In Finland the lightweight partitioning wall between dwellings or rooms is in some cases built within the span of a continuous, precast hollow core slab floor. This type of construction is most common in hotels, care homes, and apartment buildings. However, this solution has been found to result in an insufficient airborne sound insulation between adjacent rooms. It has been suggested that the low sound insulation is caused by flanking transmission via the continuous precast concrete floor. A concrete core fill of the hollow core slabs in the junction of the partitioning wall has been suggested as a solution to improve the apparent sound insulation. The finite element method was used to study the vibration reduction index of the junction between the lightweight wall and the precast concrete floor, as well as the total loss factors of the precast floors. The analysis was carried out for floors of different thicknesses, with and without the core fill. The results of the FE-analyses were used to calculate the apparent sound insulation between rooms. It was found that while the core fill does have an effect on the vibration reduction index, its effect on the apparent sound insulation was negligible.