Demand for energy efficient buildings has supported an increase in predictive performance modeling. However, operation of buildings can often be different than predictive models, creating a collective performance discrepancy referred to as the “performance gap”. Post Occupancy Evaluation (POE) can close this gap by evaluating performance, and contrasting operational data to design intention. This POE demonstrates an identifiable performance gap in a practical case study on one high-performance building. Findings suggest the case building is not meeting anticipated energy consumption with a higher than predicted energy use intensity (EUI). Additional findings indicate a leaky building enclosure, significant thermal bridging, unrealistic simulation assumptions, acoustic disturbances, and occupant thermal comfort satisfaction. This POE demonstrates that mixed-method data collection provides more information than singular analyses when attempting to identify a performance gap. It is demonstrated that qualitative data collection techniques explain quantitative findings in analysis, informing understanding of performance gap causation.