Sound insulation in a building façade plays a key role in the control of road-traffic noise. Façade performance can be described using various internationally standardized single-number quantities (SNQs) with different frequency weightings. The purpose of this study was to evaluate how 25 different SNQs explain the subjective ratings of spectrally different road-traffic sounds transmitted through a façade. Forty-three participants took part in a psychoacoustic laboratory experiment. The task was to evaluate five spectrally different road-traffic sound types transmitted through 12 simulated façade constructions. The participants rated both the loudness and the annoyance of 60 sounds. The playback levels were between 12 and 46 dB LAeq, covering the essential range of road-traffic noise usually measured inside residential dwellings. Linear correlations were determined between the SNQ values of the façades and the subjective ratings. Rw + C50-3150 explained the subjective ratings best when the five sound types were equally considered. It was also the most suitable SNQ for the standard road-traffic spectrum of ISO 717-1. The results of this study can be useful in the development of future standards.