## 1 Introduction  
Infrared thermography (IRT) has been a very successful tool for the diagnosis and monitoring of cultural heritage. In restoration projects and the planning of maintenance, IRT has been increasingly utilized since the early 1970s for its application on historical buildings [1]. It has been deployed to identify structural elements underneath the finishing, moisture issues, and other anomalies in historic buildings [2; 3; 4; 5; 6]. It is also extensively used due to its nondestructive nature, which is based on images, and it can be used on a wide surface in a short time and from a distance. Therography for moisture monitoring of buildings is well-documented in existing literature [7; 8; 9; 10; 11], and it is a very useful technique, especially for preliminary scanning and for monitoring the surface in intervals of time. It does not require sampling and, therefore, can also be applied on precious surfaces. Although it is a promising tool, one of its limitations is that when it is deployed onsite, the results are affected by different variables. Uncontrolled conditions of air temperature, relative humidity, solar irradiation, etc., surrounding the target surface, affect thermal signatures captured by the thermal camera. Especially if the location of the building is in tropical climates, such as India, the climate could prevent reliable results because of the higher humidity levels [12]. The passive approach is defined as the application of the technique without an artificial/external heating (or cooling) of the surface and it is particularly suitable for the detection of humid surfaces at certain ambient condition [13; 14; 15]. A passive approach is particularly convenient in the preliminary phase of the analysis, when the target of the investigation is any thermal anomaly that should be evaluated in a second step, comparing the information acquired by IRT, and the data resulting from the historical, visual analysis, the surveys, and any further exam on the site, and in the laboratory. Although the climatic condition of a place could generally be considered suitable for the optimal application of the IRT, it is mandatory to record the air temperature and relative humidity, solar irradiation, wind speed, etc. [15], at the very moment of the recapture. The environmental data are strategic both for the recapture and for further analysis. In fact, the analysis has the aim of detecting a false alert due to anomalies of the temperature distribution that are not dependent on the object of the investigation, but on the boundary conditions and their changes affecting the measurements. In the case of tropical climate, the regularly high RH, especially during the monsoon conditions, requires monitoring the environmental condition of the testing site. Collecting the historic climate and the local measurements in the days before the test (few days-one week, depending on the time window for testing) is very useful when considering, in advance, the possibility of success of the test, and to mitigate (if possible) the adverse condition. Moreover, in the case of building materials at balance with the environmental conditions, the surface temperatures are very similar. Therefore, any thermal anomaly that does not correspond to any visual anomaly has to be considered an alert signal in the first phase of the analysis. Qualitative observation of the temperature pattern is the first filter for detecting the anomalies, due to the visualization at false colors of the surface temperature map. Nevertheless, the qualitative analysis is only the first step for obtaining the required information from the thermograms. In fact, after the comparison between the visual and the infrared images, further analysis of the anomalies must be accomplished. The interpretation of the thermograms require an interdisciplinary work: history, building technology and science, and evaluation of damages, are necessary for evaluating each thermal anomaly.