 Indoor residual spraying, IRS, is a key malaria vector control strategy. However, it has been found that human activity can modify the treated walls after the spraying, which reduces its effectiveness. To address this issue, researchers conducted a study in southern Mozambique to measure the impact of such modifications on IRS efficacy. The study included monitoring of modifications to the treated walls, as well as the addition of new rooms and mosquito nets. It was found that modifications were common, with up to 17.2% of households modifying their walls each month. Additionally, 16.2% of households added rooms onto their compound each month. These modifications reduced the actual IRS coverage from 97% to 39%, resulting in an increase in relative mosquito bites of 43%. By integrating KAP surveys into programmatic evaluations, researchers can better understand how modifications affect IRS efficacy and adjust accordingly.