The sound insulation performance of a composed façade is usually dominated by the performance of its weakest element(s). These may for instance be windows or ventilation grids. However, when no ventilation grids are present in façades with highly insulating windows and wall elements, the connection between these may start playing a significant role in the total sound transmission. Unfortunately, very little data exists on the performance of these connections, in particular for timber frame façades. Therefore, some laboratory measurements were conducted on a real-scale highly-insulating timber frame base wall. By installing a heavy wooden dummy window in this wall, the influence of several connection details was studied: connection system, gap width, nature of the insulating material, gap shielding, sealing and airtightness, moisture barrier ... It was found that airtightness was a primordial parameter and that mounting method and the kind of sound insulating material in the gap was of minor importance.