The sub-divided unit (SDU) is the major type of substandard housing in Hong Kong. This study monitored the microclimatic conditions in eight SDUs for 24 h in summer. The mean CO₂ concentration (1012 mg/m³) under natural ventilation was much higher than the outdoor level (∼410 mg/m³), suggesting that ventilation was ineffective in most SDUs. The mean predicted mean vote (PMV) was 1.7 (warm), corresponding to 75% predicted percentage dissatisfied (PPD). None of the SDUs was thermally acceptable (PPD < 20%) at any time in the monitoring period. If the adaptive thermal comfort standard was considered, the acceptable temperature range would be between 23.4 °C and 30.4 °C in operative temperature and the thermally acceptable time would be 47%. Larger openable window size, larger external wall area and poor ventilation significantly contributed to higher maximum indoor air temperature. Copyright © 2021 Informa UK Limited.