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Old medieval churches hold objects of great historical and cultural value: organs, altars, paintings. But they have no systems for indoor climate control or the church may be heated only at services. These conditions are inadequate for the preservation of cultural heritage. The objective of this paper is to assess an adaptive ventilation (AV) solution in a church for reduction of the relative humidity (RH) in an unheated church to prevent mould growth and disintegration of wooden parts. The operation principle of the system is to ensure ventilation in the church when water vapour content in the outdoor air is lower than that indoors, to lower the RH in the church. A case study in Hangvar Church in Gotland, Sweden, was conducted to test the performance of AV to reduce the RH in the church. Field measurements showed that AV has a positive impact on the indoor RH of the church. During the measurement period without climate control, the RH in the church was higher than 70% of 98% of the time; with AV, the indoor RH was higher than 70% only 78% of the time.Building simulation was carried out to test the performance and energy consumption of AV under different conditions. The simulations showed that auxiliary heating and airflow rate both have high impact on the system performance. The higher the heating power, the more effective the system is; thus, lower airflow rates are needed. Infiltration has also high impact on the system performance: the lower the infiltration rate, the better the AV performance is.