions not covered by standards. As an added value, PM&VL7 includes additional tests characterizing the impact of environmental, chemical, biological, and mechanical factors (durability) on the reduction in safety and use of connections and the loss of vibroacoustic and thermo-humidity parameters of nZEB elements due to deteriorating factors.

5. Path of CE Marking for Polyurethane Flexible Connectors

An analysis of the certification requirements [40] enabling the determination of the CE marking path was conducted before defining the scope of research carried out by the PM&VL7 research line as part of the MEZeroE project.

5.1. Product Characteristics and Intended Use

The product type was determined: flexible polyurethane Flex&Robust connectors made of nine types of polyurethane two-component mass. The products are grouped in Table 1 depending on the type of polyurethane used.

The intended use was indicated as a repair system—the connection and strengthening of wooden, masonry, reinforced concrete, and stone structures exposed to dynamic loads, including seismic and strong wind loads.

The products are covered by two CPR [40] groups:

• 25—construction adhesives;

• 32—products for sealing joints.

5.2. Analysis of Harmonized Technical Specification

An analysis of the existing hENs and EDOs was conducted. No harmonized technical specification, neither hEN nor EDO, was found for the indicated construction products. Therefore, an analysis of hENs and EDOs for similar products was carried out, and, due to the indicated intended structural use, the AVCP systems presented in Table 2 were indicated as applicable.

Due to the lack of an existing reference document, the manufacturer is required to apply for an ETA.

5.3. AVCP

The AVCP system(s) of polyurethane polymer connectors is indicated in Table 3 under system 2+ without fire resistance and in Table 4 under system 3 for reaction to fire.

6. PM&VL7 Test Set for Polymer Flexible Connectors

The tests planned by PM&VL7 are aimed at determining the safety of connections in building partitions, treated as connections between partition elements and connections between partition plates and structural elements, along with checking the vibroacoustic and thermo-humidity comfort provided by polyurethane flexible joints. In addition, planned durability tests, together with structural, mechanical, and thermal diagnostics, aim to determine the impact of using connections on the safety and the loss of parameters of nZEB elements due to environmental impact.

Considering the possibilities and needs of both the MEZeroE project and the PM&VL7 research line, individual types of products were selected for testing, which are presented in Table 5. The most important tests were qualified for testing, the results of which will determine the continuation of the next product types. The sets of tests carried out within the individual sub-lines are presented in the following subsections.

6.1. Mechanical Tests

1. The dynamic stiffness/modulus of a composite according to EN 12697-26 [44] with modifications.

2. The breaking force of the glass fiber grid according to ISO 3341 [45]—a diagnostic feature of durability due to XE aging.

3. The matrix tensile strength and modulus of elasticity of polyurethane PS according to EN ISO 527-2 [46] and ISO 37 [47]—a diagnostic feature of durability due to XE aging.

4. The composite tensile strength and modulus of elasticity according to ISO 527-4 [46]—the warp direction of polyurethane PS reinforced with a glass fiber grid, which is a diagnostic feature of durability due to XE aging and seawater aging.

5. The initial shear strength includes two types of injection according to EN 1052-3 [48]—polyurethane PM and PST.

6. The tensile strength and modulus of elasticity of two types of adhesive materials (polyurethanes PSTF-S and PSTF-W) used in layer products according to EN ISO 527-2 [46] and ISO 37 [47]—a diagnostic feature of durability due to XE ageing.

6.2. Durability Tests

1. The resistance to artificial aging by exposure to sunlight Xe composites, injection (polyurethane PM and PST), and two types of adhesive materials (polyurethanes PSTF-S and PSTF-W) used in layer products in accordance with the requirements of EN ISO 4892-2 [49]. Diagnostics of durability due to aging included features not defined in the standard. The tests before and after aging included mechanical features described in Section 6.1 and the following:

   • Observation under an optical microscope (OM);

   • Observation under a scanning microscope (SEM);

   • FTIR analyses.

Resistance to artificial aging was determined using exposure to seawater according to an innovative procedure. Diagnostics of durability due to aging included features not defined in the standard. The tests before and after aging included mechanical features described in Section 6.1.

6.3. Acoustic Tests

1. The critical damping ratio [%] according to a modified EN 29052-1 procedure [50].

2. The airborne and impact sound insulation measured for the prepared solution according to the innovative procedure.

3. The direction-averaged junction velocity level difference for a connector or connection model according to an innovative procedure based on EN ISO 12354-1 [51] and EN ISO 12354-2 [52].

4. The dynamic stiffness (MN/m³) according to EN 29052-1 [50].

6.4. Thermal Simulations

1. Water vapor diffusion (interstitial water vapor condensation risk and intensity) according to EN ISO 13788 [53].

2. Internal surface temperature according to EN ISO 13788 [53].

3. Linear thermal transmittance according to EN ISO 10211 [54].

7. Summary

This article presents the certification path for innovative flexible polyurethane connectors tested within the MEZeroE project. Developed in the PM&VL7 research line at the Cracow University of Technology, this certification path results from cooperation with the product manufacturer FlexAndRobust Systems Ltd. It implements the key objective of the MEZeroE project—supporting the manufacturer in the field of CE marking—which accelerates the placement of the product on the market.

Polyurethane flexible connectors are designed to repair and strengthen structures, such as those made of wood, masonry, reinforced concrete, and stone, that are exposed to dynamic loads like seismic activity and strong winds. These connectors provide a durable and flexible solution for connecting and reinforcing structural components, helping to mitigate damage and improve overall structural integrity. Due to the lack of an existing reference document, the manufacturer was required to apply for European Technical Assessment (ETA).

This article also indicates the Assessment and Verification of Constancy of Performance (AVCP) systems for polyurethane flexible connectors, including system 2+ without fire resistance and system 3 regarding the reaction to fire. The scope of the research in four key areas—mechanical, strength, thermal, and acoustic— was defined using the PM&VL7 research line.

The example described in the article also highlights the potential of the service platform launched as part of the MEZeroE project at “https://mezeroe-platform.eu/ (accessed on 1 September 2024)”, which aims to overcome legal barriers related to the lack of standardization procedures enabling the rapid placement of innovative construction products on the market.

Author Contributions

Conceptualization, A.N.-M. and A.K.; methodology, A.N.-M.; validation, A.K.; formal analysis, A.N.-M.; investigation, A.N.-M. and J.M.; resources, A.K.; data curation, A.N-M. and J.M.; writing—original draft preparation, A.N.-M.; writing—review and editing, A.K.; visualization, A.N.-M. and J.M.; supervision, A.K.; project administration, A.N.-M.; funding acquisition, A.K. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

Prof. Kwiecień is an employee of the Cracow University of Technology and a partner and board member of FlexAndRobust Systems Ltd. Both the Cracow University of Technology and FlexAndRobust Systems Ltd. are partners of the MEZeroE project, within which the article was prepared. In the subject matter and scope of work included in this article, prof. Kwiecień is affiliated with FlexAndRobust Systems Ltd.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Table 1

Flex&Robust products.

Product—Polyurethane Flexible Connector	Type of Materials
Flex&Robust Composite	Polyurethane PS reinforcedwith glass fiber grid
Flex&Robust Layer	Polyurethane PSTF-SPolyurethane PSTF-WPolyurethane PM
Flex&Robust Injection	Polyurethane PMPolyurethane PST

Table 2

Systems of AVCP [40].

Product(s)	Intended Use(s)	AVCP System(s) 1
Polymer Flexible Connectors: Flex&Robust Injections; Flex&Robust Composites: polyurethanes reinforced with glass fiber mesh; Flex&Robust Layers.	Any	2+
Polymer Flexible Connectors: Flex&Robust Injections; Flex&Robust Layers.	For uses subject to regulations on reaction to fire	3

¹ The systems of Assessment and Verification of Constancy of Performance of Construction Products on a scale of 1, 1+, 2+, 3, and 4 are specified in Annex V of the CPR [40]. Individual AVCPs distinguish the scope of tasks of the manufacturer and notified bodies: product certification, production control or testing laboratories.

Table 3

Assignment of AVCP tasks under system 2+.

Tasks for the manufacturer	Factory production control (FPC) Determination of the product type on the basis of type testing (including sampling), type calculations, tabulated values, or descriptive documentation of the product Further testing of samples taken at the factory according to the prescribed test plan
Tasks for the notified factory production control certification body	Initial inspection of the manufacturing plant and of FPC Continuation of surveillance, assessment, and evaluation of FPC

Table 4

Assignment of AVCP tasks under system 3 for reaction to fire.

Tasks for the manufacturer	Factory production control (FPC)
Tasks for the notified laboratory	Determination of the product type on the basis of type testing (including sampling), type calculations, tabulated values, or descriptive documentation of the product

Table 5

Flex&Robust product used for the tests.

Product—Polymer Flexible Connector	Type of Materials
Flex&Robust Composite	Polyurethane PS matrix reinforcedby glass fiber grid
Flex&Robust Layer	Polyurethane PSTF-SPolyurethane PSTF-WPolyurethane PM
Flex&Robust Injection	Polyurethane PMPolyurethane PST

References

1. IPCC. Synthesis Report of the IPCC Sixth Assessment Report (AR6); IPCC: Geneva, Switzerland, 2023; Available online: https://www.ipcc.ch/report/sixth-assessment-report-cycle/ (accessed on 1 September 2024).

2. Agency, I.E. CO2 Emissions in 2023. 2024; Available online: https://www.iea.org/reports/co2-emissions-in-2023 (accessed on 1 September 2024).

3. Maduta, C.; D’Agostino, D.; Tsemekidi-Tzeiranaki, S.; Castellazzi, L.; Melica, G.; Bertoldi, P. Towards climate neutrality within the European Union: Assessment of the Energy Performance of Buildings Directive implementation in Member States. Energy Build.; 2023; 301, 113716. [DOI: https://dx.doi.org/10.1016/j.enbuild.2023.113716]

4. European Commission. Communication from the Commission the European Green Deal, in 640 Final; European Commission: Brussels, Belgium, 2019.

5. Regulation (EU) 2018/1999 of the European Parliament and of the Council of 11 December 2018 on the Governance of the Energy Union and Climate Action, amending Regulations (EC) No 663/2009 and (EC) No 715/2009 of the European Parliament and of the Council, Directives 94/22/EC, 98/70/EC, 2009/31/EC, 2009/73/EC, 2010/31/EU, 2012/27/EU and 2013/30/EU of the European Parliament and of the Council, Council Directives 2009/119/EC and (EU) 2015/652 and repealing Regulation (EU) No 525/2013 of the European Parliament and of the Council (Text with EEA relevance). Eur. Parliam. Counc. Eur. Union Off. J. Eur. Union; 2018; L328, pp. 1-77. Available online: https://eur-lex.europa.eu/eli/reg/2018/1999/oj (accessed on 1 September 2024).

6. European Commission. Communication from the Commission to the European Parliament, the European Council, the Council, the European Economic and Social Committee and the Committee of the Regions REPowerEU Plan; European Commission: Brussels, Belgium, 2022; Available online: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=COM%3A2022%3A230%3AFIN (accessed on 1 September 2024).

7. Amendments Adopted by the European Parliament on 14 March 2023 on the Proposal for a Directive of the European Parliament and of the Council on the Energy Performance of Buildings (Recast). 2023; Available online: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A52023AP0068 (accessed on 1 September 2024).

8. EED Directive 2012/27/EU of the European Parliament and of the Council of 25 October 2012 on Energy Efficiency. Off. J. Eur. Union; 2012; L315, pp. 1-56. Available online: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A32012L0027 (accessed on 1 September 2024).

9. Economidou, M.; Todeschi, V.; Bertoldi, P.; D’Agostino, D.; Zangheri, P.; Castellazzi, L. Review of 50 years of EU energy efficiency policies for buildings. Energy Build.; 2020; 225, 110322. [DOI: https://dx.doi.org/10.1016/j.enbuild.2020.110322]

10. Carmen, M.; Giulia, M.; Delia, D.; Paolo, B. Towards a decarbonised building stock by 2050: The meaning and the role of zero emission buildings (ZEBs) in Europe. Energy Strategy Rev.; 2022; 44, 101009.

11. Chen, X.; Vand, B.; Baldi, S. Challenges and Strategies for Achieving High Energy Efficiency in Building Districts. Buildings; 2024; 14, 1839. [DOI: https://dx.doi.org/10.3390/buildings14061839]

12. Kang, K.; Kim, D. Energy Performance and Cost-Effectiveness Assessment towards the Nearly Zero-Energy School Buildings in Mild Climates. Buildings; 2024; 14, 1147. [DOI: https://dx.doi.org/10.3390/buildings14041147]

13. Kayaçetin, N.; Hozatli, B. Whole life carbon assessment of representative building typologies for nearly zero energy building definitions. J. Build. Eng.; 2024; 95, 110214. [DOI: https://dx.doi.org/10.1016/j.jobe.2024.110214]

14. Piaia, E.; Turillazzi, B.; Di Giulio, R.; Sebastian, R. Advancing the Decarbonization of the Construction Sector: Lifecycle Quality and Performance Assurance of Nearly Zero-Energy Buildings. Sustainability; 2024; 16, 3687. [DOI: https://dx.doi.org/10.3390/su16093687]

15. D’Agostino, D.; Tzeiranaki, S.; Zangheri, P.; Bertoldi, P. Assessing Nearly Zero Energy Buildings (NZEBs) development in Europe. Energy Strategy Rev.; 2021; 36, 100680. [DOI: https://dx.doi.org/10.1016/j.esr.2021.100680]

16. Zhang, Y.; Wang, C.; Cai, R.; Lan, Z.; Shen, Y.; Zhang, D.; Tian, W.; Su, G.; Qiu, S. Experimental investigation on flow and heat transfer characteristics of lead-bismuth eutectic in circular tubes. Appl. Therm. Eng.; 2020; 180, 115820. [DOI: https://dx.doi.org/10.1016/j.applthermaleng.2020.115820]

17. Yildiz, Y.; Arsan, Z. Identification of the building parameters that influence heating and cooling energy loads for apartment buildings in hot-humid climates. Energy; 2011; 36, pp. 4287-4296. [DOI: https://dx.doi.org/10.1016/j.energy.2011.04.013]

18. D’Agostino, D.; Zacà, I.; Baglivo, C.; Congedo, P. Economic and Thermal Evaluation of Different Uses of an Existing Structure in a Warm Climate. Energies; 2017; 10, 658. [DOI: https://dx.doi.org/10.3390/en10050658]

19. Congedo, P.; Baglivo, C.; Zacà, I.; D’Agostino, D. High performance solutions and data for nZEBs offices located in warm climates. Data Brief; 2015; 5, pp. 502-505. [DOI: https://dx.doi.org/10.1016/j.dib.2015.09.041] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26629494]

20. Luo, X.; Oyedele, L. Life cycle optimisation of building retrofitting considering climate change effects. Energy Build.; 2022; 258, 111830. [DOI: https://dx.doi.org/10.1016/j.enbuild.2022.111830]

21. Barut, T.; Selçuk, S. Holistic Approach to Niche Formation: A Case on Transition to Nearly Zero-Energy Buildings in Türkiye. Buildings; 2024; 14, 1565. [DOI: https://dx.doi.org/10.3390/buildings14061565]

22. Jaysawal, R.; Chakraborty, S.; Elangovan, D.; Padmanaban, S. Concept of net zero energy buildings (NZEB)—A literature review. Clean. Eng. Technol.; 2022; 11, 100582. [DOI: https://dx.doi.org/10.1016/j.clet.2022.100582]

23. Lane, A.; Cehlin, M.; Thollander, P. Success Factors and Barriers for Facility Management in Keeping Nearly-Zero-Energy Non-Residential Buildings Energy-Efficient over Time. Buildings; 2024; 14, 242. [DOI: https://dx.doi.org/10.3390/buildings14010242]

24. Amaripadath, D.; Rahif, R.; Zuo, W.; Velickovic, M.; Voglaire, C.; Attia, S. Climate change sensitive sizing and design for nearly zero-energy office building systems in Brussels. Energy Build.; 2023; 286, 112971. [DOI: https://dx.doi.org/10.1016/j.enbuild.2023.112971]

25. Famiglietti, J.; Aprile, M.; Spirito, G.; Motta, M. Net-Zero Climate Emissions Districts: Potentials and Constraints for Social Housing in Milan. Energies; 2023; 16, 1504. [DOI: https://dx.doi.org/10.3390/en16031504]

26. Directive 2010/31/EU of the European parliament and of the council of 19 May 2010 on the energy performance of buildings. Off. J. Eur. Union; 2010; L156, pp. 75-91. Available online: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A32010L0031 (accessed on 1 September 2024).

27. Abolhassani, S.; Joybari, M.; Hosseini, M.; Parsaee, M.; Eicker, U. A systematic methodological framework to study climate change impacts on heating and cooling demands of buildings. J. Build. Eng.; 2023; 63, 105428. [DOI: https://dx.doi.org/10.1016/j.jobe.2022.105428]

28. Aruta, G.; Ascione, F.; Bianco, N.; Iovane, T.; Mastellone, M.; Mauro, G. Optimizing the energy transition of social housing to renewable nearly zero-energy community: The goal of sustainability. Energy Build.; 2023; 282, 112798. [DOI: https://dx.doi.org/10.1016/j.enbuild.2023.112798]

29. D’Agostino, D.; Congedo, P.; Albanese, P.; Rubino, A.; Baglivo, C. Impact of climate change on the energy performance of building envelopes and implications on energy regulations across Europe. Energy; 2024; 288, 129886. [DOI: https://dx.doi.org/10.1016/j.energy.2023.129886]

30. Li, H.; Wang, S.; Cheung, H. Sensitivity analysis of design parameters and optimal design for zero/low energy buildings in subtropical regions. Appl. Energy; 2018; 228, pp. 1280-1291. [DOI: https://dx.doi.org/10.1016/j.apenergy.2018.07.023]

31. Feng, W.; Zhang, Q.; Ji, H.; Wang, R.; Zhou, N.; Ye, Q.; Hao, B.; Li, Y.; Luo, D.; Lau, S. A review of net zero energy buildings in hot and humid climates: Experience learned from 34 case study buildings. Renew. Sustain. Energy Rev.; 2019; 114, 109303. [DOI: https://dx.doi.org/10.1016/j.rser.2019.109303]

32. Lis, A. Thermal modernization of building resources in line with the transformation towards a climate-neutral economy in Polish conditions. Sci. J. Marit. Univ. Szczec. Zesz. Nauk. Akad. Morskiej W Szczecinie; 2023; 76, pp. 5-16.

33. López-Ochoa, L.; Las-Heras-Casas, J.; González-Caballín, J.; Carpio, M. Towards nearly zero-energy residential buildings in Mediterranean countries: The implementation of the Energy Performance of Buildings Directive 2018 in Spain. Energy; 2023; 276, 127539. [DOI: https://dx.doi.org/10.1016/j.energy.2023.127539]

34. D’Agostino, D.; Parker, D.; Epifani, I.; Crawley, D.; Lawrie, L. How will future climate impact the design and performance of nearly zero energy buildings (NZEBs)?. Energy; 2022; 240, 122479. [DOI: https://dx.doi.org/10.1016/j.energy.2021.122479]

35. D’Agostino, D.; Mazzarella, L. What is a Nearly zero energy building?. Overview, implementation and comparison of definitions. J. Build. Eng.; 2019; 21, pp. 200-212.

36. Kraft, R.; Kahnt, A.; Grauer, O.; Thieme, M.; Wolz, D.; Schlueter, D.; Tietze, M.; Curbach, M.; Holschemacher, K.; Jaeger, H. et al. Advanced Carbon Reinforced Concrete Technologies for Facade Elements of Nearly Zero-Energy Buildings. Materials; 2022; 15, 1619. [DOI: https://dx.doi.org/10.3390/ma15041619] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35208159]

37. Report from the Commission to the European Parliament and the Council on the Implementation of Regulation (EU) No 305/2011 of the European Parliament and of the Council of 9 March 2011 Laying Down Harmonised Conditions for the Marketing of Construction Products and Repealing Council Directive 89/106/EEC. 2016; Available online: https://eur-lex.europa.eu/legal-content/en/ALL/?uri=CELEX:52016DC0445 (accessed on 1 September 2024).

38. Carlander, J.; Thollander, P. Barriers to implementation of energy-efficient technologies in building construction projects—Results from a Swedish case study. Resour. Environ. Sustain.; 2023; 11, 100097. [DOI: https://dx.doi.org/10.1016/j.resenv.2022.100097]

39. Jones, R.; Peng, D.; Michopoulos, J.; Kinloch, A. Requirements and Variability Affecting the Durability of Bonded Joints. Materials; 2020; 13, 1468. [DOI: https://dx.doi.org/10.3390/ma13061468] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32210207]

40. Regulation (EU) No 305/2011 of the European parliament and of the council of 9 March 2011 laying down harmonised conditions for the marketing of construction products and repealing council directive 89/106/EEC. Off. J. Eur. Union; 2011; L88, pp. 5-43. Available online: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A32011R0305 (accessed on 1 September 2024).

41. Proposal for a Regulation of the European Parliament and of the Council Laying Down Harmonised Conditions for the Marketing of Construction Products, Amending Regulation (EU) 2019/1020 and Repealing Regulation (EU) 305/2011. 2022; Available online: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A52022PC0144 (accessed on 1 September 2024).

42. Available online: https://www.mezeroe.eu/ (accessed on 1 September 2024).

43. Act of 16 April 2004 on Construction Products. 2004; Available online: https://isap.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=wdu20040920881 (accessed on 1 September 2024).

44. EN 12697-26 Bituminous Mixtures—Test Methods—Part 26: Stiffness; European Committee for Standardization: Brussels, Belgium, 2018.

45. ISO 3341 Textile Glass—Yarns—Determination of Breaking Force and Breaking Elongation; ISO: Geneva, Switzerland, 2000.

46. EN ISO 527-2 Plastics—Determination of Tensile Properties—Part 2: Test Conditions for Moulding and Extrusion Plastics; ISO: Geneva, Switzerland, 2012.

47. ISO 37 Rubber, Vulcanized or Thermoplastic—Determination of Tensile Stress-Strain Properties; ISO: Geneva, Switzerland, 2017.

48. EN 1052-3/A1 Methods of Test for Masonry—Part 3: Determination of Initial Shear Strength; European Committee for Standardization: Brussels, Belgium, 2007.

49. EN ISO 4892-2 Plastics—Methods of Exposure to Laboratory Light Sources—Part 2: Xenon-Arc Lamps; ISO: Geneva, Switzerland, 2013.

50. EN 29052-1 Acoustics—Determination of Dynamic Stiffness—Part 1: Materials Used under Floating Floors in Dwellings; European Committee for Standardization: Brussels, Belgium, 2011.

51. EN ISO 12354-1 Building Acoustics—Estimation of Acoustic Performance of Buildings from the Performance of Elements—Part 1: Airborne Sound Insulation between Rooms; ISO: Geneva, Switzerland, 2017.

52. EN ISO 12354-2 Building Acoustics—Estimation of Acoustic Performance of Buildings from the Performance of Elements—Part 2: Impact Sound Insulation between Rooms; ISO: Geneva, Switzerland, 2017.

53. EN ISO 13788 Hygrothermal Performance of Building Components and Building Elements. Internal Surface Temperature to Avoid Critical Surface Humidity and Interstitial Condensation. Calculation Methods; ISO: Geneva, Switzerland, 2012.

54. EN ISO 10211 Thermal Bridges in Building Construction—Heat Flows and Surface Temperatures—Detailed Calculations; ISO: Geneva, Switzerland, 2017.