In a parallel study, it has been shown by comparison of successive TV-holography campaigns that murals in the convent of Müstair (Switzerland) have progressively detached from their substrate over a time interval of 5 years (J Cult Herit 2009). Here, we focus on the dynamics of the ongoing detachment processes. In order to regularly measure small surface displacements in situ over a long duration of several years, a new method that we call Mirror Micrometry (MM) has been designed and implemented. This method monitors the reflection of a light beam by a mirror that is mounted to the surface of interest for the duration of the experiment. The mirror is designed to rotate about a pivot as small displacements in the detaching surface occur, thus deflecting the reflected light beam. Measurements over more than three years in the Convent Church and in the Holy Cross Chapel reveal various types of surface displacement perpendicular to the wall surface. These are correlated with relative humidity (RH) changes in the room climate. Reversible short-term variations of approximately ±5–10μm occur with periods of one to several weeks and relate primarily to weather changes. Reversible long-term variations of ±5–10μm correspond to averaged seasonal humidity changes. Irregular and irreversible movements in increments of 20–30μm record very localised progressive detachment steps. A semi-quantitative correlation of approximately 1μm surface displacement per 1% RH change is calculated. Based on the fact that nearby measuring points can simultaneously move in opposite directions, a geometric model is drawn to explain deformation by hygric swelling and shrinking of different shapes of detached layers.