Low insulation standards and obsolete heating systems of a large amount of buildings in Europe account for disproportional energy consumption. Within this project, the holistic renovation and the results from the monitoring activity of buildings from a field test, located in Southern Germany, are presented. The buildings, built at the end of the 1950s, have been retrofitted with seven different refurbishment layouts. The layouts differ in insulation and engineering system. An installed monitoring system collects thermal indoor environmental conditions and air quality conditions in rooms, as well as data about energy flows at delivery, distribution, storage and generation level, at high time resolution. The monitoring system allows a comparison between the real and the expected energy consumption of the buildings. The energy performance gap was identified and quantified for each refurbishment solution (with values up to 287% based on calculated savings): on average, the energy performance gap of the entire field test varied from 117% in 2011, 107% in 2012, 41% in 2013 and 60% in 2014. The occupants’ behavior has been identified as one of the causes for the energy performance gap. Further causes are mistakes in the installation, and malfunctioning of the engineering system. The importance of a monitoring system for buildings with a complex engineering system was confirmed.