To reduce carbon emissions, urgent change is needed to high-carbon human behaviours including home energy use. Previous policy failures point to insufficient integration of systemic and behavioural approaches which are too often seen as alternative and incompatible approaches to bring about change. A novel behavioural systems mapping approach was used to inform national policy recommendations for energy-saving retrofit of homes in Wales. Three participatory workshops were held with the independent Welsh residential decarbonisation advisory group (‘the Advisory Group’) to: (1) map relationships between actors, behaviours and influences on behaviour within the home retrofit system; (2) provide training in the Behaviour Change Wheel (BCW) framework and (3) use these to develop policy recommendations for interventions. Recommendations were analysed using the capability, opportunity and motivation (COM-B) model of behaviour to assess whether they addressed these factors. Two behavioural systems maps (BSMs) were produced, representing privately rented and owner-occupied housing tenures. The main causal pathways and feedback loops in each map are described. Necessary interventions to achieve national-scale retrofit included: government-led investment, campaigns and awareness-building, financial-sector funding mechanisms, enforcement of regulations and creating more streamlined and trusted supply chain services. Of 27 final policy recommendations, six addressed capability, 24 opportunity and 12 motivation. Participatory behavioural systems mapping can be used in conjunction with behaviour change frameworks to develop policy recommendations that address the behavioural determinants of complex environmental problems in a systemic way. Research is underway to refine and extend the approach through application to other sustainability challenges and methods of constructing systems maps.