Vibration methods are employed in the detection of defects in thermal insulation bricks. The technique presented in this paper concentrates on identifying the different vibration eigenmodes of the bricks. Finite element analysis and experimental results are in good agreement. They show that a horizontal crack in the bricks can shift the frequencies and the order of the eigenmodes. A likely candidate of an indicator of this defect is the sequence position of the "+ + +" extension eigenmode mode. A possible approach to the development of a detection system is proposed based on these results.