The decarbonisation of the European building stock by 2050 represents one of the most ambitious challenges of the European Green Deal. Following the implementation of the revised Energy Performance of Buildings Directive (EPBD), the focus has shifted from estimated energy consumption during the design phase to measured performance during building operation. This transition is anchored in the new ZEB standard. Under the recast, all new buildings must be ZEBs by 2030 (and public buildings by 2028), meaning they must have a very high energy performance and emit zero on-site carbon emissions from fossil fuels. In this new paradigm, the so-called ‘performance gap’ [1 - 3], which is the discrepancy between efficiency targets predicted by simulation models and the building’s actual performance, is no longer merely a technical issue but a barrier to achieving carbon neutrality.

Reducing this gap is essential for at least three systemic reasons:

• Climate targets and quantification: To comply with the milestones set by the new EPBD, it is crucial that every kilowatt-hour saved on paper translates into actual savings. The spread of monitoring through Building Automation and Control Systems (BACS) in non-residential buildings reflects the need for reliable data. Where the gap cannot be eliminated due to variability in use or weather variability, its precise quantification becomes a key asset for energy management and adjusting large-scale policy interventions.
• Indoor Environmental Quality (IEQ) and well-being: A façade that does not perform according to design affects not only the energy balance but also the thermal and hygrometric and visual comfort of occupants. Unexpected thermal fluctuations or poorly managed solar gains degrade occupant experience, negatively influencing concentration, productivity, and psychophysical well-being.
• Reduced maintenance: A well-functioning system that is regularly monitored is less costly to operate and maintain because issues and failures can be detected at an early stage.

The challenge of complex façade systems    

While overall building energy consumption (heating, cooling, lighting) can be reliably assessed through utility bills or metering data, characterising the contribution of a single envelope component remains complex. Isolating the thermal contribution of the façade from other building systems requires detailed knowledge of the heat transfer processes occurring through the building skin.

Modern dynamic and adaptive façade systems amplify this complexity. In the pursuit of ZEB status, architects increasingly rely on active skin systems such as smart glazing units (electrochromic or thermochromic glass), ventilated cavities (double-skin façades), and adjustable shading systems to transform the façade into an active, dynamic system rather than a passive one. In such systems, heat transfer is no longer a linear process governed solely by temperature differences and incident radiation, but depends on variable factors such as natural convective flows (stack effect within the cavity), solar incidence angle, and software-driven control strategies.

Traditional design indicators, typically calculated under steady-state conditions (e.g., U-value, g-value, SHGC) [4, 5], are inadequate for in-situ verification. These parameters fail to capture the dynamic behaviour of façades that continuously adapt their optical and thermal properties in response to external stimuli or automated control systems. Moreover, laboratory boundary conditions differ substantially from actual operating conditions, where external temperatures and solar irradiance vary continuously. Using a fixed g-value to design an adaptive façade effectively neglects the temporal variability of its performance over time.

The role of façade commissioning (Cx)    

Cx is a systematic process aimed at ensuring that building systems are designed, installed, and tested to meet the owner’s project requirements (OPR) [6]. For the building envelope, the process includes several key phases:

• Design phase: The definition of performance targets and acceptance criteria.
• Laboratory testing: Mock-up testing to validate theoretical performance under controlled conditions.
• In-situ testing: On-site tests to verify installation quality on the completed building.

Current façade commissioning practices, in situ, focus primarily on physical integrity aspects: air tightness (e.g., blower door tests), water tightness, wind resistance, and acoustic performance. Although crucial for durability and safety, these tests are instantaneous snapshots only and do not assess the façade’s long-term energy management. 

A systematic procedure for verifying the post-installation thermal and solar performance is largely missing. In the case of complex façades, this gap is critical: increased construction and operational complexity significantly raise the risk that actual performance deviates from design expectations.

Continuous commissioning (CCx): evolution of the commissioning process    

In the context of rapid digital transformation, the spread of IoT solutions [7] now enables continuous monitoring of the building envelope to reduce the performance gap and prevent critical degradation. This technological evolution is supported by a shift towards service-based business models, where recurring revenue streams, similar to the concept of Façade-as-a-Service [8], can sustain operational management activities.

Within this framework, the CCx paradigm, or ongoing commissioning, extends performance verification far beyond building handover. Although not yet standard practice, this approach enables long-term operational trend analysis and the implementation of fault detection and diagnostics (FDD) strategies to identify sudden deviations in adaptive systems. While traditional commissioning evaluates façade performance as a one-time event, CCx represents a data-driven quality management process that persists throughout the building lifecycle. Collected data can feed predictive maintenance models and digital twins, transforming the façade into a dynamic virtual replica capable of simulating and optimising thermal and solar behaviour in real time through machine learning algorithms trained on historical climate data.

However, effective implementation of CCx requires sensor systems that meet strict architectural and functional integration criteria. Devices must be compact to avoid impacting aesthetics or occupant comfort, favouring wireless, low-power communication technologies that simplify maintenance and calibration. A careful balance must also be achieved between sensor autonomy and data management, lower sampling frequencies often prove optimal, ensuring long-term operation while avoiding unnecessary data overload for seasonal trend analysis.

Despite these opportunities, significant barriers remain to widespread adoption in the European market. High upfront costs and uncertain return on investment discourage building owners from investing in long-term monitoring systems. Additional challenges include fragmented post-handover responsibilities and a lack of data analysis expertise within the construction value chain. A persistent cultural barrier also remains: the façade is still often perceived as a passive element, which limits awareness of the benefits of continuous operational optimisation, particularly for complex systems.

The FAIR project approach: towards new dynamic indicators

Within the framework of the FAIR project, an innovative methodology is being developed to overcome the intrinsic limitations of traditional building diagnostics. The primary objective is to define a protocol for in-situ measurement of thermal and solar performance that moves beyond static parameters such as fixed g-values or SHGC. While standardised, these indicators are notoriously difficult to isolate accurately in uncontrolled outdoor environments and are inadequate for characterising modern façades whose behaviour changes dynamically across the day and through the seasons. In real operating conditions, variables such as shifting solar incidence angles and fluctuating indoor/outdoor air temperatures make laboratory-based calculations unrepresentative of actual performance.

The research focuses on defining measurement procedures for dynamic solar indicators capable of providing a high-resolution assessment of façade behavior over a full 24-hour cycle under actual dynamic conditions. These indicators quantify the actual energy transmitted through the envelope by accounting for hourly variations in solar gains and the adaptive response of the façade systems. Building Performance Simulation (BPS) tools have been used in prior research [9, 10] to define and assess these indicators, referred to as Daily Integrated SHGC and Maximum Solar Gain Ratio, demonstrating variability based on contextual factors such as orientation, location, season, and the operation of façade systems. 

To translate these theoretical indicators into practical measurement methods, an extensive measurement campaign was carried out on an actual office building featuring a ventilated south-oriented double-skin façade. High-precision, laboratory-grade instruments were used, including PT100 sensors, heat flux meters (HFM), and pyranometers.

_{Figure 1: Installation of the sensors used for the thermal assessment procedure.}

It is important to emphasise that this phase represents detailed, short-term characterisation. Because this setup utilises high-fidelity sensors and requires specific energy independence and data-logging precision, it is designed as a point-in-time diagnostic tool rather than a permanent installation. This high-resolution snapshot allows for the precise calibration of the façade performance model, but due to the cost and complexity of the sensor network, it is inherently limited in duration.

This method aims to enhance standard commissioning by adding a performance verification stage. It is intended to supplement checks on physical integrity and installation quality with tests of thermal and solar behaviour, empirically confirming that the façade meets energy targets set during design.

Long-term monitoring and implementation of continuous commissioning within the FAIR project    

In parallel with the development of high-resolution indicators, the FAIR project has explored the feasibility of long-term façade monitoring by viewing it as a natural extension of the initial diagnostic phase. While the short-term characterisation provides a performance baseline, long-term monitoring, based on the CCx approach, is essential to track performance trends and degradation over the building's entire lifecycle.

The objective of this stage was to test the effectiveness of scalable, off-the-shelf IoT sensors under actual operating conditions in the previously mentioned case study. This phase demonstrates that monitoring can evolve from an expensive, research-only activity into a routine building management tool. By utilising compact, non-intrusive wireless devices, we monitored parameters such as cavity air temperatures, indoor environmental conditions (CO₂, humidity), and user interactions with dynamic systems.

_{Figure 2: Installation process of the wireless IoT sensors.}

By extending the monitoring period, the research captured long-term phenomena that a short-term test would miss. For instance, an analysis of a south-facing ventilated cavity revealed consistent thermal stratification, where rising air temperatures in the upper sections of the façade led to increased thermal loads for the top floors. Furthermore, observing prolonged daily thermal cycles allowed for an assessment of the physical strain on materials like sealants and gaskets, which is vital for proactive maintenance strategies.

This long-term approach also highlighted the human factor, revealing how occupants often interact with windows and shades in ways that contradict energy-efficiency goals. Ultimately, these findings reinforce the idea that the façade must be managed as an active, interconnected system. By viewing long-term monitoring as an extension of the initial detailed characterisation, the industry can bridge the gap between design intent and operational reality. The next frontier lies in developing automated data analysis strategies that can translate large long-term datasets into the systemic corrective actions required by the continuous commissioning paradigm.

Discussion

The shift from design-centric verification to operational, performance-based evaluation is no longer optional, but a requirement driven by the EPBD recast and the transition towards ZEB. Traditional steady-state indicators are inherently unable to capture the non-linear and time-dependent behaviour of dynamic and adaptive building envelopes, where even minor deviations in performance can compromise compliance in highly optimised systems.

The findings of the FAIR project indicate that bridging this gap requires a combined approach. High-resolution, short-term diagnostic campaigns enable the accurate calibration of façade performance under actual conditions, while long-term monitoring through scalable IoT systems allows the tracking of performance evolution over time. This dual methodology makes it possible to identify phenomena that remain invisible to conventional commissioning practices, such as thermal stratification in ventilated cavities or the cumulative stress induced by repeated thermal cycles on façade components.

However, the value of this approach lies not only in data acquisition but in its integration within a continuous commissioning framework. By coupling monitoring with fault detection and diagnostics, predictive maintenance strategies, and digital twin environments, the façade can be effectively managed as a dynamic and measurable asset throughout its lifecycle. This shift enables a transition from reactive to proactive management, where performance deviations are identified early and addressed systematically.

From an application perspective, this approach generates tangible benefits across the value chain. Facility managers and building owners can improve operational reliability, reduce maintenance risks, and extend the service life of façade systems. At the same time, manufacturers and designers can leverage in-use long-term performance data to validate and refine their solutions, while asset managers and policymakers benefit from more robust datasets to support lifecycle optimisation and evidence-based regulatory strategies. Despite these advantages, widespread adoption still depends on overcoming economic constraints, clarifying post-handover responsibilities, and strengthening expertise in data analysis. Addressing these barriers will be essential to fully unlock the potential of continuous commissioning as a standard practice in the façade sector.

Conclusions

In summary, the construction industry is moving towards increased transparency and real-time monitoring. The FAIR project demonstrates how an improved approach to performance verification can ensure that buildings meet future ZEB requirements. By integrating detailed performance verification and long-term monitoring after construction, stakeholders across the value chain can close the gap between how buildings are supposed to perform and how they actually do. This approach not only protects the environment but also boosts the value of the buildings themselves, keeping pace with changing rules and expectations.

References

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[4] National Fenestration Rating Council, ‘NFRC 200 - Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence,’ 2023.

[5] Comité Européen de Normalisation, ‘EN 410:2011 - Glass in building - Determination of luminous and solar characteristics of glazing,’ 2011.

[6] S. Plesser, O. Teisen, and C. Ryan, ‘Quality Management for Buildings Improving Building Performance through Technical Monitoring and Commissioning,’ 2019. Accessed: Mar. 17, 2026. [Online]. Available: https://www.rehva.eu/hvac-guidebook-repository/rehva-guidebook-29  

[7] M. Arnesano, B. Bueno, A. Pracucci, S. Magnagni, O. Casadei, and G. M. Revel, ‘Sensors and control solutions for Smart-IoT façade modules,’ in 2019 IEEE International Symposium on Measurements and Networking, M and N 2019 - Proceedings, Institute of Electrical and Electronics Engineers Inc., Jul. 2019. doi: 10.1109/IWMN.2019.8805024. 

[8] J. F. Azcárate-Aguerre, A. C. den Heijer, M. H. Arkesteijn, L. M. Vergara d’Alençon, and T. Klein, ‘Facades-as-a-Service: Systemic managerial, financial, and governance innovation to enable a circular economy for buildings. Lessons learnt from a full-scale pilot project in the Netherlands,’ Front. Built Environ., vol. 9, 2023, doi: 10.3389/fbuil.2023.108407.

[9] R. Gazzin, G. De Michele, S. Avesani, G. Pernigotto, and A. Gasparella, ‘Simulative applications of novel indicators for the characterization and performance evaluation of transparent facades,’ 2024, doi: 10.13124/9788860462022_67.

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