The adoption of the German Passivhaus Standard in the UK has grown rapidly in recent years. Stimulated by the shift towards energy efficient design and rising fuel costs, the concept is perceived as a potential means of meeting energy and carbon targets through an established, reliable methodology. However the performance of the Standard in terms of adequate indoor air quality and thermal comfort in a UK climate remains under-researched. This paper describes the use of the Passivhaus Standard in a UK context, and its potential implications on indoor environmental quality. A case study is presented, which included indoor air quality measurements, occupant diary, building survey and occupant interviews in a Passivhaus social housing project in Northern Ireland. The study found issues with indoor air quality, the use and maintenance of Mechanical Ventilation with Heat Recovery (MVHR) systems, lack of occupant knowledge and the perception of overheating in the case study dwellings. The findings provide a much needed insight into the indoor environmental quality in homes designed to the Passivhaus standard; which can be disseminated to aid the development of an effective sustainable building design that is both appropriate to localised climatic conditions and also sensitive to the health of building occupants.