Innovations in Building Diagnostics and Condition Monitoring: A Comprehensive Review of Infrared Thermography Applications

Abstract

Infrared thermography is a non-destructive technique that uses infrared radiation to visualize surface temperature variations. It is a versatile tool that can be used to detect a variety of problems in buildings, including insulation deficiencies, moisture intrusion, structural compromise, and electrical faults. The review paper discusses the fundamental principles of infrared thermography, the different types of infrared approaches, and the condition monitoring of buildings using infrared imaging techniques. It also discusses research showing how infrared thermography has been applied to recognize and solve different building-related problems. The article highlights the potential for infrared thermography to advance while also acknowledging its current limits. Infrared thermography is predicted to become an even more effective technique for building diagnostics with the development of more sensitive cameras and the incorporation of artificial intelligence.

1. Introduction

The demand for sustainable and efficient building techniques has never been larger in an era of growing urbanization and increased infrastructure complexity. A vital element of modern society is the physical environment, which includes anything from extensive industrial facilities to residential and commercial buildings. The rising thermal stresses on these structures, however, have increased along with the expansion of urbanization. As the urban landscape develops, buildings must adapt to handle the growing concerns of resource scarcity, environmental sustainability, and population congestion [1,2,3,4,5,6]. Condition monitoring and building diagnostics have become crucial techniques for ensuring the long-term performance and safety of buildings. They play a vital role in inspecting, maintaining, and improving the infrastructure that underpins the built environment. The art and science of assessing a building’s structural integrity and functionality is known as building diagnostics. It involves a systematic analysis of systems, structural components, and building materials to identify any flaws, vulnerabilities, and potential hazards [7,8,9,10]. In contrast, condition monitoring focuses on continuously observing and assessing the physical condition of a building or its individual components. These processes are now crucial for preventing structural failures, improving energy efficiency, and extending the lifespan of buildings [11,12,13,14].

In recent years, there has been a remarkable transformation in the field of building diagnostics and condition monitoring, driven by the integration of cutting-edge technologies and innovative methodologies. The substantial progress in non-destructive testing and evaluation (NDT&E) techniques has played a vital role in directing this shift. Among the advanced NDT&E techniques available to researchers and practitioners, IRT has emerged as a versatile and non-invasive technology, allowing full insights into the structural integrity and performance of buildings [8,15,16,17].

IRT has emerged as a non-destructive, non-intrusive, and non-contact methodology that facilitates the visualization of thermal patterns, known as thermograms, across the surfaces of objects, entities, or systems. This visualization is achieved by employing specialized equipment, such as an infrared camera, designed for infrared (thermal) imaging [18,19,20,21]. Over the past few decades, the utilization of NDT&E technology has become increasingly prevalent, with its applications continuously expanding to research and development (R&D) in various industries, structural health monitoring, material characterization, manufacturing quality assurance, energy cost reduction, surveillance, night vision, agriculture, medical science, and many more [22,23,24,25,26,27,28,29]. In recent times, there has been a growing demand for the quantitative assessment of buildings and structures [20,30,31,32,33,34]. In response, the use of IRT technology has proven to be valuable as a measurement tool, particularly for assessing heat dissipation [35,36,37,38]. IRT, a cornerstone of NDT&E methodologies, has risen to prominence as a versatile and invaluable tool for comprehensively assessing the health of buildings [39,40,41,42,43]. Characterized by its non-invasive and non-contact nature, IRT presents a unique approach to diagnosing hidden issues that may otherwise elude conventional inspection methods [44,45,46,47]. By capturing and converting the thermal radiation emitted by objects into visual representations, this technique unveils temperature distributions across various building components, exposing subtle anomalies and irregularities that often foretell more significant problems.

This study has a multifaceted set of aims and offers several innovative aspects. Firstly, the review paper aims to present a comprehensive and up-to-date synthesis of the existing literature, encompassing a wide range of studies. This synthesis will provide readers with a more complete understanding of the diverse applications of IRT in building diagnostics and condition monitoring. Secondly, the paper focuses on emerging trends and cutting-edge technologies within the field, offering a forward-looking perspective that prepares readers for the future of IRT applications in the context of building diagnostics. Lastly, the paper will address the challenges and limitations associated with the use of IRT, while also highlighting potential directions for future research and development in this domain.

2. Methods

To fulfill the aims outlined in the introduction section, a systematic literature search was conducted, starting with the acquisition of diverse, relevant publications from reputable academic databases and search engines, including Google Scholar, ResearchGate, IEEE Xplore, and ScienceDirect. A well-defined set of keywords, such as “infrared thermography,” “building inspection,” and “condition monitoring,” was employed to ensure comprehensive and relevant coverage of the research field. In Google Scholar, these keywords yielded the following approximate numbers of publications: “Infrared thermography” (6230 publications), “Building inspection” (466 publications), and “Condition monitoring” (10,900 publications). These figures provide an initial indication of the extensive literature available on these topics and serve as a foundational point for the systematic literature review. Subsequent steps in the review process will involve the screening and selection of relevant publications to align with the research objectives and criteria. To ensure that the search results closely aligned with the research focus, a combination of the “AND” and “OR” operators was applied to combine and broaden the search keywords. The “AND” operator, as seen in “Infrared Thermography AND Building Inspection,” was used to pinpoint publications discussing both IRT and building inspection, resulting in a more focused subset of literature, with approximately 83 publications. In addition, the “OR” operator was utilized to expand the search scope. For instance, the query “Building Inspection OR Condition Monitoring” retrieved publications related to either building inspection or condition monitoring, amassing a more extensive pool of relevant sources, totaling about 10,600 publications. Furthermore, a combination of these operators, exemplified by “Building Inspection OR Condition Monitoring AND Infrared Thermography,” was employed to further diversify the search results, resulting in approximately 6370 publications. This combination retrieved publications related to either building inspection or condition monitoring while still emphasizing the relevance of IRT, ensuring a comprehensive collection of pertinent sources for the research.

The inclusion criteria focused on articles published from 2000 to 2023 and were selective based on relevance to the topic, research design, and language. Additionally, exclusion criteria were applied to avoid patents, ensuring that the focus remained on academic publications rather than patent documents. Quality assessment was performed using a customized evaluation framework. Data extraction encompassed a range of crucial information, including the type of building under examination, the specific thermographic technology employed, and the innovative applications within building diagnostics and condition monitoring. Data synthesis was conducted using both qualitative and, where applicable, quantitative analyses to identify prevailing trends, technological breakthroughs, and methodological innovations in the field.

In the process outlined above, it is important to note that, in addition to the inclusion and exclusion criteria, papers that have been cited a significant number of times were also considered in the review. This approach helps to ensure that the literature review encompasses influential and well-regarded publications within the research field. The consideration of highly cited papers adds an extra layer of rigor to the review, as these papers often represent seminal works, key insights, or widely recognized contributions to the subject matter. Including such papers can enhance the comprehensiveness and depth of the literature review, providing valuable insights and context for the research objectives.

In conducting this review, it is important to acknowledge potential limitations, including the possibility of publication bias and the inherent variations in study methodologies found across the literature. Ethical considerations were taken into account concerning data handling, analysis, and reporting, with a focus on upholding the integrity of the review. The review process involved the use of Mendeley, a robust reference management software, to ensure systematic citation management.

3. Background

3.1. Basics of Infrared Thermography

All objects with a temperature above 0 K (i.e., −273 °C) emit electromagnetic radiation in the infrared region of the electromagnetic spectrum. Infrared radiation (wavelength in the range of 0.75–1000 µm) is positioned in-between the microwave and visible part of the electromagnetic spectrum. This vast range can be further subdivided into near infrared or NIR (0.76–1.5 µm), medium infrared or MIR (1.5–5.6 µm), and far infrared or FIR (5.6–1000 µm). In 1800, Sir William Herschel discovered infrared radiation and the recording of the first thermal image was performed by his son John Herschel, which added new dimensions to the temperature measurement, quality equipment, and technical knowhow [48]. In thermal radiation theory, the blackbody is considered as a hypothetical object which absorbs all incident radiation and radiates a continuous spectrum according to Planck’s law and can be expressed by Equation (1) as follows [49,50,51,52].

$${\mathrm{L}}_{\mathsf{\lambda }}=\frac{{\mathrm{C}}_1}{{\mathsf{\lambda }}^5[{\mathrm{e}}^{\frac{{\mathrm{C}}_2}{\mathsf{\lambda }\mathrm{T}}}-1]}$$

(1)

where $\mathsf{\lambda }$ is the wavelength of the radiation (µm), ${\mathrm{L}}_{\mathsf{\lambda }}$ is the power radiated by the blackbody per unit surface and per unit solid angle for a particular wavelength (W m⁻² µm⁻¹ sr⁻¹), T is the temperature in absolute scale (K), and ${\mathrm{C}}_1$ and ${\mathrm{C}}_2$ are the first and second radiations constants, respectively. On integrating Planck’s law over all frequencies, Stefan–Boltzmann’s law is derived and can be expressed by Equation (2) [48,50,53].

$$\frac{\mathrm{q}}{\mathrm{A}}=\mathsf{\epsilon }\mathsf{\sigma }{\mathrm{T}}^4$$

(2)

where q is the rate of energy emission (W), A is the area of the emitting surface (m²), T is the absolute temperature (K), $\mathsf{\sigma }$ is the Stefan–Boltzmann’s constant ($\mathsf{\sigma }$ = 5.676 × 10⁻⁸ W m⁻² K⁻⁴), and $\mathsf{\epsilon }$ is the emissivity of the emitting surface for a fixed wavelength and absolute temperature T. For a perfect blackbody, emissivity is unity, but for real surfaces it is always less than unity [48]. The wavelength of the peak of the emission spectrum is also related to the absolute temperature of the emitting surface by Wien’s displacement law and can be expressed by Equation (3) [48,54,55].

$${\mathsf{\lambda }}_{\mathrm{m}\mathrm{a}\mathrm{x}}\mathrm{T}=2897.7 \mathrm{\mu }\mathrm{m} \mathrm{K}$$

(3)

From an experimental approach aspect, IRT techniques are classified into two major categories: passive and active. The passive approach is generally used in the research on materials that are at a different temperature compared with the ambient, while in the case of an active approach, an external excitation source, such as optical flash lamps, halogen heat lamps, mechanical ultrasonic vibration, or a hot and cold air gun, is employed with the intention of inducing thermal contrast. In the domain of active thermographic NDT&E, pulsed thermography and lock-in thermography are the two most frequently utilized approaches. [17,56,57,58,59]. Active thermography and passive thermography are two distinct techniques employed in thermal imaging and thermographic analysis for various applications, including building diagnostics, industrial inspections, and material testing [60,61,62,63]. Let us delve into the differences and characteristics of both.

In passive thermography, the objective is to capture the naturally occurring thermal radiation emitted by an object’s surface without applying any external energy source. This technique is used when the features of interest naturally have a temperature difference compared to their surroundings. Passive thermography is primarily qualitative, aiming to identify discontinuities or anomalies within the material. It is effective in the short-wave infrared (SWIR) range (3–5 µm) and the long-wave infrared (LWIR) range (8–12 µm). This method finds applications in diverse fields, including process monitoring, condition monitoring, predictive maintenance, structural health monitoring, medical imaging, building thermal efficiency assessments, process monitoring, forest fire detection, road traffic monitoring, agriculture, biological sciences, and more [18,52,64,65,66,67]. However, passive thermography is limited in NDT&E applications where a substantial natural thermal contrast is required.

Active thermography involves introducing controlled energy to the object being inspected to induce thermal contrast and highlight specific features. The energy can be applied as pulses or modulated continuous forms. Various energy sources such as halogen lamps, optical flash lamps, and hot or cold air guns can be used. The introduction of energy disrupts the object’s thermal equilibrium, creating a detectable thermal response that reflects underlying defects or anomalies. Active thermography allows both defect identification and quantitative analysis of anomalies. It is particularly useful in NDT&E applications for detecting structural layers like delamination in the automotive and aerospace industries, investigating interior structures, identifying deeper material irregularities, detecting and characterizing defects such as metal corrosion, cracks, coating wear, debonding and impact damage, determining material properties such as thermophysical properties like diffusivity and thermal conductivity, adhesion strength, anisotropic material characterization, and many more [18,40,67,68,69].

Choosing between active and passive thermography depends on the application and required information. Passive thermography is suitable for surface-level inspections and detecting anomalies, whereas active thermography is advantageous for assessing subsurface defects and material properties by introducing controlled thermal stimuli.

Both passive and active thermographic techniques may be static and dynamically inspected. In the static approach, the camera, object, and heat source remain steady, making it ideal for an in-depth analysis of the timing, purpose, and stability of the items. On the other hand, dynamic inspection is significant for complicated or huge objects, offering faster and more complete evaluations where timing is addressed and the apparatus is in changing conditions. Dynamic methods may involve laser scanning with a static item and camera, or utilizing a mobile setup with a camera and light mounted on a robot. Being particularly distinguished between active and passive thermography approaches, these dynamic and static configurations include different processes and tools, each with specific benefits and limitations for thermal diagnostic applications.

Table 1. Comparative characteristics of IRT approaches [68,70,71,72].

Approaches	Techniques	Excitation	Advantages	Disadvantages
Passive	-	Solar natural cycle	No external excitation source; lower equipment complexity; safe for both operator and object under inspection; can be used for continuous monitoring	Dependence on ambient conditions; ineffectiveness on non-emissive materials; slower inspection times; lower sensitivity to subsurface defects; limited detection depth.
Active	Pulsed	Pulse	Simple heating mode; fast inspection time; high power energy; beneficial for flat defects	Affected by non-uniform heating; inversion techniques are complex; not suitable for the inspection of complex structural components; detection depth is limited.
	Lock-in	Modulated	Little impact of non-uniform heating, environmental reflections, emissivity variations, and nonplanar surfaces; low power thermal waves; depth inversion is straightforward.	Requires a test for every inspected depth; long heating time; varied optimal frequency according to material properties and defect depths; affected by blind frequency and low frequency.

provides a comprehensive overview of various infrared approaches, summarizing their strengths and weaknesses.

3.2. Principles of IRT

IRT operates as a potent technique for analyzing thermal patterns and temperature variations across diverse surfaces. Its efficacy is underpinned by a spectrum of performance parameters that collectively shape its precision and applicability, rendering it a crucial tool in building diagnostics [48,73,74].

Spectral Range: The spectral range defines the portion of the infrared spectrum within which an infrared camera functions optimally. Objects emit thermal radiation more prominently at shorter wavelengths as their temperatures rise. For ambient temperature observations, the long wave band (7.5–14 μm) is favored due to objects predominantly emitting within this range. Moreover, measurements conducted within this band remain unaffected by sunlight radiation, making it apt for outdoor scenarios. Conversely, short wave systems (2–5 μm) are advantageous during overcast conditions and at night [48].

Spatial Resolution: The ability of an infrared camera to distinguish between objects in its field of view is known as spatial resolution. This parameter hinges on factors such as the object-to-camera distance, lens system, and detector size. Smaller field-of-view lenses and larger detector arrays enhance spatial resolution. However, spatial resolution diminishes as the object-to-camera distance increases. Notably, a lens system with a smaller field of view, such as 10° × 7°, provides superior spatial resolution compared to a wider lens, such as 20° × 16° [48].

Temperature Resolution: Temperature resolution reflects the smallest temperature difference discernible by an infrared camera within its field of view. Factors like object temperature, environmental conditions, and object-to-camera distance influence this parameter. Quantities like noise equivalent temperature difference (NETD), minimum resolvable temperature difference (MRTD), and minimum detectable temperature difference (MDTD) serve as metrics for temperature resolution assessment. Contemporary cooled cameras exhibit NETD values below 0.025 °K at room temperature [48].

Temperature Range: An infrared camera’s capacity to measure temperature falls within a designated temperature range. Common ranges span from −20 °C to 500 °C. Employing specialized filters can extend this range to reach temperatures up to 1700 °C [48].

Frame Rate: Frame rate is the frequency at which an infrared camera captures frames per second. Cameras equipped with higher frame rates are invaluable for monitoring dynamic events and moving objects. A standard frame rate is 50 Hz (50 frames per second) [48].

Yet, the selection of an infrared camera for building diagnostics transcends these parameters alone. Inherent attributes such as power consumption, physical dimensions, weight, image processing capabilities, calibration precision, storage capacity, compatibility with computer interfaces, cost, and available service support collectively contribute to the camera’s suitability for specific applications [75,76,77].

The integration of these performance parameters and inherent attributes underscores the meticulousness of IRT as a diagnostic tool for buildings. By aligning an infrared camera’s specifications with the unique requisites of building assessments [78,79,80], professionals ensure the accurate identification of thermal anomalies, structural irregularities, and energy inefficiencies. Thus, IRT stands as a cornerstone in modern building diagnostics, enabling practitioners to uncover insights that inform maintenance practices, optimize energy efficiency, and ensure the structural longevity of buildings [81,82,83].

In building diagnostics, IRT serves as a valuable tool because it visualizes temperature differences that are indicative of structural issues, energy inefficiencies, or hidden defects. For instance, anomalies in thermal patterns can signify compromised insulation, air leakage, moisture intrusion, or even electrical problems within walls [23,84,85,86]. By capturing and analyzing these temperature variations, professionals can detect underlying problems that might otherwise remain unnoticed through traditional visual inspections [87].

The principles of IRT rest on the fundamental physics of thermal radiation and the conversion of emitted infrared waves into visual data through specialized cameras [88]. By utilizing this technology, building inspectors and maintenance professionals can uncover vital information about the condition of buildings, enabling informed decisions and targeted interventions that enhance energy efficiency, prevent damages, and ensure the longevity of structures [56,89,90].

3.3. Condition Monitoring

Condition monitoring refers to the practice of overseeing the state of machinery and operations. The primary objective of condition monitoring is to prevent unforeseen breakdowns, enhance plant accessibility, and minimize potential risks [48,91,92,93,94]. This approach facilitates the identification of issues prior to significant machinery or component failures. For instance, the degradation of oil and gas pipelines, and concealed leakages within valves and pressure vessels, among others, can result in severe consequences such as explosions and fires. The aim of condition monitoring is to mitigate these risks by proactively detecting and addressing problems [70,95,96,97,98,99].

The inspection and monitoring of structural conditions are an essential part of the life-cycle management of civil engineering systems. In recent decades, extending the useful service life of structures and infrastructures has become of crucial importance, due to cultural, social, and economic factors [100,101,102,103]. The development of NDT&E techniques is given priority in the field of inspection methodologies. These techniques are especially appropriate when dealing with constructions and infrastructures [104,105,106].

The modern built environment stands as a testament to human ingenuity, technological advancement, and architectural creativity. However, alongside these triumphs lies a complex web of challenges that demand effective solutions. Buildings, as dynamic systems influenced by a myriad of factors, require constant vigilance to ensure their longevity, functionality, and energy efficiency [54,61,62,63]. Within this context, the role of diagnostic techniques becomes paramount in safeguarding structural integrity and optimizing operational performance [107,108,109,110].

The essence of IRT lies in its foundation on the principles of thermal physics. Objects, whether human-made or natural, emit thermal radiation because of their temperature. This radiation, primarily manifesting within the infrared spectrum, is inherently invisible to the human eye [111,112]. However, specialized infrared cameras, equipped with sophisticated detectors and optics, could translate this invisible radiation into tangible images known as thermograms [113,114]. These thermograms provide a window into the thermal behavior of building materials, components, and systems [112,115,116]. Figure 1 depicts an example of using IRT to investigate the effects of individual climatic parameters such as solar irradiation, wind, environmental infrared radiation, and past outside air temperatures. The building under study was constructed in 1939 in Winterthur, Switzerland, and this research was conducted by B. Lehmann et al. in 2012. Figure 1a presents a regular photograph of the building, while Figure 1b shows the corresponding infrared image, revealing temperature variations. The bright areas in Figure 1a indicate sections of the facade without insulation, which are clearly distinguishable via thermal mapping, as seen in Figure 1b [30].

The key principle that governs the success of IRT is emissivity [117]—a fundamental property that quantifies the ability of a surface to emit thermal radiation. Variations in emissivity across different materials influence the thermal patterns observed in thermograms, enabling professionals to pinpoint areas of concern [44,117]. By understanding the principles of emissivity and its impact on temperature measurements, practitioners can distinguish genuine anomalies [118] from emissivity-driven variations, ensuring accurate diagnostic assessments.

The significance of IRT in building diagnostics is underscored by its wide range of applications, each addressing critical aspects of structural health and energy efficiency [56,119]. One of its primary applications lies in the detection of thermal anomalies, commonly associated with energy inefficiencies such as poorly insulated areas or air leaks. By identifying these irregularities, building owners and managers can implement targeted improvements to enhance energy conservation and reduce operational costs [120,121].

Moisture intrusion, a pervasive challenge in building maintenance, is another domain where IRT excels. The ingress of moisture can lead to structural degradation, mold growth, and compromised indoor air quality [104,122,123,124,125]. Through its ability to detect temperature variations caused by moisture-related issues, IRT aids in the early identification of potential water intrusion sites. This timely intervention prevents escalating damage, promotes healthier indoor environments, and preserves the structural integrity of buildings [126].

Furthermore, IRT plays a pivotal role in assessing insulation performance—a factor integral to energy efficiency. Inadequate or damaged insulation can result in significant energy losses and discomfort for occupants. IRT accurately identifies such insulation deficiencies, guiding insulation enhancement efforts and improving overall building performance. Technology continues to advance, and so does the capacity of IRT to revolutionize building diagnostics [20,127,128]. High-resolution cameras provide detailed thermograms, enabling the identification of subtle irregularities. Enhanced sensitivity allows the detection of minute temperature differentials, broadening the spectrum of detectable issues. Emerging multi-spectral imaging extends the boundaries of data capture, facilitating comprehensive diagnostics by considering a wider range of information beyond the thermal domain [59,129,130].

The rise of IRT as a potent and indispensable diagnostic tool for buildings is indisputable. Its non-invasive nature, capacity to unveil hidden anomalies, and ability to inform strategic decisions for maintenance and energy efficiency position it at the forefront of modern building diagnostics. As this review delves deeper into the principles, applications, advancements, challenges, and prospects of IRT, it is evident that this technology’s influence will continue to shape the sustainable and resilient buildings of tomorrow [131,132].

3.4. Applications in IRT in Building Diagnostics

In the domain of building diagnostics, IRT emerges as a cornerstone technology with the capability to identify structural irregularities and evaluate thermal performance. By harnessing the principles of capturing thermal radiation emitted by materials, IRT transforms the inspected area into a visual representation where colors correspond to distinct temperature gradients [133,134,135]. This technique necessitates the controlled application of thermal stimuli, either in the form of heating or cooling, to specific regions of interest, thereby inducing corresponding temperature fluctuations on the surface [136,137]. The resultant surface temperatures are inherently influenced by the thermal attributes of the material, encapsulating elements such as thermal resistance and inertia traits. Notably, the presence of moisture confined within porous materials introduces variations in thermal conductivity, consequently diminishing thermal resistance and culminating in the manifestation of detectable thermal bridges [138]. Such thermal anomalies manifest as cooler zones in the captured infrared images. Furthermore, environmental factors like wind or sunlight augment the evaporative cooling impact on damp surfaces, thereby intensifying the discernibility of moisture-related irregularities [45,115,139].

In the realm of building diagnostics, the efficiency and expedience of IRT in comprehensive in situ examinations are particularly evident. This facilitates the identification of both overt and concealed material defects, moisture-induced complications, and thermal aberrations. While IRT scanning furnishes an encompassing overview, the acquisition of quantitative data from representative areas assumes paramount importance for the accurate interpretation of the identified issues [140]. Employing the strategy of sequential thermal imaging to scrutinize problematic zones facilitates a quantitative evaluation of these locales. This methodology entails the acquisition of successive infrared images during alternating heating and cooling intervals, culminating in the production of differential thermal images that accentuate the surface temperature differential [129,131,141]. By monitoring the transformations in the physical and thermal properties intrinsic to the defect region, these differential images enable the facile tracking of anomalies. Additionally, the temporal evolution of temperature under the influence of heating and cooling is amenable to analysis through graphical representations, offering insight into surface temperature changes over time. The rate of warming up or cooling down, denoted as the rate of warming (RW) or the rate of cooling (RC) for each specific area, provides empirical links to material thermal inertia characteristics [142,143]. By comparing these rates with those characteristics of sound materials, relative interpretations of thermal inertia can be effectively deduced.

IRT exhibits acute sensitivity to the thermal attributes of materials. Given the inherent correlation between these attributes and the physical and mechanical properties of materials, IRT assumes the role of an adept instrument for meticulous monitoring and the quantification of alterations [144]. Moreover, the synergy arising from the integration of IRT with complementary techniques, as exemplified by ultrasonic testing, enhances the precision of non-destructive in situ examinations. This amalgamation has demonstrated its efficacy in evaluating structural robustness, yielding correlations between the extent of deterioration, thermal inertia characteristics, and ultrasonic pulse velocity measurements [121,145,146]. Consequently, IRT emerges as an indispensable tool within the ambit of building diagnostics, delivering insights into material behaviors, thermal anomalies, and the structural integrity of edifices [147].

Accurate data acquisition is paramount for reliable IRT inspections. Environmental conditions, surface emissivity [148,149], and camera settings influence the quality of thermograms. Adequate preparation, including accounting for ambient temperature and humidity, contributes to accurate results. Furthermore, robust image analysis techniques, such as filtering and temperature calibration, enhance the clarity and precision of thermograms. Quantitative analysis methods enable detailed temperature measurements and comparisons across different areas of the building [150,151].

To ensure the consistency and reliability of IRT in building diagnostics, adherence to industry standards and guidelines is essential. Prominent standards like ASTM E1186 and ASNT SNT-TC-1A provide comprehensive practices for conducting IRT inspections. These standards cover aspects such as equipment calibration, data collection procedures, and reporting protocols, ensuring that inspections are conducted professionally and consistently. Technological advancements have significantly enhanced the capabilities of IRT [152,153,154]. Higher resolution cameras provide detailed thermograms, facilitating the identification of subtle anomalies. Increased sensitivity enables the detection of minute temperature differences, making the technique more effective. Multi-spectral imaging, an emerging advancement, expands the data captured beyond the thermal spectrum, enabling comprehensive diagnostics [155,156]. While IRT is a powerful technique, its effectiveness can be maximized through integration with other diagnostic methods. Combining IRT with visual inspections, ultrasonic testing, and moisture meters provides a holistic view of building conditions. This synergy results in a more comprehensive and accurate assessment, allowing for better-informed decisions regarding maintenance and repairs [48,104]. Despite its advantages, IRT faces challenges, including limitations in penetrating certain building materials. Future research aims to address these challenges through the development of more advanced infrared technologies and the integration of complementary diagnostic techniques. The evolution of machine learning and artificial intelligence could further enhance the data analysis process and improve the accuracy of diagnostic assessments.

Real-world case studies exemplify the practical utility of IRT in building diagnostics. These examples highlight how technology has identified hidden issues, prevented damage, and improved energy efficiency. Each case underscores the value of early detection and proactive intervention made possible through IRT. Lucchi delved into detailed applications of energy audits, encompassing the detection of thermal bridges, insulation defects, air leakage, moisture, indoor temperature, U-value measurements, and human comfort assessments. Further, Nardi et al. focused on quantitative IRT for U-value measurement, indicating its role in representing building energy efficiency [13]. Similarly, Bienvenido-Huertas et al. highlighted IRT as a pivotal in situ method for assessing U-value. Beyond energy audits, IRT finds a crucial application in civil infrastructures and buildings for detecting delamination, voids, and high moisture content. Lourenço et al. explored cutting-edge techniques of IRT to identify delamination and moisture beneath ceramic cladding facades, showcasing the efficacy of quantitative and qualitative survey methods [154]. Moreover, Sirca Jr. and Adeli investigated experiment conditions and reviewed IRT’s utility in concrete defect detection, both in laboratory tests and field surveys, further emphasizing IRT’s efficacy in infrastructure and building inspections.

The methodology aspect of IRT encompasses diverse approaches, characterized by their features and principles. The classification includes passive IRT, active pulsed IRT, and active lock-in infrared. Milovanovic and Pecur concentrated on active IRT for concrete infrastructures, providing insights into physical backgrounds, equipment, and post-processing methods [13]. Furthermore, Garrido and coauthors contributed comprehensive reviews on IRT methodologies, covering both data acquisition and post-processing. Their data acquisition review introduced various IRT approaches, comparing recent studies in terms of experimental setups, target materials, IRT modes, and analysis schemes across different defect types and applications [154]. In the post-processing domain, they outlined theories and exemplar studies on analysis algorithms, evaluating their advantages and disadvantages. Together, these reviews illuminate the panorama of both traditional and contemporary IRT methodologies.

An analytical lens directed towards research trends, based on the statistical data of past studies, offers valuable insights into IRT’s expanding horizons. For instance, Fox et al. dissected trends in IRT related to energy-related building defect detection, shedding light on correlations between different methodologies [154]. Similarly, Kylili et al. conducted a statistical analysis of IRT in building facades, encompassing measurement methods, analysis schemes, and analysis types. These statistical reviews provide objective evidence of the growing literature focused on IRT’s applications in both infrastructures and buildings [154]. Likewise, the evolution of thermal cameras, highlighting their characteristics and pivotal advancements, is explored in

Table 2. List of infrared studies and its respective camera characteristics.

Researchers	IR Camera Characteristics	Year	Data Analysis	Main Findings	Aim
D.J. Titman [60]	Portable thermal imagers	2001	Locating anomalies in thermal insulation, detecting structural issues	Valuable NDT&E technique for assessing structural conditions	Promote thermography’s diverse applications in civil engineering while examining its pros and cons in different situations.
S.M. Ocaña et al. [157]	640 × 480 px. res, 1.3 mrad, FoV 24° × 18°/0.3 m	2003	Comparison of the thermal performance of two buildings in two separate inspections: one in the late evening and the other in the early morning	The novel method encloses several strengths including low cost, automation, flexibility, accuracy, and reliability, and it is also scalable. Its challenge is to extrapolate this process to images captured from a robotic aerial system so that the thermographic information of the most remote parts of the building can be also obtained	Assess the usefulness of IRT as a technique for detecting the thermal performance of buildings in rural areas of Spain.
Carosena Meola et al. [158]	136 × 272 px. at 12 bit res., 8–12 $\mathsf{\mu }$m spectral range	2004	Limitation in detection of defects in composites by IRT	LT allows for better defect detection. The contrast decreases with decreasing the defect size and with increasing the depth	Study how factors like diameter, depth, and thickness affect defect visibility in carbon/epoxy and glass/epoxy composites using IRT.
Laurent Zalewski et al. [159]	8–13.5 $\mathsf{\mu }\mathrm{m}$ spectral range, 320 × 240 px. res., −20 to +60 °C temperture range, 0.4 mrad	2008	Development of a simple accurate three- dimensional numerical method that could be used for the design of specific installations and parametric studies	IRT located moisture problems, mortar disaggregation, bricks cracking and problems in the adherence between the two materials. Useful in the detection of different materials, for example, areas with repaired mortar different than the original one	Establish and improve reliable models for understanding thermal bridges within a wall involving a metal frame, insulating materials, and air gaps. The ultimate goal is to reduce the impact of thermal bridges through parametric studies.
W.L. Lai et al. [160]	320 × 240 px. res., 3.6–5 $\mathsf{\mu }$m spectral range, 1 mrad, <0.1 °C at 30 °C temperature resolution	2009	Durability assessment using IRT of the effects of exposing CFRP. concrete beams to elevated water temperatures	High water temperatures cause deterioration of the adhesive bonding Layer of CFRP-concrete composites due to disruption of the polymer matrix chains of the epoxy resin	Examine the durability of externally-bonded CFRP-concrete beams under varying water temperatures. The research aims to assess how temperature exposure affects the adhesive bonding layer and failure modes.
D.G. Aggelis et al. [161]	320 × 240 px. res, uncooled microbolometer detector, 7.5–13 $\mathsf{\mu }$m spectral range	2010	Subsurface damage characterization of concrete structures using a combination of IRT and ULT	Vertical cracking is typically of a very thin shape; thus, the IR camera is required to have a high thermal sensitivity ULT enables the characterization of the depth of the crack in more detail	Evaluate the effect of subsurface cracks on steel fiber-reinforced concrete using IRT and elastic wave measurements.
Jeff R. Brown and H.R. Hamilton [162]	320 × 240 px. res., uncooled microbolometer detector, 8–12 $\mathsf{\mu }$m spectral range	2010	Characterization of FRP applied to concrete using single pixel analysis	Effective method for minimizing the effects of non-uniform heating and establishing the relative depth of fabricated defects of the fiber-reinforced polymer/concrete interface	Assess the feasibility of using IRT as a non-destructive evaluation tool for identifying and characterizing defects in FRP composites bonded to concrete.
F. Cerdeira et al. [134]	-	2010	Feasibility of IRT in identifying defects in the concrete used to stack the stone panels	IRT has proven to be a suitable nondestructive technique for identifying defects in the cement wash on a wall, such as the lack of adherence of the stone panels. On the contrary, IRT provides reliable results when the panels are thinner than 30 mm	Explore the feasibility of using IRT to non-destructively inspect building facades and detect wall surface defects under specific thermal conditions.
Paris A. Fokaides and Soteris A. Kalogirou [163]	7.5–13 $\mathsf{\mu }\mathrm{m}$ spectral range, 320 × 240 px. res, FoV 25° × 19°/0.4 (m), 50 mK thermal sensitivity	2011	Determination of the overall heat transfer coefficient (U-value) in building envelopes	10–20% absolute deviation between the notional and the measured U-values	Determine the U-Value of typical building constructions in Cyprus using IR thermography and validate the results against relevant standards and other measurement techniques.
I. Martinez et al. [164]	Uncooled focal plane array (UFPA) detector, 8–13 $\mathsf{\mu }\mathrm{m}$ spectral range, 320 × 240 px. res, 50 mK thermal sensitivity	2011	Physicochemical characterization of the original mortar used during the construction of the bell gable of a church built in 1672	Daybreak inspection of buildings whose walls are not structural and with a short thickness, i.e., modern buildings, provides more information. The inspection of traditional buildings should be preferred to be conducted during the evening	Assess the level of deterioration and the physico-chemical properties of the existing mortar, including its binder type, hydraulicity, and carbonation degree. This evaluation will inform the appropriate choice of retrofitting material.
Francesco Asdrubali et al. [165]	320 × 240 px. res., microbolometer without cooling detector, 7.5–13 μm spectral range	2012	Development of a methodology that expresses the thermal bridge effect on building envelopes, using only the information captured from thermographic surveys and the subsequent analytical processing	Verification of the incidence factor of the thermal bridge though laboratory thermographic investigation	Evaluate the effect of thermal bridges on the global dispersions of buildings, and develop a quantitative factor for the purpose.
Janet F.C. Sham et al. [166]	±2 °C accuracy, 0.08 °C thermal sensitivity	2012	Assessment of continuous surface temperature monitoring technique for investigation of nocturnal sensible heat-release characteristics by building fabrics	Verification that sensible heat release estimated by continuous surface temperature monitoring technique is consistent with sensible heat calculated using the traditional internal energy equation. IRT is a reliable tool that can be employed for verification purposes	Study building materials’ energy release during cooling to reduce urban heat island effects.
W.L. Lai et al. [167]	320 × 240 px. res., 3.5–5 $\mathsf{\mu }$m spectral range, 1 mrad, <0.1 °C at 30 °C temperature resolution	2013	Durability assessment of FRP. concrete beams using IRT and quasi-static direct shear test for monitoring the intermediate cracking processes	The adhesive bonds are weakened at elevated temperatures due to the heat distortion temperature of the epoxy resin reached during the exposure making the interfacial bonds more vulnerable	Introduce a new method using direct shear testing and IRT to analyze the intermediate state of CFRP-concrete composites during shear loading, focusing on the impact of water intrusion and elevated temperatures on IC debonding.
D. González - Aguilera et al. [168]	UFPA, 7.5–13 um spectral range, 640×480 px. res., FoV 21.7°×16.4°/0.6 (m)	2013	Development of a novel image-based thermographic modeling for assessing energy efficiency of building facades	The most sensitive variable in IR thermography is the reflected apparent temperature and the assumed emissivity of the building surface	Present an original contribution to the automation of thermographic 3D modeling of buildings using a low-cost methodology supported by proprietary software.
K.E.A. Ohlsson and T. Olofsson [35]	8–14 $\mathsf{\mu }$m spectral range, 160 × 120 px. res., 3.3 mrad, NETD<0.08K at 30°C	2014	Valuation of measurement errors and the comparison of thermography results against a reference method	An improvement in the measurement of the density of heat flow rate (q) using thermography	Improve a procedure for measuring the 2-dimensional pattern of the density of heat flow rate (q) across a building element’s surface using thermography.
Laurent Ibos et al. [169]	LWIR camera, 7.7–9.2 $\mathsf{\mu }$m spectral range, 320 × 256 px. res., 1.3 mrad	2015	Analysis of thermal resistance values and uncertainties	Comparison of different methods for estimating thermal resistance and the impact of insulation on the measurements	Estimate the thermal resistance of building walls using various methods and evaluate the effectiveness of different measurement techniques.

. This tabulated summary will provide a clear, concise overview of the thermal imaging landscape, showcasing the remarkable progress made in this field.

Rodgers et al. conducted a 1999 study focused on predicting heat transfer in printed circuit boards under natural convection. They utilized an Intrametric Model 760 camera with an HgCdTe sensor sensitive to 3–12-micron wavelengths and a low detectivity (<0.2). In 2003, Clark et al. employed an AGEMA Thermovision-900 camera with an InSb sensor sensitive to 2–5.6-micron wavelengths and a sensitivity of 0 for monitoring the condition of concrete and masonry bridges. These studies highlight IRT’s versatility, from analyzing electronic heat transfer to assessing the health of civil engineering structures [48]. In 2004, Venkatraman and colleagues conducted a study focused on predicting the tensile failure of 316 stainless steel specimens. To achieve this, they utilized an AGEMA Thermovision-550 infrared camera equipped with a PtSi (Platinum Silicide) sensor sensitive to wavelengths in the range of 3.6–5 microns. This sensor’s sensitivity was exceptionally high, with a low detectivity of less than 0.1. The researchers employed IRT to gain insights into the behavior of the stainless-steel specimens under stress and identify potential failure points [48]. In 2008, Speka and colleagues undertook a study aimed at controlling the laser welding process of amorphous polymer materials. For this purpose, they employed a FLIR THERMACAM S40 infrared camera. This camera was equipped with an uncooled microbolometer sensor, which possessed sensitivity to the 7.5–13-micron wavelength range. The microbolometer’s detectivity was measured at 0.08, indicating its efficiency in capturing thermal data. Through IRT, the researchers monitored the temperature changes and heat distribution during the laser welding process to ensure precise control and quality in polymer welding applications. In the same year, Bagavathiappan et al. conducted a study involving the condition monitoring of exhaust system blowers. They employed an AGEMA Thermovision-550 infrared camera equipped with a PtSi sensor sensitive to wavelengths ranging from 3.6 to 5 microns. Like the previous study, the sensor boasted a low detectivity of less than 0.1. IRT allowed the researchers to identify potential issues or abnormalities in the exhaust system blowers’ thermal behavior, contributing to maintenance and performance optimization [48]. In 2010, Wang and colleagues focused on investigating the necking phenomenon in fiber drawing processes. They utilized a FLIR THERMACAM A40M infrared camera equipped with an uncooled microbolometer sensor sensitive to wavelengths in the range of 7.5–13 microns. This sensor’s detectivity was measured at 0.08, indicating its capability to capture minute temperature variations. By applying IRT, the researchers could visualize and analyze the necking phenomenon during fiber drawing, shedding light on material behavior under mechanical stress. In 2011, Lahiri et al. conducted a study involving the inspection of friction stir welds in aluminum and tungsten inert gas-welded stainless-steel joints. They employed a FLIR SC 5200 infrared camera equipped with an InSb (Indium Antimonide) sensor sensitive to wavelengths in the range of 2.5–5.1 microns. This sensor possessed an impressively low detectivity of less than 0.025. Through IRT, the researchers were able to non-destructively inspect the welded joints, identifying any potential defects or irregularities that might impact the joints’ structural integrity. In 2011, Fokaides and Kalogirou focused on the determination of the overall heat transfer coefficient (u-value) in building envelopes. For this purpose, they utilized a FLIR T360 infrared camera equipped with an uncooled microbolometer sensor sensitive to wavelengths in the range of 7.5–13 microns. The sensor’s detectivity was measured at 0.06, indicating its sensitivity to thermal variations. Through IRT, the researchers quantified heat transfer characteristics in building envelopes, aiding in energy efficiency assessments and thermal insulation optimization. In 2012, Naderi et al. conducted a study focusing on monitoring dissipated thermal energy and damage evolution in glass/epoxy materials. They employed a MIKRON M7500 infrared camera equipped with an uncooled microbolometer sensor sensitive to wavelengths in the range of 7.5–13 microns. This sensor’s detectivity was measured at 0.06, highlighting its suitability for thermal analysis. Through IRT, the researchers could track the dissipation of thermal energy in the glass/epoxy materials and identify potential damage mechanisms. Figure 2 depicts an innovative technique developed by Youngjib Ham and Mani Golparvar-Fard, which combines digital and thermal images in their study focused on the rapid 3D modeling and visualization of energy performance in existing buildings. This approach is designed to provide a deeper understanding of the role of IRT in building analysis. Digital images are used to capture accurate geometric information about the buildings, while thermal images record the immediate energy performance data [88]. Similarly, Figure 3 depicts an autonomous drone-based IRT system developed by Chris Henry et al. for the automatic detection and localization of defective PV modules in a PV power station. The drone employed in this system is specially configured with a dual camera setup. It includes a thermal camera, capable of capturing temperature-related data, and an RGB camera, which captures standard color images [170]. This system can be effectively employed for comprehensive inspections and assessments of various building components, making it a valuable tool for early issue detection and efficient maintenance in building-related contexts. The integration of digital technology and thermal imaging enhances the accuracy of 3D models, which also opens the door to a wide range of applications in various fields. Also, in 2012, Zhang and colleagues focused on monitoring the temperature variation in bridge-wires used in electric explosive devices. For this purpose, they utilized a FLIR A40 infrared camera equipped with an uncooled microbolometer sensor sensitive to wavelengths in the range of 7.5–13 microns. The sensor had a detectivity of 0.0. IRT allowed the researchers to ensure the consistent thermal behavior of the bridge-wires, contributing to the safety and reliability of electric explosive devices [48].

Various articles underscore the critical importance of delamination detection in various materials, such as concrete and building facades, within the context of deteriorating infrastructures and buildings. To address the research gap, a more detailed analysis of the physical and thermal properties influencing the detectability of delamination using IRT is warranted. Notably, factors like delamination size, depth, and material composition, alongside their variation across different infrastructures and buildings, are essential considerations that warrant exploration [171,172]. Understanding how these factors interact with IRT’s capabilities can enhance the accuracy and reliability of delamination detection.

The impact of measurement conditions on IRT’s detectability of delamination is an area of particular significance. While the introduction acknowledges the limitations of passive IRT, a deeper exploration of environmental conditions such as solar irradiation, ambient temperature, and wind, and their interplay with different delamination characteristics, can offer valuable insights. Investigating the complex relationship between these conditions and their effects on passive IRT’s accuracy in detecting delamination could pave the way for optimizing measurement protocols [154]. The research should delve into the variability in experimental conditions that has led to inconsistent results in previous studies. By elucidating specific experimental parameters that significantly influence outcomes, the study can contribute to standardizing or controlling conditions for IRT-based delamination detection. Identifying key factors contributing to variations and proposing strategies to address them will enhance the reliability and comparability of IRT findings [173,174,175].

4. Results

IRT, an invaluable NDT&E technique, has emerged as an essential asset for building diagnostics and diverse industrial applications. Its ability to capture thermal patterns and temperature variations across surfaces provides invaluable insights into the condition of structures and systems [18,176]. IRT operates on fundamental principles like emissivity and spectral range, allowing it to visualize temperature differences indicative of issues such as compromised insulation, hidden defects, moisture intrusion, and energy inefficiencies. It comes in two primary approaches, passive and active thermography, each suited to specific inspection needs, from identifying surface-level anomalies to probing deeper into subsurface defects. In building diagnostics, IRT excels in its capacity to detect thermal anomalies and facilitate early intervention, ultimately enhancing energy efficiency and ensuring the structural integrity of buildings [177,178]. Ongoing technological advancements, including higher-resolution cameras and multi-spectral imaging, continue to broaden its diagnostic capabilities, making IRT a cornerstone in shaping more sustainable and resilient structures [57,121,145]. Furthermore, its non-invasive and non-contact nature lends itself well to condition monitoring across various industries.

The comprehensive analysis of IRT presented in the text underscores its pivotal role as a thorough and versatile diagnostic tool for buildings. Professionals can maximize its utility by aligning the specifications of infrared cameras with the unique requirements of building assessments, ensuring the precise identification of thermal anomalies, structural irregularities, and energy inefficiencies. IRT’s fundamental principles, grounded in thermal radiation physics and the conversion of emitted infrared waves into visual data, empower building inspectors and maintenance professionals with crucial insights into the condition of buildings [73,114,116]. The integration of IRT with complementary techniques, such as ultrasonic testing, enhances the precision of NDT&E examinations, allowing for a more comprehensive understanding of structural health. The article also emphasizes the significance of adhering to industry standards to ensure consistent and professional IRT inspections [179]. Real-world case studies further underscore the practical utility of IRT in building diagnostics, demonstrating its ability to identify hidden issues, prevent damage, and improve energy efficiency [168,180]. These examples highlight the value of early detection and proactive intervention, made possible through IRT, in ensuring the longevity and sustainability of buildings. Additionally, the article recognizes the need for a more detailed exploration of factors influencing delamination detection, including delamination size, depth, material composition, and measurement conditions. By addressing these factors, future research aims to enhance the accuracy and reliability of IRT-based delamination detection, contributing to advancements in building diagnostics and infrastructure assessments [18,154]. Overall, the article reinforces the pivotal role of IRT in shaping the future of building diagnostics and condition monitoring, offering insights that inform maintenance practices, optimize energy efficiency, and ensure the structural longevity of buildings [150]. Figure 4 depicts a highly developed multi-sensor image processing system intended to produce accurate 3D representations of structures. To gather thorough information on the structure, this system smoothly combines three different types of sensors.

In order to completely evaluate the state and thermal properties of a building, a variety of cutting-edge technologies are used in the inspection process. A thermal camera, a terrestrial laser scanner (TLS), and a visible light camera are some of these equipment types. The thermal camera is crucial in recording photos of the surface temperature of the building, which enables the identification of thermal variations inside the building. The TLS also painstakingly generates a perfect 3D point cloud that provides a precise representation of the building’s shape and spatial dimensions. The visible light camera, on the other hand, records typical visible light photos that show how the structure appears. There are numerous crucial steps in the image processing process. The three sensors first combine to obtain thorough information about the structure [181,182,183]. After that, multi-sensor image preprocessing is applied to the captured pictures to remove any distortions or noise that could have developed during data collection. The following phase is multi-sensor image matching, which involves lining up the pictures from the three sensors to make it easier for the computer to identify similar regions in each image. Finally, multi-sensor image registration is used to combine the registered pictures into a single, meaningful representation by utilizing the data from all three sensors. This innovative method produces several hybrid products, most notably the “thermal orthophoto” and the “thermal 3D model”. The visible light picture and temperature information from the thermal camera are seamlessly combined to create a thermal orthophoto. The combination makes it easier to identify sections of the building with different temperatures, which is helpful for activities like finding temperature anomalies. The thermal 3D model, on the other hand, uses information from the thermal camera to overlay temperature data onto a 3D point atmosphere, therefore mapping the building’s thermal characteristics. With the help of this depiction, it is simpler to identify any probable hot or cold regions inside the building and to comprehend the thermal features of the structure. Essentially, this cutting-edge multi-sensor technology transforms building inspection by offering a thorough and in-depth evaluation of both its physical structure and thermal behavior [43,181,184].

The multi-sensor image processing system is useful for a variety of activities, including 3D modeling and mapping, building inspections, maintenance, energy audits, security, and surveillance operations, and planning for disaster response. It is a useful tool in a variety of sectors due to its adaptability and capacity to deliver complete data. Figure 5 depicts a grayscale thermal illustration of a wall with warmer portions appearing white and cooler areas looking black. The surface temperatures of the wall at two different locations are given in the figure’s accompanying text: 15.6 °C at the top and 12.2 °C at the bottom.

Figure 6 depicts thermal bridging detection using an IRT survey. Geometrical thermal bridging develops at the intersections of several building components when a window meets a wall. Due to the potential differences in thermal conductivities between the various materials, heat may transfer from one to the other, from one that is warmer to one that is cooler. The difference in temperature between the wall’s top and lower parts clearly suggests that heat is leaking through the structure. Several factors, including the wall’s construction type, thickness, and insulating capacities, are likely responsible for this heat loss.

Building owners may improve the energy efficiency of their structures by making educated decisions using this information. For instance, they could decide to update to a more energy-efficient heating and cooling system or insulate the wall. Overall, the photo demonstrates how well IRT surveys work at detecting where heat is lost from a structure. These studies can considerably improve a building’s energy efficiency and save energy costs.

Linear thermal bridging happens along continuous building envelope components like metal beams or columns. Regardless of regions of insulation, these components can transfer heat from the inside of the structure to the outside [181]. Both geometrical and linear thermal bridging may be found using the IRT survey. The owner of the building can take action to increase the building’s energy efficiency by determining the regions of heat loss. Building corners, where the walls and roof converge, are potential locations of geometric thermal bridging, whereas the roof’s metal beams likely experience linear thermal bridging [181]. Figure 7 depicts an application of IRT for the detection of air leakage within the building envelope. This study was conducted by Milad Mahmoodzadeh et al. in a laboratory room situated within a building constructed in 1948, located in Victoria, British Columbia, Canada. The result demonstrates both the qualitative and quantitative impacts of varying differential pressure (∆P) on surface temperature, and the detection of air leakage at the interface of aluminum and wood frames. Qualitatively, the surface temperature near the interface appears colder than in other areas, and higher ∆P values result in even lower temperatures. Air leakage areas are detectable at lower ∆P, but may be obscured by thermal bridging in building assemblies. Higher ∆P values are more effective in distinguishing air leakage from thermal bridges or other thermal anomalies. Notably, this study found that thermograms at 25 Pa were necessary for reasonable air leakage detection with thermography, contrary to some published standards suggesting 5–10 Pa. Quantitatively, IRT analysis reveals that the majority of the surface temperature decrease (∼60% of ∆T) occurs between 0 and 25 Pa, with a diminishing impact on surface temperature at higher ∆P levels, as the surface approaches outdoor air temperature [185].

Figure 8 depicts a result of a thermographic assessment that mapped the thermal features of a city using drone-assisted IRT conducted by K. Fabbria, and V. Costanzo. Drone-assisted IRT, a relatively new technical invention, has applications in a variety of fields. Notably, it assists in building inspections by detecting heat loss, enabling increased energy efficiency and cost savings. This equipment also helps to detect leaks and roof degradation during roof inspections. In the field of solar panel evaluations, drone-assisted IRT is used to identify hotspots and potential problems. This technology is used in agriculture for crop health monitoring, pest and disease detection, and irrigation assessment. Furthermore, in the field of public safety, drone-assisted IRT assists in the search for missing people in distant places, identifies victims of fires and natural catastrophes, and detects dangerous items. The thermal map produced by drone-assisted IRT indicates a temperature spectrum spanning from 21.8 °C to 37.1 °C. Buildings and roads often have the greatest temperatures owing to solar heat absorption, whereas parks and green spaces have the lowest temperatures due to heat reflection. This map uses a color-coded scheme, with blue representing the coldest temperatures (typically below 21.8 °C), green representing cooler temperatures (typically between 21.8 °C and 27.0 °C), yellow representing warm temperatures (between 27.0 °C and 32.0 °C), orange representing hot temperatures (typically between 32.0 °C and 34.3 °C), and red representing the hottest temperatures (typically above 34.3 °C). Drone-assisted IRT develops as an effective and adaptable technique capable of gathering valuable information on surface thermal properties [186].

Figure 9 depicts an application of IRT for detecting delamination on a building’s wall during a heating cycle. This research was conducted in 2022 by Ko Tomita and Michael Yit Lin Chew, within the premises of the National University of Singapore, Singapore. In Figure 1a, a visible image of the building wall is presented, while Figure 1b displays the corresponding thermal image. The thermal image highlights temperature variations, with arrows indicating areas of delamination. Delamination serves as insulation and disrupts heat flow, causing the surface temperature above these areas to be higher than that of the surrounding region. Consequently, delamination areas manifest as positive thermal contrast or hot spots in the thermal images [154].

5. Discussion

IRT is extensively and methodically examined in this paper, shedding light on its essential role in the field of NDT&E. As it explores IRT’s varied applications, with a particular focus on its key position in developing diagnosis and across numerous industrial sectors, IRT emerges as a reliable diagnostic tool. This article clarifies the underlying concepts of IRT by systematically describing its two thermography modes, passive and active, in a straightforward and convincing manner. Its exceptional capacity for spotting thermal abnormalities, including inadequate insulation, concealed structural flaws, moisture intrusion, and energy inefficiency, is highlighted. These irregularities can take many different forms, including thermal patterns and temperature changes across surfaces. Additionally, the article notes how IRT is changing because of ongoing technical improvements, with multi-spectral imaging and higher-resolution cameras constantly improving its diagnostic capabilities. Underscoring the significance of matching the infrared camera specs to the precise evaluation requirements will enable the accurate detection of thermal anomalies, structural irregularities, and energy inefficiencies, which is crucial for professionals in the area. The paper emphasizes how the integration of complementary techniques may enhance the precision and depth of NDT&E, resulting in a more comprehensive understanding of structural health. The material is strengthened with real-world case studies, which provide concrete evidence of IRT’s practical use and efficacy in identifying latent problems, preventing possible structural damage, and improving energy efficiency in real-world building settings. The study carefully outlines the numerous advantages of IRT, such as early issue detection and early intervention, but it also carefully identifies possible areas for development. This illustrates the dedication to developing the area of building diagnostics and infrastructure evaluations and for more research into the variables impacting the delamination of the detection, such as delamination size, depth, material composition, and measuring circumstances. To further support its credibility and usefulness in the field, this text continues to stress the significance of adhering to industry standards to ensure the consistency and quality of IRT inspections.

Emissivity, a fundamental parameter crucial for accurate temperature measurements in building inspection, quantifies an object’s ability to emit and absorb infrared radiation, varying with the material being measured. It dictates how effectively a material emits thermal radiation, with common values ranging from 0.90 to 0.98 for materials like concrete, wood, and most building surfaces, while metals tend to have lower values, approximately 0.10 to 0.30 [117,187,188,189]. To ensure precision in temperature readings, awareness of specific emissivity values for examined materials is essential, thereby ensuring accuracy in infrared measurements for building diagnostics and condition monitoring. Reflectivity, the counterpart of emissivity, must also be considered to avoid inaccuracies when dealing with highly reflective materials. Furthermore, environmental factors such as temperature and humidity can influence thermographic inspections, making it crucial to document these conditions. Regular camera calibration and time-of-day awareness are necessary, given their impact on thermal patterns due to solar influence. Adequate temperature differentials between the interior and exterior, along with steady-state conditions, are necessary prerequisites. Minimizing solar radiation and precipitation is imperative, and high-quality, well-calibrated thermal imaging cameras are recommended for inspections. A clear line of sight to building surfaces, adherence to safety precautions, and the involvement of trained experts are prerequisites. Detailed record-keeping is indispensable for subsequent analysis, as is the interpretation of thermal patterns, temperature comparisons against baseline values, and follow-up inspections. Moreover, environmental factors, building characteristics, and the inspection’s specific purpose collectively contribute to determining the optimal conditions for a successful thermographic building inspection.

It is evident that the field of IRT has experienced noteworthy innovations across cameras, procedures, and applications. High-resolution and multispectral IRT cameras have emerged, providing unprecedented clarity and versatility in thermal imaging. Additionally, the integration of uncooled microbolometer technology has resulted in more compact and accessible IRT devices. In terms of procedures, automated analysis and AI-assisted interpretation have reduced analysis time and improved fault detection accuracy, while real-time monitoring and reporting capabilities have enabled proactive maintenance. Furthermore, the integration of IRT with building information modeling has enhanced decision-making in building management, and innovative applications, such as health monitoring during the COVID-19 pandemic, showcase IRT’s adaptability to evolving challenges. These advancements collectively underscore IRT’s pivotal role in modern building diagnostics and condition monitoring.

Thermography in building applications provides valuable insights into the thermal performance and condition of structures. However, it has limitations that must be carefully considered for accurate assessments. It primarily provides surface temperature data, missing internal issues, and it is affected by environmental factors like temperature, humidity, and wind. It is less effective on materials with similar thermal properties and is sensitive to distance variations. Subjectivity in image quality and interpretation, as well as factors like emissivity variations and the presence of reflective surfaces, can lead to potential false positives or negatives. Moreover, certain materials such as glass, water, plastics, and coatings can obstruct or alter infrared radiation, making inspections more complex. The high costs associated with equipment, training, maintenance, and calibration can be a barrier to adoption [16,190]. These challenges highlight the need for a comprehensive approach, combining technical expertise, environmental awareness, and material knowledge alongside other diagnostic techniques in building applications.

6. Future Trends

The future directions for the use of thermography in buildings are promising and multifaceted. Thermography is expected to significantly enhance energy efficiency by identifying thermal leaks and insulation deficiencies, aiding targeted retrofitting efforts, and thereby reducing energy consumption and greenhouse gas emissions. Additionally, thermography will play a pivotal role in predictive maintenance, extending the lifespan of building assets and ensuring their uninterrupted functionality. In the realm of green buildings, it will verify energy-efficient designs and monitor renewable energy installations, while in the context of smart buildings, it will integrate with management systems for real-time temperature data, optimizing systems, ensuring occupant comfort, and reducing costs. Post-pandemic, thermography will contribute to health and safety by monitoring indoor air quality, detecting issues like mold growth, and enhancing fire safety by identifying electrical overheating and fire hazards. Integration with drone technology will expand for efficient building inspections, and machine learning will streamline analysis. In urban planning, thermography will optimize designs for temperature control and climate resilience [16,186]. Efforts to improve accessibility through cost-effective devices and training and certification programs will ensure competent use across various applications, ultimately enhancing the quality, sustainability, and wider availability of this technology in our built environment.

7. Conclusions

IRT certainly has become an essential tool in the field of building diagnostics. It is considered advantageous in identifying a variety of problems within buildings, including improper insulation, moisture penetration, structural damage, and electrical faults. This is due to its unique capacity to perceive non-invasive surface temperature fluctuations. This ability is crucial for avoiding catastrophic failures and enabling quick energy-saving measures. IRT’s capabilities have also been enhanced by the use of modern image processing technologies and artificial intelligence methods, which enable automatic decision-making through real-time pseudo-color-coded visuals that clearly show an object’s state and highlight defects. In addition to civil constructions, electrical installations, equipment, material deformation under various loads, corrosion damages, welding processes, and applications in sectors including nuclear, aerospace, food, paper, wood, and plastics, IRT’s adaptability embraces a variety of subjects. IRT has the potential to become even more powerful as technology progresses, especially with the development of more sensitive cameras and the integration of artificial intelligence. This development offers less downtime, reduced maintenance costs, reduced accident risks, increased production, and growth across industries. IRT is the most frequently used method in the field of NDT&E. It effectively addresses issues associated with preventing failures and enhancing structural and component reliability by providing accelerated inspection rates, increased resolution and sensitivity, and the capacity to detect defects over the span of structure, component fabrication, and the operational lifetime.

Furthermore, future work will focus on conducting energy audits in buildings using a combination of comprehensive reviews and experimental investigations. The synergy of innovative technology, such as IRT, and advanced energy auditing techniques will not only contribute to the sustainability and energy efficiency of buildings but also aid in the overall enhancement of building diagnostics and maintenance.