An investigation of the thermal comfort issues of a building was carried out to identify areas for improving the building's energy efficiency. A qualitative investigation of perceived comfort in a building acted as a foundation to highlight any patterns of indoor comfort that could help identify critical areas requiring a more detailed study: which includes a series of measurements and site surveys combined with computational fluid dynamics (CFD) modelling to help generate a more detailed understanding of the comfort control and associated energy concerns of the building. The investigation shows the office building displaying inadequate control of the internal environment; resulting in poor thermal comfort and energy inefficiency. The studied building was built under building regulations in force in 1990; where Part L energy conservation regulations were in existence. This study suggests that assessments of thermal control systems of all existing commercial buildings should be considered a vital, initial step in helping to reduce the CO2 emissions associated with commercial buildings. It should also be considered as a repeatable process to help maintain and continually improve a building's energy efficiency.