Poor acoustic insulation of building façades is often caused by poor mounting conditions and poor final set-up of openable windows. For the first, when wrong materials are used to obturate the gap between the window and the construction, the subsequent decrease in acoustic insulation can be observed in a wider range of frequencies. A poor final set-up of the window will usually lead to whistle noises inside the rooms, especially next to busy roads. Consequently, the aims of this investigation are to evaluate different methods to detect acoustical failures in openable windows, and to present an efficient approach to optimise the sound insulation of a façade. By means of experimental measurements in the acoustic laboratory on a standard openable window, two different and complimentary approaches are compared to evaluate its acoustical quality and failures. More specifically, the first is based on the standard sound insulation measurement according to EN ISO 140-3, and the second consists of ultrasonic measurements. The experimental results demonstrate that the standard sound insulation measurements enables to evaluate the quantitative impact of different parameters governing acoustical failures, such as the use of the correct gasket, continuity and correct amount of pressure applied on the gasket. In addition, it was observed that the ultrasonic measurements enable a quick qualitative assessment that is very efficient to determine the exact position of the acoustic leaks in the façade. Finally, a combined use of standard and ultrasonic measurements is proposed to deliver an efficient approach to detect acoustical failures of a façade. The ultrasonic approach is a quick and cost-effective way to instantly track deficiencies of a window on site, whereas the standard measurement in the laboratory is more time consuming but needed to guarantee the performance of a building element.