Impact of the external thermal insulation composite system (ETICS) on the sound reduction index improvement is widely discussed topic. ETICSs are already several years commonly used to reduce buildings thermal loses. However, its application can negatively influence facades sound insulation properties. In the high frequency range an additional thermal insulating layer improves the sound insulation property. However, in the low frequency range the sound insulation spectra rather often decrease. This decrease is caused by a mass - spring - mass resonance of the ETICS system. Location and the depth of resonance dip depend on the ETICS composition. Several prediction models and measurement-based case studies were published already. In this contribution, an analytical model is used to predict the sound reduction index, using a measured dynamic stiffness of the ETICS system. A number of different thermal insulating layers are analysed having similar thermal properties but different material dynamic stiffnesses and loss factors. The comparison of theoretically predicted sound insulation spectra and the impact of the dynamic stiffness measurement technique are presented.