The efficacy of fac¸ade insulation in providing an improved indoor noise environment and in reducing indoor noise annoyance was examined in a socio-acoustic before-and-after study with a control group. An average equivalent noise reduction inside the dwellings of 7 dB was obtained from the fac¸ade insulation. Whereas 42% of the respondents were highly annoyed in the before-situation, this dropped to 16% in the after study. The conclusion is therefore that the fac¸ade insulation provided a substantial improvement in the indoor noise environment. The advantage with respect to indoor noise annoyance, of having the bedroom facing the least noise-exposed side of the dwelling corresponded to a 6 dB noise reduction. The changes in annoyance from noise reduction due to the fac¸ade insulation were in accordance with what would be expected from the exposure-response curves obtained in the before-situation. A total of 637 respondents participated in the before-study. Of these, 415 also participated in the after study. Indoor and outdoor noise exposure calculations for each of the dwellings were undertaken before and after the fac¸ade insulation was implemented.