By using industrialized building, pre-installed modules can be delivered to the construction site where they are easily mounted. This method has many advantages compared to on-site manufacturing. In general, the variability of sound insulation is large in lightweight constructions. The reasons behind this variability are not known and therefore the construction is over-dimensioned to make sure the requirements are met. If the causes of this variability can be found and controlled, the production cost will be reduced and the sound quality improved. This is possible, since much of the construction takes place under controlled circumstances. Field measurements of 31 apartments in a four-storey building have been made. Both airborne and impact sound insulation were measured. The apartments on the highest floor achieved significantly better sound insulation. This is an indication that preload is a factor which contributes to high flanking transmission. The structural coupling between modules will therefore be studied in depth. This paper will present the results from the measurements and propose further research.