Massive timber buildings are often equipped with concrete or cement screed floors to increase the mass of the structures and to improve the sound insulation between rooms. However, the use of concrete is more and more often seen as an unfavourable option due to the increasing need to decrease the carbon dioxide emissions of the construction sector. A recent field study performed in a mock-up CLT-framed building showed that the sound insulation can drastically be improved without concrete by mounting the upper layers of the floor and the suspended ceiling elastically to the load-bearing timber slabs. In comparison with the conventional screed solution, the vertical airborne and impact sound insulation was improved with the elastically mounted floor up to 13 and 20 dB in terms of the single-number quantities DnT,w and L’nT,w. This study aims to investigate the sound insulation performance of the conventional and elastically supported floors by conducting computational assessments which were compared with the field measurement results. Furthermore, the assessments were broadened to study the sound insulation between rooms in the horizontal direction. These computations based on FEM and parametric models reveal the superior performance of the elastically mounted floor both in horizontal and vertical directions.