This article aims to present the influence of infrared thermography on masonrywalls to detect pathological manifestations. A systematic review was carried outthrough research with automatic search and snow-balling, selection and siftingof articles to restrict them to the desired theme. After that, infraredthermography in the pathological manifestations was studied along with thethermal properties and their behavior, thermal bridges, temperature differenceand air infiltrations. In general, some care must be taken during the executionof experiments and measurements. It has also been shown that infraredthermography is a complex technique and should be used.

Walls can be evaluated in order to verify better quality for thermal performance.There are several forms of measurements according to the functionality or parameterto be observed, such as mechanical resistance, water absorption, capillarity,thermal performance, geometric characteristics. Those are some aspects that can beevaluated to obtain a satisfactory performance of the walls in a building.

The Brazilian performance standard NBR 15575 (ABNT,2013) and the base standard on Thermal Performance, NBR 15220 (ABNT, 2005), complement each other: the firstdefines performance as "behavior in use of a building and its systems"while the second approaches the thermal concepts, properties and theircalculations.

Infrared thermography is a non-invasive and non-destructive survey technique. Itscapture is done through devices that show infrared radiation, through mechanisms ofeasy and fast use, though in a complex way. The use of the technique has become morefrequent given its fast, precise and non-contact nature which makes possible for itto be used in a wide range of cases (Kylili et al.,2014). Infrared thermography uses a camera to measure the emittedinfrared radiation of an object and convert it into a thermal radiation pattern,which is invisible to the human eye, in a visible image (Clark et al., 2003).

Several researchers have applied infrared thermography techniques for various uses(Bagavathiappan et al., 2013) such asemissivity measurement and determination of global heat transfer coefficient, thusdemonstrating a positive potential (Porras-Amores etal., 2013). O'Grady (2017a) bringsimportant information in his research: about 40% of the energy consumed in Europecomes from buildings. The previous study on thermal behavior of walls avoids errorsin the construction phase. Once built, its on-site verification enables to findpossible pathologies and/or design deficiencies that lead to a reduction in itsthermal performance.

In Argentina, about one third of the energy produced is for the management ofbuildings, half of which is directed to heating and cooling. In addition, more than30% is lost due to insufficient thermal insulation or even roofs and walls that arelikely to overheating in summer and presenting heat leaks in winter (Marino et al., 2016).

According to the Green Building Council Brazil(2015), based on the national energy balance of 2015, about 50% of allthe demanded electricity was for buildings. The consumption of electricity inBrazil, excluding losses, reaches 516.6 TWh: 258 TWh of the total, or the equivalentof BRL 60 billion, are consumed only by buildings. According to the EIA (2018), in the year of 2017 in the UnitedStates, about 39% of total energy produced was consumed by households and commercialsectors. In the European Union countries, the tertiary and residential sectorsconsume about 41% of all energy produced, 55% of which is heat. Similarly, inSerbia, where about 50% of the total energy consumed goes to the buildings, 60% ofit is heat (Tanic et al., 2015). In view ofthis information, it is important to study the thermal behavior of walls.

Rural buildings in China consume a lot of energy and have poor thermal performancedue to the type and situation of building materials (Diao et al., 2018). Thus, the detection and quantification of heatlosses through buildings become relevant given their extreme importance forsociety.

There is still a lack of studies on the subject, making it difficult to research andobtain a better understanding of the scope regarding infrared thermography. As asubject that has more than 25 years of relevant studies, researchers are investingin this topic intentionally to explore the full extent of the usage of infraredthermography. In the light of foregoing, this work aims to perform a systematicreview of the existing research on the usage of infrared thermography in order tostudy the parameters, their properties, and influence on the thermal performance ofwalls.

Infrared thermography was first used for purposes other than civil construction.Its principles were discovered by accident while scientist William Herchel wastrying to solve an astronomical problem, in the 1800’s (Barr, 1961). Over the years, the technique was improved foruse in several sectors (Lucchi, 2018). In1830, Melloni, an Italian investigator, discovered that NaCl, in naturalcrystals large enough to be transformed into lenses and prisms, became the maininfrared until the 1930's, the era of synthetic crystal (Flir, 2017). The first quantum detector was developedbetween 1870 and 1920 based on the interactions between the radiations,increasing the precision and considerably reducing the response time (Smith et al., 1958). The thermography wasgreatly improved during World War II, showing the importance of the technologyespecially at night. The propagation of the infrared images in the constructionsector occurred in the 2000’s, with the use of barium-strontium titanate andmicrobolometer (Lucchi, 2018). In thelast years, its use has increased dramatically, mainly in restoration, buildingconstruction, and survey works (Kylili et al.,2014; Bianchi et al., 2014).In addition, it is important to note that the use of this technique has beenassociated with a reduction in size equipment, cost reduction and resolutionimprovements, sensitivity and accuracy, operability and portability (Meola, 2012). The use has grownconsiderably over the last 15 years, mainly for civil engineering andrestoration of historic buildings, thus facilitating a diffusion of Europeanlegislation not only for energy efficiency but also for energy auditing ofbuildings (Lucchi, 2018). However, evenafter 30 years since the beginning of its use, it has not yet been extensivelyexploited (Grinzato et al., 2002; Albatici and Tonelli, 2010).

Around the world, the use of infrared thermography has been diffused for someyears. There are standards regarding the subject such as those of ASTM, ISO andthe European Union, which regulate the use of infrared thermography in buildingsand their properties. Their use is widely recommended (ASTM, 2013a, ASTM,2015a, ASTM, 2013b, ASTM, 2015bISO, 2008, ISO, 2015, EN,1999).

In Brazil, there is no standard for the topic, and it is often necessary toresort to international standards or adaptations of uses in other areas.

The Brazilian standard that has some aspect regarding the use of the infraredthermography is the NBR 15575 (ABNT,2013a), which is divided into 6 parts and approaches aspects for a goodbuilding performance, including the thermal conditions. However, it does notrefer to any field tests to verify this performance. Some Brazilian standardsthat refer to infrared thermography include NBR 15572 (ABNT, 2013b), NBR 15763 (ABNT, 2009) and NBR 15866 (ABNT,2010), which cover techniques for its use.

According to Marques and Chavatal (2013)the thermal behavior of a house depends substantially on the interactiveactivity between the external walls, ceiling and floor. Nowadays, around theworld, the walls are constructed with numerous materials in several layers(Robinson et al., 2017). In Brazil,most of the buildings still use traditional materials such as concrete, ceramicblocks and plaster. However, researchers are exploring other materials such asEVA (Silva et al., 2012) and vegetablefibers (Savastano Junior and Pimentel,2000), in different percentages inserted in traditional materials, toaid in their behavior without removing their characteristics.

According to Maldague (2001), infraredthermography is divided into two main techniques: active and passive. Lerma et al. (2018) say that the techniquesdo not meet the substrate in order to avoid damage or future recoveries.

The passive technique is one in which the temperature measurement is done undernormal conditions, in objects that have their own thermal energy or somehowstore energy by a natural source of heat, with temperature difference betweenthe object studied and the environment (Kyliliet al., 2014, Viégas, 2015).In the technique of active infrared thermography, an external source ofartificial energy is required, generating a temperature variation over theobject (Viégas, 2015). The use of passivethermography will depend on the energy available in nature, and can often sufferfrom wind, shade, weather and environmental conditions. As the principle ofactive thermography is the use of artificial heat sources, the use of lamps inthe environment can be considered an alternative.

In the active thermography there are some techniques that are differentiated bythe nature of the applied stimuli: heating lamps or ultrasound (Kylili et al., 2014). They are named asPulsed, Lock-in, Pulsed-Phase (Maldague, 2001 apud Rocha,Póvoas, 2017), Laser Spot Array Thermography (Pei et al., 2016), PrincipalComponent Thermography (Milovanovicet al., 2016; Rajic, 2002),among others.

Since infrared thermography began to be applied in civil construction, it hasbeen used in the monitoring of buildings both quantitatively and qualitatively(Grinzato et al., 2002). Thequalitative analysis is considered a technique of infrared thermography thatprovides instantaneous reports, since the focus is the profile and not thevalues (ITC, 2014 apud Viégas, 2015),comparing the value relative to local access in relation to a point (Bagavathiappan et al., 2013).

In the quantitative thermography it is possible to define the severity of thesituation for the studied object. The first analysis to be done must be thequalitative, since the quantitative one allows the numerical quantification ofthe evaluated parameters. If this order is not followed, the procedure ischaracterized only as a comparative analysis (ITC, 2014 apud Viégas, 2015). The data quantitativeanalysis allows a precise determination of the temperature in a point or aregion (Bagavathiappan et al., 2013).

In order to measure the thermal performance of buildings, there are methods suchas the laser spot thermography (LST) (Pei etal., 2016), heat flux meters (HFM) (Danielsky and Fröling, 2015)infrared thermovision (Albatici and Tonelli,2010), among others.

These techniques are used for the measurement of thermal bridges (O'Grady, 2017a; Bianchi et al., 2014; BRÁSet al., 2014), air infiltration (Lerma et al., 2018), thermal transmittance (Simões et al., 2014; Donatelli et al. 2016), thermal emissivity (Abatici et al., 2013; Ciociae Marinetti, 2012) and other properties.

The difference between the materials and their humidity, the emissivity to beanalyzed, the noise caused by the reflective temperature readings are some ofthe factors that interfere in the analysis of the infrared thermography.

Global warming has brought increase in temperature. In this respect, the constructionsector seeks improvements in energy efficiency through alternatives that avoid thethermal discomfort of buildings (Cani et al.,2012). The European Directive 2010/31/EU (European Parliament and ff The Council, 2010) provides a description ofhow energy efficiency of buildings has a role in achieving near zero consumption.Aversa et al. (2017) state that "forthis to occur, energy analysis or auditing is an effective and rapid tool for newconstructions, projects and in decision-making on the energy renovation of existingbuildings often characterized by inefficiencies that lead to waste of energy".Due to the launching of the Performance Standard in Brazil, the NBR 15575 (ABNT, 2013), thermal comfort is presented indiscussions. Thermal comfort is defined as the mind condition that expresses theuser’s satisfaction with an environment (Ghahramaniet al., 2018).

Lerma et al. (2018) worked on a paper topromote a discussion on the opportunities and constraints of using activeinfrared thermography to detect air leaks. The potential is evaluated in aqualitative approach, comparing the thermograms of passive and active infraredthermography. In addition, there is a quantitative approach, testing methods fornumerically interpreting thermograms. An experiment was carried out in a room ofa 1980 construction, in the Northwest of Portugal. The experiment was performedin 8 days with different climatic conditions and the measurement was done bothon the internal and external side. In the qualitative analysis it was detected,in the active approach, that air infiltrations begin to be visible when thepressure difference is 25Pa. As for the passive approach, the pressuredifference must be greater for leaks. In the quantitative analysis, twodifferent positions of the camera were used to detect air leaks: theperpendicular camera (PP) and the parallel camera (PL) to the hand shutterroller. The first technique detected air leaks through the pressure differenceand the second detected the colder locations as air leakage points. The resultsshowed that in the quantitative analysis the PP scenario allowed for a moredetailed discussion. In the qualitative analysis, the active thermography showedthe results clearly.

Grinzato et al. (1998) employed amethodology that resulted in a discussion on the detection and evaluation ofimperfections in buildings. In order to detect air leaks, quantitative infraredthermography was applied on a solid wall with a crack in plaster, producingimages before and after leaks for verification. It has been found that a thermalstimulus would be useful in detecting defects, be it solar irradiation, airflow,or radiant flux from an artificial source. The main disadvantage of thetransient analysis is the considerable increase in processing time, hardlyachieved without exclusive equipment.

Thermal bridges are defined as "any and all building envelope in which thethermal resistance is significantly altered relative to the current envelopezone" (ISO, 2008 apud Castro, 2010). Changes in thermalresistance can be caused by full or partial penetration of the building envelopeby materials with a different thermal conductivity, by varying their thicknessand/or by a difference between internal and external areas, which occurs atwall/floor/ceiling junctions (Castro,2010).

Asdrubali et al. (2012) performed aquantitative analysis, using infrared thermography, with a comparativeexperiment between an isolated and a non-isolated thermal bridge. The articleproposes a methodology to perform a quantitative analysis of some types ofthermal bridges, through simple thermographic surveys and subsequent analyticalprocessing. The selected thermal bridge was given by the difference of thestructure and the window glass. This wall was placed between 2 rooms, withtemperature difference of 20ºC. Two analyzes were considered. The differencebetween the incidence factor of thermal bridges in relation to the twocomparisons is 1.606 for the isolate, and 2.000 for the non-isolate. Theinfluence factor calculated in situ is equal to 2.111 and the incidence factorof the thermal bridges calculated by the FLUENT program is equal to 1.262.Therefore, there is a reduction in thermal loss of the thermal bridge by about40%. For a better performance, simulations were performed for a global heat lossin the winter. A heat loss of 4684W was found, 13.4% of which due to the thermalbridge. The correction of this thermal bridge would reduce the loss of heat to avalue of 4307W and the incidence to 8.8%.

Bianchi et al. (2014) used a quantitativeanalysis of the infrared thermography in the field measurement with theobjective of evaluating the energy losses through a 10m² building. The externalwalls, ceiling and floor were evaluated. For this, a comparison was made between9 incident factors of calculated and identified thermal bridges. Overall theanalysis shows that thermal bridges increase heat loss through building by 9%.The main results show that the procedure is a reliable tool to quantify theincidence of thermal bridges. O'Grady et al.(2017a and 2017b) applied aquantitative approach and showed the loss of heat by the thermal bridges throughthe temperature difference and the thermal transmittance. Grinzato et al. (1998) performed in their researchexperiments on three different types of walls: concrete, rock wool and concretesandwich panel with a rod crossing the insulation layer. The purpose was toverify the behavior of the thermal bridge in the use of quantitative infraredthermography. See Table 1 for moreinformation.

Jorge (2011) shows that the walls areelements constructed to separate the environments. When the thermal energy isconsidered, it can be observed and quantified through the thermal properties. Asany object, the walls have mechanical, chemical, and thermal properties. Amongthe thermal properties, the thermal transmittance, thermal diffusivity, thermalresistance, thermal capacity, heat transfer coefficient, and conductivity standout.

Aversa et al. (2017) proposes theexperimental study on the thermal behavior of opaque walls. They used activethermography stimulated with the objective of evaluating the effectiveness in adynamic behavior for walls prototypes as well as verifying its success forapplication in situ. The authors compare a brick wall with a prototype wall withhemp fibers. It was clearly noted that hemp fibers contribute with the decreasefactor (ratio between periodic thermal transmittance and thermal transmittance)from 0.87 to 0.92 for the walls with the fibers. In addition, the fibersincreased the estimated time difference. It is concluded that different resultswere found. The next step should be measurement in situ.

Grinzato et al. (2002) used infraredthermography and calculated the thermal diffusivity of a brick sample from anold building in massive masonry, located in the Historical Arsenal of Venice.The authors performed six tests to aid in the mapping of humidity. First, aquantitative analysis was carried out with continuous monitoring and then aqualitative analysis mapped the moisture distribution due to the evaporationwater cooling effect. The highest thermal diffusivity found was 5.2800 x107 m²/s and the lowest was 5.1288 107 m²/s. Theresults showed a successful application to the mapping of moisture for theconnection between the walls and the knowledge of the thermal diffusivity inbricks and plaster.

Robinson et al. (2017) aimed to study asimple and low-cost method to estimate the effective thermal diffusivity instructural walls of buildings. For this, they used infrared thermography as anexperimental and low-cost method to calculate the thermal diffusivity of theconcrete wall under controlled conditions. The greatest difficulty found in thiswork was the control of heat loss through the lateral limits of the section,being calculated in situ, since in controlled environment, the lateral limitswere isolated. This inexpensive experiment combined with a mathematical modelresulted in a concrete diffusivity of 7.2 m2/s ± 0.27m2/s, which is sufficiently precise. For this experiment the laterallimits were isolated, but it was concluded that there is a great loss of heatfor these limits.

Danielsky and Fröling (2015) investigateda quantitative methodology to analyze the thermal performance of buildingenvelope in a non-stationary state condition, including two phases. They didexperiments with wood wall exposed to external conditions to calculate thecoefficient of heat transfer by convection; the value of 2.63 W/(m2K)was found. The external parameters used were wind speed, humidity, and snowfall.In addition, the heat flow through the wall was assumed to obtain stable statecondition only sparsely and for short periods. HFM and infrared thermographywere used for the calculation of both the heat transfer coefficient andconductivity. The results of 4% and 3%, respectively for the conductivity andthe global transfer coefficient, were found compatible with differences betweenthe methods, suggesting that the thermography method is more accurate.

Donatelli et al. (2016) used activethermography for two prototype walls under controlled environmental conditionsand calculated the thermal transmittance in situ, comparing with the thermaltransmittance calculated by a computer program. The results showed that thetemperature measurements on software (FEA) are identical to those of a realwall, and that the procedure allows the measurement of temperature in prototypewalls throughout the year without climatic interference.

O'Grady et al. (2017a) elaborated a studywith an efficient, non-destructive method, based on an outside infraredthermographic survey, to determine the performance of the thermal bridge. Forthis, they compared the values of the thermal properties, mainly of the thermaltransmittance, obtained by the quantitative infrared thermography, with thevalues of a hot box. A computer program was used to adjust the results. Thethermal transmittance of these 2 methods with 3 different wind speeds wascalculated and compared. For the thermal transmittance, the external convectivecoefficient was determined using the Jürges approximation and the Nusseltnumber. The results of this study demonstrated the suitability of bothapproaches for calculating the value of thermal transmittance; however, theJürges approach is less time consuming. Infrared thermography is an effectivetool for the determination of thermal transmittance.

O'Grady et al. (2017b) propose the use ofa non-invasive and easy-to-use method to provide quantitative measurements ofthe actual thermal performance in the thermal bridge. They studied thermalproperties and used quantitative infrared thermography in addition to anexperimental program designed to quantify the thermal bridges and tested in acalibrated and controlled hot box. They used the calculation of the thermaltransmittance and the temperature variation. Three samples were taken, sample 1had the highest value found: 0.441 W/(mK) by hot box and 0.436 W/(mK) bythermography. It can be concluded that after being tested in the laboratory andpresenting excellent results for the external conditions, the observations willbe a challenge for the precision of the measurements by the infraredthermography.

Datcu et al. (2005) used quantitativeinfrared thermography to measure walls in order to improve the measurement ofambient temperature, both internal and external. The authors used an infraredmirror, which allows large measurements of surface temperature by infraredthermography under near-ambient conditions with greater accuracy. To validatethe method, an experimental study was performed on a multilayer wall, whichsimulated an isolation pattern. The methodology addressed in the work allowed toquantify the average radiation around the object using a highly reflective anddiffusive aluminum mirror. Then, two heat sources were used: one with 24W/m2 and the other with 48 W/m2. The results werecompared to the results of the FLUENT program for the internal environment; asfor the external environment, the wall temperature was compared with the windowand the heat sink.

Lai et al. (2015) used quantitativepassive infrared thermography to analyze the external wall of a skyscraper. Fourconcrete walls with different coatings were tested. The methodology was usedwhen there were changes of heat flux and solar intensity. They usedthermographic cameras and a computer program for analysis. Porras-Amores et al. (2013) used wall and surfacemeasurements to locate the air temperature inside the building. The studyfocuses on the design of the system, its characterization, and quantification ofits accuracy in different configurations. They applied a quantitativethermography to develop a precise measurement technique. An experiment was donein the garage and underground. Small variations in temperature were observedlongitudinally.

As previously shown, infrared thermography may be used in combination with othermethods for comparison of values and structures.

The method has a great applicability in the identification of air leak points. Theuse of active or passive thermography will generate different results. The activetechnique shows air leaks clearly. External stimuli aid in detection of air leaks,which may be highlighted as advantageous in the use of infrared thermography. Amongthe uncertainties identified were (1) the difficulties in the longer processing timeof the transient analysis, which requires a unique equipment, and (2) theinterpretation of the graph data and pressure versus temperature histograms. Forfuture research, the comparison between the thermal images of passive infraredthermography and the active one in a quantitative approach would be very useful.

The advantages found in thermal bridges are simple and effective evaluations of theireffect in the thermal energy behavior. Simplicity in the geometry of the buildingcontributes to measured and calculated values. Given the uncertainty of energyconsumption in the configuration with thermal bridges, the singular error due to theanalysis of each thermal bridge must be taken into account. The incidence factor ofthe thermal bridge, analytically defined, depends on the internal temperature ofboth the air and the wall for the infrared thermographic camera to read. Among theapplications on thermal bridges identified through the measurements, it should behighlighted the possibility of making interventions to improve the insulation. Inaddition, it is a useful method to analyze, refine and validate specially designed3D simulation tools for the evaluation of energy performance in buildings, sincethey can evaluate thermal fields of internal and external walls.

Regarding the thermal properties, the thermal transmittance calculation was the mostdiscussed topic, approaching several methods to calculate and compare the results.However, there are significant differences between the calculated thermaltransmittance and the in-situ measurement. Moreover, some studies have emphasizedthat in situ measurement of thermal properties would be best performed in winter. Alaboratory study indicates that the procedure implemented is aimed at measuringprototype walls throughout the year, without concern for climate change. Theadvantages of infrared thermography are the multidisciplinarity and integration ofthe results. Among the uncertainty, which was repeated in some studies, one may citethe way the applied methodology would behave or its result in normal conditions,that is, without laboratory control. The difficulties were quite specific, bothregarding the use of infrared thermography in historical buildings due to severalenvironmental factors, as well as the heat losses not controlled by the laterallimits of the section under test. there were no restrictions on applicability forthis topic.

The approach of infrared thermography for temperature measurement was also used,which succeed as a comparative method. Infrared thermography has the advantage ofdisplaying images with different identifications, to measure the surface temperaturein a large area of ​​an element under construction. Thus, it provides morerepresentative data in relation to point measurements. The difficulties were in thequantitative monitoring through conventional thermography. It presents problems inthe measurement of surface temperature and air conditions inside the building. Inaddition, it can be applied to various surfaces.

In general, most of the work on infrared thermography approached quantitativeanalysis. The active approach is also widely explored. It was noticed that there isa multidisciplinarity between the topics covered, since some authors used infraredthermography to talk about more than one subject, enriching and complementing theirstudies.

Some authors used computer programs, mainly for the measurement of thermalproperties, when there were experimental studies facilitating the comparison betweenexperimental and theoretical values. The researches that used successful experimentswith prototypes and controlled conditions contribute with important considerationsthat the next step would be the measurement in situ. Then, it is expected thatinfrared thermography will be increasingly exploited and bring better performancesand energy savings in buildings.