Indoor heatwave risk in Bangladeshi buildings and Rohingya shelters to 2050

South Asia faces a growing threat from compound heat events, where extreme temperatures coincide with power outages that limit indoor cooling. Yet understanding indoor heat exposure in Bangladeshi buildings under such conditions remains limited. Here we assess heat exposure and retrofit options across building archetypes and Rohingya camp shelters under present and projected climates. Using 5,760 EnergyPlus simulations of seven building archetypes in seven regions and the Cox’s Bazar camps, we quantify dangerous wet-bulb globe temperature hours during heatwave weeks under passive, fan-assisted, and air-conditioned modes with scheduled outages. A two-step machine learning surrogate predicts dangerous hours for any design within the sampled ranges. A climate-shift model trained on 17,280 further simulations and a multi-objective genetic algorithm then evaluate envelope retrofits and investment portfolios. Exposure was highest in Dhaka and in the most crowded buildings, and air exchange mattered most among envelope variables. Air conditioning removed 70% of mean exposure, yet 24% of air-conditioned simulations still recorded dangerous hours because cooling stopped during outages. Projected warming doubles the lowest achievable national risk by 2050, from 1,118 to 2,364 occupancy-weighted person-hours per week. Bamboo-tarpaulin camp shelters show a 5.3 to 5.6 times risk increase by 2050, whereas brick shelters with adequate ventilation record no dangerous hours. These findings show that retrofits must combine ventilation with reliable power to protect heat-vulnerable populations as the climate warms. Heat adaptation should therefore link building standards, warnings based on humid heat, occupancy-based investment, and thermally safer shelter materials.