Mediaeval churches hold substantial historical and cultural values, organs, altars, paintings that need to be preserved. Many of these churches have no climate control systems installed. Thus the indoor climate is mainly determined by the outdoor climate. Focus in this study is on the possibilities to improve indoor climate in originally unheated churches to prevent and protect churches from mould growth and disintegration of wooden parts. A case study in the Church of the Holy Cross in Harju Risti, Estonia, was conducted to test two different indoor climate control systems: dehumidification and an air-to-air heat pump. The systems were controlled with relative humidity to prevent the indoor humidity from dropping low enough to start wood cracking and salt drying in the walls. Based on field measurements, simulation models were calibrated and used to test different climate control systems and their impact on churches' indoor climate and energy consumption. Our study showed that the adaptive ventilation performs well in high indoor humidity conditions, annual energy consumption of conservation heating with a heat pump is the lowest, and dehumidification is most effective during the cold period. (C) 2015 Elsevier B.V. All rights reserved.