The acoustic performance of a floating floor is often evaluated according to the Cremer-Vèr model. This model predicts an increase of 40 dB per decade of frequency above the resonance frequency for locally reacting floating floors. This approximation provides a reliable estimation only if the thickness of the underlay of the floating floor is negligible with respect to the wavelength of longitudinal waves present in this underlay. When the longitudinal wavelength is smaller than the thickness, standing waves appear in the underlay and the insulation effectiveness will be reduced. This paper highlights the effect of these standing waves for floating floors with thermal insulation layers by means of experiments and an adapted analytical model. For typical layer thicknesses (5-10 cm), standing waves can occur already from 500 Hz onwards.