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Computing a 2D colour map of average U-values pixel-by-pixel could become a challenging task in terms of complexity and time, especially for entire façades under the influence of anomalies. In a quantitative IRT test, a thermal image with a resolution of 320 × 240 pixels involves 76,800 elements with different TWALL for each instant “t”. This research aims to create a thermographic 2D U-value map for the characterization of heavy walls in a stationary regime. The method was divided into three steps: (i) metrology; (ii) assessment of how mesh discretization affects the image quality by MATLAB; (iii) development of a 2D map by SURFER. The results demonstrated that all 2D maps were a great reproduction of the original image, considering as optimum a TWALL mesh comprised of 1600 elements of 8 × 6 pixels. The automated data-processing method only took 20 min and image quality losses were estimated at 6.65%, This work was financially supported by: UID/ECI/04708/2019 - CONSTRUCT - Instituto de I&D em Estruturas e Construç ̃oes funded by national funds through the FCT/MCTES (PIDDAC)., Peer Reviewed, Postprint (author's final draft)

Within the European framework, most of residential buildings do not satisfy the minimum thermal specifications. In fact, the renovation rate across the EU is estimated at 1% per year. To fulfil with the goals stated by European Directives 2010/31/EU and 2012/27/EU, it is necessary to ensure a minimum energy performance gap. From a thorough literature review, it was detected that the thermal behavior of a building is often underestimated or neglected during its construction and operation stages. For this reason, an accurate non-destructive testing (NDT) should be required, improving the shortcomings given by the current modelling tools and diagnostic techniques. The purpose of this thesis was to develop a method for determining in-situ the thermal behavior of façades under steady-state conditions using quantitative internal infrared thermography (IRT). After drawing up a numerical model to estimate the thermal transmittance (U-value) as a key parameter of the built quality, the dissertation continued with a validation process that was executed in two typical Spanish walls from different construction periods. This allowed: (i) refining the proposed method; (ii) exploring the boundaries conditions; (iii) assessing the influence of tabulated values set by international standards for wall emissivity and convective heat transfer coefficients among other aspects. The results revealed lower deviations related to the theoretical U-values (1.24 to 3.97%) for test durations of 2-3 hours. Furthermore, the results demonstrated that the use of tabulated values might entail high deviations (40%) in heavy multi-leaf walls with low U-values. Broadly, construction project documents for existing buildings, especially the oldest ones, are not available. Hence, this method may provide information about the building envelope for future refurbishment. In the case of new buildings, the method might allow the thermal behaviour of building façades to be checked according to the design parame, Dins el Marc Europeu, la majoria dels edificis residencials no satisfan les especificacions tèrmiques mínimes. De fet, la taxa de renovació a Europa és estimada en 1% anual. Per complir amb els objectius de les Directives Europees 2010/31/UE i 2012/27/UE, és necessari assegurar una bretxa energètica mínima. A partir d'un estat de l'art exhaustiu, es va detectar que el comportament tèrmic d'un edifici sovint és subestimat o negligit durant les etapes de construcció i operació. Per aquest motiu, una prova no destructiva i precisa hauria de ser requerida, per tal de millorar les deficiències donades per les actuals eines de modelització i diagnosi d'edificis. El propòsit de la tesi era desenvolupar un mètode per determinar in-situ el comportament tèrmic de les façanes sota condicions estacionàries mitjançant la termografia quantitativa interna (IRT). Després d'elaborar un model numèric per estimar la transmitància tèrmica (U-value) com a paràmetre clau de la qualitat construïda, la dissertació va continuar amb un procés de validació executat en dues parets típiques espanyoles de diferents períodes de construcció. Això va permetre: (i) refinar el mètode proposat; (ii) explorar les condicions de contorn; (iii) avaluar la influència dels valors tabulats establerts per les normatives internacionals per l'emissivitat de la paret i els coeficients de transferència de calor per convecció. Els resultats van revelar baixes desviacions respecte als valors teòrics de transmitància tèrmica (1.24 a 3.97%) per duracions de test entre 2 i 3 hores. A més a més, els resultats van demostrar que l’ús de valors tabulats podria implicar altes desviacions (40%) en parets compostes. En general, els projectes de construcció d'edificis existents antics no estan disponibles. Per tant, aquest mètode podria proporcionar informació sobre la façana per futures rehabilitacions. En el cas d’edificis nous, el mètode podria permetre verificar el comportament tèrmic de les parets d’acord amb els paràmet, Postprint (published version)

Within the European context, most of the current residential building stock does not fulfil minimum thermal requirements and needs to be refurbished urgently. Quantitative internal infrared thermography can provide valuable information about the in-situ thermal transmittance of existing buildings for their future refurbishment. This paper aims to establish how operating conditions and thermophysical properties might affect the accuracy of the measured U-value using this technique. To assess the most influential operating conditions, one experimental room with a heavy single leaf-wall was chosen to develop the research in quasi steady-state conditions, with a wide temperature difference range (3.8<¿T<21°C). A statistical analysis demonstrated that the variance in thermal transmittance could mainly be predicted by changes in the outer air temperature. To analyze the impact of the thermophysical properties, specifically the heat capacity per unit of area, four unoccupied residential buildings with heavy multi-leaf walls were tested (6<¿T<10°C). The results mainly showed that the quantitative internal infrared thermography method is more accurate in heavy multi-leaf walls with high kappa values, reaching maximum deviations of 0.20%., Peer Reviewed, Postprint (author's final draft)