Glass facade is a popular feature of office buildings for its aesthetic outlook and deep natural light penetration. In a tropical setting, the main setback of glass facade is the excessive admittance of solar heat gain, which adversely impacts the indoor cooling load. In Singapore, air cooling takes up about 52% of energy consumption of office buildings. Retrofitting existing glass facade with a functional inner glass, also called retrofit double-glazing, is the quick and economic solution to cut down solar heat gain and improve building energy efficiency. It is important to note that, although retrofit double-glazing cuts down the thermal transmittance, it also reduces the visible transmittance, which may lead to higher lighting electricity demand in order to maintain the indoor visual comfort. Little work has been done to assess the overall energy impact of the double-glazing. In our work, we attempt to investigate the energy performance of retrofit double-glazing taking into consideration of its impact on the lighting and cooling demand. Firstly, we model the annual lighting electricity demand through daylight autonomy analysis of a typical office room with single and double-glazing settings. The daylight model is validated with field measurement data collected at an office room situated on rooftop with glass facades on three walls. Secondly, we model the annual cooling load of the room through thermal analysis. Last but not least, we derive the overall energy performance of double-glazing based on the daylight and thermal analysis. Different double-glazing configurations, room orientations and geographical locations will be examined. This study provides a guideline for assessing the feasibility of retrofitting double-glazing for office buildings in the tropical region.