The quality of the indoor environment of buildings is one of the key properties of buildings and has a significant impact on the health of people and all living organisms. Currently, in EU countries, the issue of low quality of the indoor microclimate of buildings is associated primarily with insufficient ventilation and high levels of CO2. Low or too high relative humidity also has a negative effect. Stabilization of the indoor environment can generally be carried out by targeted forced air exchange and treatment using air conditioning and humidification units, but this process is relatively energy-intensive, and the environment created in buildings does not always have a completely positive effect on the human body. One option is to use building materials/elements in the interior that have the ability to bind and release moisture into their structure, thereby actively influencing the indoor environment of buildings. The ideal relative humidity of the environment from a health perspective ranges from 40 to 60%. The aim of this paper is to present the results of research focused on studying the hygroaccumulation effect of natural-based materials and their influence on the humidity stability of the indoor environment of buildings. To stabilize the indoor environment of buildings, environmentally friendly, sorption-active materials on a natural basis were selected - clay material with natural fibers. The article presents the results of a study that focused primarily on the sorption/desorption activity and moisture behavior of the developed materials, in order to optimize the indoor microclimate.