The goal of this analysis was to evaluate the energy consumption and cost impacts of installing new glazing units as part of an overall retrofit of the sloped glazing system at the top of the Tower. The process started with an initial site visit, review of as-built drawings, and confirming operational/HVAC assumptions with the building manager. Using this information, a whole-building energy model of the upper floors was created, including descriptions of VAV air distributions systems, central plant equipment, and daylighting controls. Detailed simulation inputs for the existing and various new glazing options were generated using the WINDOW/OPTICS programs, and the impacts of each selection on HVAC and lighting energy consumption evaluated.