This paper outlines a methodology for the calibration of detailed building energy simulation (BES) models using an analytical optimisation approach. The approach combines evidence-based model development with statistical Monte-Carlo based optimisation techniques. The first stages of the proposed calibration methodology are applied to a 700m2 naturally-ventilated library building using short-term monitored BMS and sensor data. The paper concludes with a discussion of how this methodology differs from existing approaches and the benefits it offers over traditional calibration techniques.