Recent studies show that the relative humidity and the indoor air temperature constitute an important determinant of the quality of indoor air. Hygrothermal microclimate has a significant impact on occupant’s health and their comfort. The study presents the results of experimental measurement of indoor air temperature and relative humidity in selected apartment in prefabricated panel house situated in Ostrava, Czechia. The contribution describes and analysis the relation between indoor air temperature [°C] and relative humidity [%] in this apartment. The experimental object is selected with respect to the housing stock in the Czech Republic. A third of the housing stock in the Czech Republic is composed of prefabricated panel houses. Regeneration and revitalization of these buildings were in the focus of interest during recent years. Building modifications, such as thermal insulation of building envelope or window replacement, lead to a significantly higher level of airtightness of these objects. Humidity and indoor air temperature are measured in 10-minute cycles for two periods. The values of temperature and humidity are measured for the non-heating and the heating season. The length of each experimental period is 30 days. The mean value of indoor air temperature is 22.21 °C and average relative humidity is 45.87% in the non-heating period. The values of 22.62 °C and 35.20% represent average values for the heating period. A slight increase of the average temperature of the indoor environment (+1.85%) is observed. The decrease of the relative humidity is evident at first glance. The relative humidity of the internal environment is approximately 10% lower in the heating period. Long-term decline of relative humidity below 30% brings many problems. It is necessary to take measures to increase of relative humidity in residential prefabricated building. The aquarium appears to be ineffective. The solution may be forced artificial ventilation or humidifiers.