To achieve the 80% statewide greenhouse gas reduction target mandated by California Executive Order S-3-05, the existing building stock must become 40% more energy efficient by 2030. Meeting the target for existing buildings will require deep energy retrofits to the 465-million square meters of existing commercial building stock in the next 15 years. While studies have shown that energy retrofits leading to reductions as high as 28% are possible in a cost-effective manner, and that “deep” energy retrofits can achieve even greater reduction, these recommendations are often poorly applicable to historic buildings for two reasons. First, the recommendations nearly always promote incremental improvements to a sealed building envelope and efficiency upgrades to mechanical HVAC and lighting systems. This approach overlooks the possibility of meeting energy reduction targets simply by re-activating the passive strategies present in many historic structures, which have largely been de-activated over time following past renovations which introducing air conditioning and fluorescent lighting technologies. Second, many retrofit recommendations would alter the character of historic structures to an extent that would be unacceptable to preservationists, the public, and existing historic preservation laws. ❧ Because a large portion of the existing commercial building stock in Downtown Los Angeles was built before 1950 and originally designed to rely on the local climate for most Indoor Environmental Quality (IEQ) needs, an alternate approach to deep energy retrofits is needed which begins by examining the energy reductions achievable through re-activation of these original passive strategies. ❧ To address this need, a simulation-based framework was developed to compare annual energy and IEQ outcomes from re-activation of multiple passive strategies. Detailed building information on LA’s Subway Terminal Building (with observational analysis of ten additional pre-1950s buildings) was used as a basis to develop the initial baseline model and identify original passive design strategies (e.g. exterior solar control, daylighting, and natural ventilation). Energy and daylight simulations using the EnergyPlus and Radiance engines are used to quantify annual performance outcomes from a parametric exploration of retrofit combinations that replicate and improve upon the original passive design intent of the historic building type. Compared to the baseline model, implementation of the best set of passive retrofits was found to yield a reduction in Energy Use Intensity (EUI) of 29%. Due to the typical design conventions of this building type, a parametric model was developed to extrapolate the potential of re-activating passive strategies for additional historic buildings. Analysis of the metric thermal autonomy revealed that thermal comfort conditions can be met in pre-1950s buildings for large periods (up to 67%) of occupied hours utilizing only passive conditioning. Analysis of the metric daylight autonomy revealed that floor plates could… Advisors/Committee Members: Konis, Kyle (Committee Chair), Carlson, Anders (Committee Member), Noble, Douglas (Committee Member), Lesak, John (Committee Member).