A careful design and construction of energy efficient buildings is essential to reduce both the global energy consumption and carbon dioxide emissions. The desired outcome can be achieved by increasing the construction of low energy buildings, which are designed to sustain desired indoor temperature with a minimal energy input, and by improving the energy performance of existing buildings. Some buildings underperform by gaining or losing more heat than needed. This study introduces a framework for investigating building energy performance. Thermography investigation, building modelling, characterization of thermal bridges and future prediction for overheating are encapsulated in the proposed framework. A sport changing facility, which was designed as a low energy building, serves as a demonstrator for the application of the framework. The energy investigation framework revealed that the facility is underperforming. According to the building model, the main reason for the poor building performance is thermal bridging (presence of steel members), which increases gas consumption and wall heat-loss by 18% and 11%, respectively. Other contributors to heat lose/gain are cracks in the building envelop, weak mortar joints and uninsulated hot water pipes. Furthermore, the future temperature data, which is input to the building model, suggests that the entire facility is under the risk of overheating.