The thermal performance of double-pane windows depends on the type and composition of the gas filling. For improvement of thermal insulation today often argon or krypton is used instead of air. However, leakage may occur during the lifetime of the window, which needs to be quantified for quality control. Because the velocity of sound in a binary mixture is a function of the fractions of both gases, the air content in the rare-gas filling can be derived from sound-velocity measurements. We found that the sound velocity, determined interferometrically from a change in distance between the panes and the corresponding change in travelling time of an ultrasonic signal, can be measured to an accuracy of within about ±1%. This technique thus can be used to quantify the thermal performance of rare-gas-filled windows in a non-destructive manner.