Energy performance modelling can provide insights into the efficiency and sustainability of commercial buildings, and also the achievement of certification standards such as USGBC LEED. However, the results from the modelling must be validated via a post-construction evaluation, which quantifies any discrepancies between the predicted energy usage and the actual energy consumed. In this study, an existing office building was examined to test how well the model predicts energy usage. The results from the model were compared with the actual usage of gas and electricity over two years (2010-2011). Our study showed a 123% higher gas usage, and a 36% lower electricity, compared with the simulation. This difference presents that occupant behaviour and building construction practices have significant impact on the energy usage of a building. For instance, the large discrepancy among gas usage is due to the office building's thermal envelope, which identifies the spots at which heat leaks out of the building, thereby forcing the heating unit to work more. Additionally, the post occupancy evaluation study identified that indoor environmental conditions impact on energy consumption of the building.