Active and passive thermography to detect Moisture in Building Material

Dampness is one of the main problems of preservation of ancient building. Among non-destructive methods for quantitative evaluation of damages of buildings materials, thermographic applications are based on the measure of thermal properties variations. Those variations highly depend on water content inside these materials. Water presence in the superficial layer of the wall produces a cooling effect due the evaporation process, which affects the surface temperature. The evaporation rate can be properly calculated when all the energy exchanges across the wall surface are known The solution of the energy balance equation gives theoretical temperatures that describe the cooling during evaporation and it allows calculation of evaporation rates. A good agreement with experimental values by weighing test has been found in lab tests. Furthermore the thermographic passive measurements of the surface cooling allow to estimate the water content. The same values of water content can be obtained by an active approach. In the paper the reference method is considered. It describes dependencies of thermal effusivity and diffusivity on water content with respect to dry material. The distribution of moisture in building porous materials is derived from thermal effusivity. In the paper are also reported the experimental results obtained in estimating water content by front-surface temperature. Authors describe the contribution due to the different kinds of heat exchange, and determine reliable values of water content and evaporative flux using both active and passive techniques.