•Long-term field data for 11 existing multifamily buildings are presented.•Analysis of calculated and actual energy savings resulting from renovation of buildings.•The actual payback time was in the range between 3.1 and 104.8 heating seasons. Thermal renovation of existing buildings is one of the most popular actions to decrease the energy consumption for heating and cooling. However, to the best of the authors’ knowledge, there are no long-term field studies that present the influence of hydraulic rebalancing of the heating system after the thermal renovation of the building’s envelope on the level of achieving the calculated energy savings. This work presents the results from a field study that collected data on actual operational energy over several heating seasons, from 11 similar multifamily buildings in Poland. All buildings were thermally renovated by insulating their envelope, while for some of them the works were completed by a hydraulic rebalance of their heating system. The modernization activities were implemented with a different sequence. For another group of buildings, the hydraulic balancing of the heating system was not performed after the envelope thermal renovation. This offered an excellent opportunity to compare the actual energy performance of the renovated buildings and quantify the achieved energy savings resulting from different practices. Energy audits were performed in all buildings in order to calculate the energy use of the buildings before and after the renovations. These estimates were then compared with the actual energy savings from the monitored energy use for all renovated buildings. Accordingly, the actual energy savings range between 8.8% and 74.8% of calculated energy savings, depending on the different renovations. The actual payback time for the analysed modernization actions was longer than the calculated one, ranging between 3.1 and 104.8 heating seasons.