Evaluating the indoor environment, health, carbon emission, and economic outcomes of retrofitting social housing properties to improve energy efficiency in a low-income multi-ethnic population: a quasi-experimental study with process evaluation

BACKGROUND: Housing-related illness costs the NHS £1billion/year and indoor air quality (IAQ), thermal comfort, and damp/mould are linked to mental, respiratory and cardiovascular health. Homes also contribute to climate change with 20% of UK carbon emissions linked to inefficient heating methods. The Government is funding a retrofit programme to improve energy efficiency of social housing, which accounts for 17% of UK homes. Retrofitting can improve energy efficiency, making homes warmer and cheaper to heat, particularly benefitting low-income groups such as social housing residents. However, there may be trade-offs such as reduced ventilation and deterioration of IAQ which could negatively affect health and increase inequalities. These are poorly understood and there is an urgent need to identify how retrofit can be implemented to maximise environmental, health, and economic co-benefits to inform policy. AIM: To understand how improving the energy efficiency of social housing impacts on indoor conditions, health, and economic outcomes. RESEARCH QUESTIONS: What is the impact of retrofit on indoor temperature, humidity, IAQ, carbon emissions, and short and long-term resident health? What are the trade-offs, and how do contextual/system factors influence outcomes? Is retrofit value-for-money? METHODS: WP1: Controlled pre/post quasi-experiment in 210 retrofit and 210 matched control homes. Indoor temperature, humidity, IAQ (PM2.5, NO2, CO2) and energy use monitored over 18 months including two winter periods. Self-report thermal comfort, behaviours, and health in the winter pre, and post retrofit. Controlled multilevel interrupted time series and repeated multilevel models will compare outcomes for retrofit homes vs. controls. WP2: Matched cohort study with linked routine health and housing data: 6500 retrofit homes matched 1:1 with control homes using exposure density sampling between 2022-28. Acute health-care episodes will be aggregated at household level for respiratory, cardiovascular, and mental health up to 5 years post retrofit. Negative binomial regression will estimate incidence rate ratios for retrofit vs.control. Subgroup analysis will explore impacts for vulnerable residents. WP3A will estimate changes in indoor conditions and carbon emissions for a range of archetype properties using building performance software. WP3B will perform a health impact assessment to quantify lifetime economic and health impacts resulting from changed exposures measured as quality adjusted life years. Estimated benefits and cost offsets will be considered relative to retrofit costs. WP4: Process evaluation including interviews (20 residents, pre/post retrofit; 10 stakeholders, two time points) and participatory methods (10 peer researcher led citizen science projects with 200 residents). Thematic analysis will explore acceptability, barriers, enablers, unanticipated outcomes, and contextual influences. Two participatory workshops will create and refine a system map and programme theory identifying key system dependencies and leverage points. TIMELINE: 54 months(M). WP1 complete M33; WP2 M53; WP3A M48; WP3B M53; WP4 M48; Reports: Interim M36, Full M54. IMPACT/DISSEMINATION: For policy/industry: briefing notes, guidance, dissemination events. For researchers: academic and conference papers. For communities: lay summaries, community events. Outputs will be disseminated widely using national networks, media links and established social media channels.