The article describes a case study of a collegiate basilica located in northern Poland and listed in the register of monuments of the Kuyavian-Pomeranian Voivodeship under the identification number 601148. This building dates back to the 14th century and has been rebuilt many times in its history, serving both a sacred and penitentiary function. Currently, the facility is used only for religious purposes. Its owner and user set himself the goal of maintaining the temperature inside at a level of approximately +16C all year round, which required insulation of the external partitions, but this was only possible from the inside due to the historic nature of the façade, which is protected by law. As part of the article, a thermal-humidity analysis of vertical envelopes was carried out regarding the possibility of moisture accumulating in them after thermal modernization and its impact on their technical condition over time. The above-mentioned analysis was carried out with the support of the WUFI 2D program to assess the possibility and amount of moisture accumulation inside the envelopes and on their internal surface, as well as the Physibel Trisco program regarding temperature distribution in the cold season.