Summer indoor overheating risks in temperate climates: a case study of vulnerable households in Aotearoa New Zealand

Climate change is intensifying indoor overheating in temperate climates that have historically experienced mild summers, posing growing public health risks. Vulnerable households are disproportionately affected due to the intersection of physiological vulnerability, poor-quality housing, and constrained adaptive capacity. This study analysed indoor temperature data collected from 542 vulnerable households across New Zealand during summer months between 2020 and 2024. Hourly indoor–outdoor relationships were modelled with linear regression and used to project indoor conditions under two IPCC pathways: a moderate-emissions future (SSP2-4.5) and a high-emissions future (SSP5-8.5). Overheating risk assessment against CIBSE TM59 shows that currently 38% of homes overheat during daytime and 56% during nighttime. By 2100, these shares are projected to rise to 40% (SSP2-4.5) and 43% (SSP5-8.5) for daytime and 76% and 89%, respectively, for nighttime. Projections indicate a prolonged nighttime exposure: homes with >25% of sleep hours in an overheated state increase from 6% at baseline to 23% by 2100 (SSP5-8.5). These estimates are conservative because TM59 does not account for New Zealand’s high humidity and does not specify nighttime thresholds for vulnerable groups. This study highlights the need for evidence-based, locally derived overheating criteria for New Zealand and explicit summer energy and thermal-performance provisions in housing standards, coordinated with energy and health policy to support a just transition.