Envelope impact on indoor thermal resilience across different housing types during extreme heatwaves and power outages

Extreme heatwaves have intensified reliance on mechanical cooling systems and increased the risk of power outages. However, the quantification of indoor thermal resilience during extreme heatwaves and power outage conditions remains limited and underexplored, specifically for the inequalities in thermal resilience across income groups. This study investigates the influence of the envelope characteristics on thermal resilience across low, medium, and high-income housing units. Specifically, Philadelphia, PA is selected as a case study due to the frequent presence of heatwaves and higher social vulnerability. A simulation model, validated through thermal chamber experiments, was employed to assess indoor thermal performance during extreme heatwaves and power outage conditions. Following validation, the U.S. Department of Energy's residential prototype models were modified to create three representative envelope configurations in Philadelphia, corresponding to three low-, medium-, and high-income groups: i) uninsulated walls and windows for a low-income unit; ii) R-11 insulated walls and double-pane windows for a medium-income unit; and iii) R-15 insulation with low-emissivity double-pane windows for a high-income unit. Results indicate that low-income units consistently experience higher short-term and energy stress across historical, mid-term, and future-term climate scenarios. Their peak cooling load, 15.45 kW, exceeds that of middle- and high-income units by 35 % (9.88 kW) and 53 % (7.26 kW), respectively. During post-outage recovery, thermostat setbacks cause a surge up to 89.77 kW, nearly a sixfold increase compared to the 22 °C setpoint load. Thermal safety also varies considerably, with low-income units reaches the ‘Danger’ heat index threshold (≥41 °C) in under 4 h, compared to 6 h for middle-income and 8 h for high-income units. These findings demonstrate the critical role of envelopes in moderating peak demand, enhancing thermal comfort, and strengthening grid resilience during extreme heatwaves.