Moisture detection through thermographic measurements of transpiration

Damage due to moisture and particularly to evaporation is one of the major causes of decay of wall surfaces in ancient buildings. The evaporative rate of water in building materials can be related to the alteration (chips, gallets) caused by salts crystallization when the water evaporates through the surface of the wall. Current and future usage of NDT heavily depends on the possibility to precisely measure physical variables which present large sensitivity to small variations of water content. A NDT thermography allows us to exactly determine the evaporation rate because of both the high value of water latent heat and the high sensibility of thermographic devices. The research has been carried out both in the laboratory and on the field measuring relative humidity and temperature in a frescoed wall of the castle of Malpaga (Northern Italy). In laboratory a climatic room has been set up using a thermovision system and a temperature & RH% probes, to analyze the evaporative phenomena. A mathematical model, although approximate, is proposed to describe the energy balance of the surface where evaporation is present. The model has been applied to the fresco to correlate the temperature to the evaporation rate. This method allows us to correlate the decay, due to the capillary raise of water in the masonry, to the transpiration phenomena.