© 2019 Retrofitting is one of the most important policy measures for timely decarbonisation of the residential sector due to slow turnover of the housing stock. Ireland is an interesting case study given the high reliance on oil as a fuel, the dispersed pattern of residential housing and the relatively poor energy efficiency performance of the existing housing stock. Decarbonising residential space and water heating has proved challenging in the Irish context. These energy service demands are generally inflexible and resilient to reduction due to a range of considerations including external weather conditions, fuel price and the rebound effect. This paper examines and challenges the suitability of popular retrofit combinations as they apply to nine distinct building archetypes in Ireland's housing stock portfolio. An archetype simulation model is used to evaluate the potential for improved energy efficiency gains within the existing retrofit program. We introduce a new methodology that provides insights into sub-optimal retrofit choices. The five most common retrofit combinations are simulated for each of the nine archetypes. The results show that the alternative retrofit combination differs by archetype and that additional energy efficiency gains of up to 86% can be achieved due to alternative retrofit choices. We believe there is room to improve building energy efficiency standards in Ireland through the implementation of a bespoke building retrofit grant scheme which delivers better informed retrofit choices and more effectively considers the pre-existing condition of a building as part of the initial application process. The implications of this analysis are explored and insights for policy are also provided.