This paper covers the initial work in a project where the focus is to improve impact sound insulation for a lightweight module based building system. The size of each module could typically be a small apartment (one room student apartment) or a part of a larger apartment (like two bedrooms or a living room). Modules are prefabricated in a factory, indoors, and are then assembled together at the building yard to form a complete multi-storey building up to fives floors.This building technique often suffers from poor impact sound insulation, especially at low frequencies. A number of parameters that potentially affect the impact sound have undergone experimental testing performed on a setup of two modules, one upper and one lower. Examples of tested parameters: single vs. double layer of floor plaster boards, grey rag boards vs. underlayer foam under the parquet flooring, varying load (weight) on the module and varying kind of vibration insulation in between the modules.