The global rise in air and ocean temperatures and the recent weather extremes demonstrate that climate change is unequivocal. The UK has legislatively committed to an 80% reduction in CO2 levels by 2050 based on the 1990 level. Its residential sector is responsible for 30% of the C02 emissions where by space heating is responsible for 66% of domestic energy consumption. 75% of dwellings already occupied will still be in use by 2050 with new dwellings only adding 1 % per year to existing stock levels. The aim of this research is to develop a matrix by which decisions can be made to help decide on the best use of resources to reduce CO2 emissions for different types and ages of dwellings in Northern Ireland. To derive a constructional baseline of the dwellings in Northern Ireland, the background calculations used to generate all lodged EPCs undertaken to date were obtained and analysed as part of this research. The SPSS Statistics programme was used to recode and extract all the EPC construction data. Floor plans of dwellings built from pre 1919 to date were obtained from Building Control. IES (Ve) was then used to dynamically simulate the performance of the dwellings and the effect of subsequent possible alterations to their thermal envelope. Costs were estimated for their refurbishment and replacement. A novel decision making matrix was then developed to categorise each type I age of dwellings and determine what was the best use of resources i.e. retrofit energy saving features or demolish and rebuild the dwellings to reduce C02 emissions. Of 80 variations, 64% of emissions were generated from 13 dwelling types I ages. To make deep cut CO2 savings, most of these dwellings ought to be fully refurbished rather than replaced. This research has highlighted that the indiscriminate and ad hoc approach to retrofit is not going to optimise CO2 reduction. There should be a focus placed on the dwellings which can readily achieve deep cut emission savings and specifically retrofitting features to achieve the best value for money.