The floating floor technology in buildings is increasingly used to achieve national acoustic and thermal insulation requirements, combining different compositions of materials and components with proper technical features. However, the analytical models nowadays available do not allow to estimate with the due accuracy the actual acoustic performance of floating floors with a thick or heavy resilient layer from involved material properties. The physical model presented in this paper, based on the vibration transmissibility theory, improves the prediction of the effect of a floating screed on the impact sound insulation, as a function of frequency, by taking into account the thickness-resonance wave effects in the resilient layer. The model is based on the exact analytical solution of the one-dimensional wave equation in elastic media. Theoretical results are compared with existing computational models and with experimental data of impact sound insulation.