The UK government has set targets to reduce carbon emissions by 80% (from the 1990 baseline) by 2050 (Defra, 2008). Buildings contribute around 50% of the energy used in the world and of the buildings that will be standing in 2050, 87% already exist. Domestic dwellings account for between 25 and 27% of CO2 emissions (Boardman, 2007) and it is suggested that improvements in energy efficiency will reduce CO2 emissions in existing buildings.Although the energy intensity of industrial economies fell before the recession, the absolute energy use attributed to UK households (and associated CO2 emissions) continued to increase (Druckman et al. 2010). One explanation for the failure to reduce energy consumption is that occupants ‘take-back’ some of the potential energy savings. This ‘rebound effect’ for energy efficiency improvements can occur both directly or indirectly and the indirect rebound effects remain largely unexplored (Druckman et al. 2010).Research also suggests that taking part in ‘simple and painless’ behaviours will lead to increases in motivation for the individual to adopt other (possibly, more ambitious) related behaviours; known as 'spill over' (Thøgerson and Crompton, 2009).The main aim of this research is to investigate the spillover and rebound effects of occupant behaviour after energy efficiency improvements have been carried out in their homes.It is proposed that a longitudinal study will be conducted to examine the behaviour of occupants in existing properties in Wales which are due to have energy efficiency improvements carried out by the Welsh Government (Arbed Phase 2).Under the Arbed phase 2 scheme, 1560 homes per annum (for three years) will have energy efficiency improvements carried out. All of the participants will be social housing tenants. The control group will also be eligible for the Arbed phase 2 energy efficiency improvements, but these improvements will be carried out at a later date.The participants will be asked to complete a questionnaire before and after the energy efficiency improvements are made. A selection of participants will also be asked to complete an additional questionnaire one year later. A sub-sample will have physical monitoring (indoor air temperature recorded and gas and electricity meter readings) carried out in the heating season of 2012/2013 and 2013/2014. The first wave of data collection is due to be carried out in February 2012 and the data will be analysed using SPSS.The findings from this research should help to contribute to understanding how people respond to energy efficiency improvements and whether they ‘take-back’ some (or all) of the energy savings made and/or whether their behaviours spill over to other related or non-related behaviours.