Longitudinal assessment of indoor environment in existing UK social housing dwellings requiring energy retrofit

The UK’s social housing stock is widely recognised to be in poor condition, yet there remains a significant lack of empirical data on the indoor environments experienced by residents—many of whom are vulnerable due to financial hardship or health conditions. This study presents a longitudinal assessment of indoor environmental conditions in 23 EPC D- or E-rated social housing dwellings in Warwickshire, monitored over two consecutive winters (2021/22 and 2022/23). Temperature, relative humidity (RH), and carbon dioxide (CO 2 ) concentrations were continuously recorded in bedrooms and living rooms, while a subset of 13 homes was additionally monitored for particulate matter (PM 2 . 5 , PM 10 ) and volatile organic compounds (VOCs) in living rooms. Findings reveal a marked deterioration in thermal conditions during the second winter. Average living room temperatures fell by 0.7°C (from 19.2°C to 18.5°C) and bedroom temperatures by 1.9°C (from 19.0°C to 17.1°C), likely due to reduced heating use in response to rising energy costs and poor insulation. Indoor air quality (IAQ) was similarly concerning: CO 2 concentrations exceeded the 900 ppm benchmark for over 95% of occupied hours in nearly all rooms across both seasons, with average levels rising from 1193 to 1519 ppm. Limited ventilation and increased time spent indoors, as residents sought to conserve warmth, contributed to these conditions. Elevated PM and VOC concentrations were associated with occupant characteristics (smoking and pet ownership) and household activities, providing further evidence of insufficient air exchange. These results highlight the urgent need for holistic retrofit strategies that address both energy efficiency and IAQ. Prioritising improvements to building fabric and ventilation before heating system upgrades will be essential to safeguard occupant health, comfort, and well-being. The study provides rare empirical evidence and offers insights to inform policy and retrofit design for the UK’s most vulnerable households.