Lime plaster is one of the key sustainable building materials which is also effective as a passive cooling strategy. Since it is being used for ages now, it is the most compatible material for old heritage structures. Thus, majorly used in conservation projects. In this work, the hygrothermal performance of lime plaster is observed through simulations and surveys. Residential surveys are carried out for occupied old naturally ventilated spaces. The inner surface temperature, relative humidity, and moisture content of the walls are measured. Simultaneously, the indoor air temperature, relative humidity, globe temperature, and air velocity are measured. In hygroscopic materials like lime plaster, realistic conditions like mold growth cannot be predicted through simulations. A simulation of thermal and hygrometric behavior of historical buildings is a challenge. An inaccurate simulation may lead to inadequate conclusions, which could lead to inappropriate and dangerous actions for the building’s heritage conservation. An attempt is made to use the survey readings and observations to predict the phenomenon of mold growth in a space while conducting simulations. Using a combined approach of simulation and survey, it was possible to predict the mold risk of the space.