In buildings of cultural heritage, such as churches, it is quite difficult to ensure and to maintain the constant values of the indoor climate. Keeping a comfortable indoor environment throughout the year is a challenge due to special requirements for the preservation of the integrity of the interior decoration (frescoes, icons, stucco molding), strict canons to the exterior appearance, the presence of sharp peaks in the increase of hygrothermal parameters due to a peculiarly different number of visitors during church holidays compared to the rest of the year. Three churches described in this paper are situated in Russia and Latvia and despite different geographical position have similar problems: violation of the thermal insulation layer, problems with the provision of heat and humidity mode of condensation. The paper suggests possible ways of monitoring the state of the indoor climate of churches and possible solutions for its maintenance.