The demand for high performance buildings is on the rise. As a result, several new building standards have emerged including the Passive House Standard, a rigorous energy-use standard based on a super-insulated and very tightly sealed building envelope. A common challenge with passive house designs is that they tend to overheat. This study explores the use of phase change materials (PCMs), which store heat as they melt and release heat as the solidify, to reduce the number of overheated hours and improve thermal comfort for a case study passive house duplex located in Portland, Oregon, USA. In this study, a newly constructed passive house duplex was thoroughly instrumented to monitor indoor environmental quality metrics and building energy use. One unit of the duplex was outfitted with 130 kg of PCM while the other unit served as a control. The performance of the PCM was evaluated through analysis of observed data and through additional computer simulation using an EnergyPlus whole-building energy simulation model validated with observed data. The study found that installation of the PCM had a positive effect on thermal comfort, reducing the estimated annual overheated hours from about 400 to 200.