Triangulation laser displacement sensors were applied to the continuous in-situ monitoring of the dimensional response of the wooden altarpiece in the Church of Santa Maria Maddalena in Rocca Pietore, Italy, to variations in indoor temperature and relative humidity (RH) between December 2002 and March 2005. Further, a small inductive sensor was used to monitor the width of a crack in one of the elements. The measurements demonstrated that only the external layer of wood, several millimetres thick, continually absorbs and releases water vapour following external variations in RH. For massive elements, this leads to gradients in the moisture content through the wood, a restraint of the dimensional change and a development of stress, which is the main threat to the integrity of the wood and the decorative layer. Particularly strong RH variations and related high stress levels were produced by the intermittent heating system based on the inflow of warm air. To incorporate requirements for preservation, heating systems must provide a localized comfortable temperature in the area occupied by people without changing the natural climate of the church as a whole.