The thermal performance gap in buildings is defined as the difference between the theoretical and the actual energy consumption for heating, and is known to undermine energy retrofit strategies and policies. This study examines the performance gap in retrofitted buildings using the Swiss Cantonal Energy Certificate for Buildings (CECB) database, using a sample of 1172 buildings for which both theoretical and actual metered consumption were known. We found an average negative performance gap of -23% for pre-retrofit buildings (actual consumption smaller than calculated) and instead a good approximation of actual consumption with theoretical consumption after retrofitting (a positive gap of 2%). A regression analysis on the energy performance certificate input parameters characterizing the building led to the conclusion that these are poor predictors of actual consumption compared to the theoretical calculation: parameters such as the energy label and the thermal proprieties of the envelope (U-values) have minor explanatory power for the actual consumption despite explaining a high degree of change in the theoretical consumption. Analysis of the indicator Energy Savings Deficit (ESD) shows an overestimation (of 37%) of the achievable savings on the basis of the theoretical consumption, whereas the prediction of savings using measured consumption before retrofit resulted in a good agreement with the actual savings (3.6% overestimation). This implies that energy savings can be estimated rather accurately by comparing the actual current consumption with the expected theoretical consumption defined by the certificate after retrofit.