It is estimated that 80% of the 2050 building stock already exists and given targets of an 80% reduction of greenhouse gas emissions by 2050, there is a clear need to develop and evaluate retrofitting strategies that reduce energy consumption whilst achieving resilient and healthy indoor environments. This paper presents the results of a building performance evaluation of a retrofitted, Passivhaus certified dwelling in the Orkney Islands (Scotland) during the heating season. The study involved testing of the Mechanical Ventilation with Heat Recovery (MVHR) system, sound assessments, U-Value measurements, energy monitoring, a thermographic survey and indoor environmental monitoring over a three week period. The dwelling had sought to address some ventilation issues identified in other projects by the inclusion of carbon dioxide sensors as part of the MVHR control strategy. The thermal performance of the building envelope and sound level measurements of the MVHR system satisfied the Passivhaus minimum requirements, with acceptable indoor environmental quality observed throughout the monitoring period. However, the results highlighted concerns regarding energy and noise of the MVHR system in boost mode and identified some thermal weaknesses at window seals, and maintenance of the MVHR system in a social housing context. The findings of this study can be used to highlight potential problems and good practice, with the aim of reducing the gap between design intentions and measured performance in future retrofit projects.