Sustainable energy efficiency retrofits as residenial buildings move towards nearly zero energy building (NZEB) standards

9 million housing units in Ireland are required to be retrofitted for the existing Irish national housing stock to be considered nearly zero energy building (nZEB). This paper assesses optimum retrofit packages aimed at improving the building material thermal efficiencies and energy demand of gas-heated semi-detached and end-terraced houses in Ireland. The cost-optimal methodology framework for calculating cost optimal levels of minimum energy performance requirements for buildings and building elements is used to determine the optimum retrofit packages from both a householder perspective and a societal perspective. The results of the cost-optimal approach are compared to the results from a weighted framework approach which incorporates life cycle environmental and cost indicators from both a householder perspective and a societal perspective. The results found that the ranking of energy efficiency retrofit designs based on multiple life cycle environmental and cost indicators differ compared to cost optimum designs based on only life cycle energy and cost indicators. The cost-optimal approach was found to be effective for identifying retrofit packages that are among the best packages even without accounting for multiple environmental indicators. However, once multiple indicators were considered, the hierarchy of the optimal retrofit packages changed. In the Irish context, the environmental impact of the Irish electricity grid was found to play a significant role in the hierarchy of the retrofit packages examined. While the cost-optimal method should not be relied on solely for identifying the optimum retrofit package solution, it could be employed as part of the multistage assessment methodology for narrowing down design solution sample sizes. Using the remaining array of retrofit packages solutions, the optimum retrofit package could be identified using multiple indicators.