A methodology, based on the solution of the inverse heat transfer problem, for the detection and evaluation of flaws in buildings is discussed. The temperature varying in space and time is recorded by a thermographic equipment and each point belonging to the inspected area is analysed quantitatively. Data are processed to give a map of defects of the wall, based on the most suitable local thermal parameter. The thermal-physical aspects of different defects are studied, along with the description of simplified models to interpret surface temperature data. The building envelope is examined mainly in transient thermal regime. Testing procedures using periodic or pulse heating of the surface are described. The theoretical analysis is used to predict temperature evolution and properly design the test. A detectability comparison among different procedures is performed. Experimental results are reported for insulation deficiencies and thermal bridges evaluation, air leakage detection and moisture content mapping.