The insulated glass unit (IGU) system is widely used in the Northern Hemisphere to improve the energy performance of buildings and the thermal comfort of their occupants. However, it has been introduced in the Brazilian market without a proper thermal assessment. Proper glass choice is essential to reduce energy consumption, as the most intense heat exchange occurs through the windows. This research aimed to investigate the influence of IGU use on conditioned office buildings’ energy performance for nine different climates in Brazil, including tropical (Aw, Af, and As) and temperate (Cfa and Cfb) climates. The energy consumption using air-filled IGU was compared to its non-insulated version through computer simulation in EnergyPlus. This sample demonstrated that IGU could generate annual savings in cooling consumption in tropical climates (up to 2.8%) when the outside temperature is constantly higher than the thermostat temperature. However, IGU models resulted in annual cooling consumption up to 9.3% higher in temperate climates by hindering the thermal load dissipation through the façade. The observed sample demonstrated that the IGU could inhibit the dissipation of the indoor thermal load through the façade, which increases energy consumption for cooling compared to models with the same glass but non-insulated. Either in the tropical or the temperate climates analyzed, the use of IGU seems not to be the ideal approach to improve the thermal performance and reduce the cooling energy consumption of highly glazed office buildings in Brazil.