The moisture content of brick walls is investigated to establish the causes of deterioration of mosaics in the Basilica of Santa Maria Assunta in Torcello, Venice. Discontinuities in the mosaic due to restoration made in different centuries, the presence of detached zones, and the height reached by the capillary rising damp on the wall of the Last Judgement are studied using thermovision. Humidity due to capillary rise is also identified by weighted samples from cored samples taken from masonry. On the west wall, high values of humidity were found at a 10-12 m height due to water infiltration coming from the rotted roof and gutters. Cored samples taken from the east wall of the central apse show the capillary rise to be more pronounced than in the other walls. This is probably due to the presence of stagnant water in the crypt lying immediately beneath the mosaics of the central apse.