Finding the ventilation sweet-spot: A validated CFD trade-off between airborne-infection risk, thermal comfort, and cooling energy in a tropical classroom

Post-pandemic high-occupancy classrooms renew concern over airborne pathogen transmission, yet in tropical climates ventilation for pathogen dilution competes with thermal comfort and latent cooling load. Existing studies typically optimise these objectives in isolation, largely for temperate climates. This study uses a Computational Fluid Dynamics model, verified by a grid-convergence analysis and validated, within measurement uncertainty, against field air-velocity and temperature measurements in a real Malaysian university classroom, to evaluate six air change rates (ACH 3.8 to 22.7) against three objectives: airborne-infection risk (Wells--Riley), thermal comfort (ISO 7730 draught rate and adaptive comfort temperature), and cooling-energy demand. Infection risk fell steeply up to ACH 11.3 to 15.1 (about a 75% reduction at ACH 15.1 relative to ACH 3.8) but only marginally thereafter (a further 8 percentage points by ACH 22.7, only 3 of them from the final step from ACH 18.9), while the cooling load rose 6.0-fold and local air speeds exceeded the ISO 7730 draught limit; near-occupant temperatures of about 24.3°C lay roughly 5°C below the local adaptive comfort temperature (29.3°C), indicating overcooling that is better addressed through the supply-air set-point. An air change window of approximately 11 to 15 per hour emerges as the cost-effective optimum, capturing near-maximal infection control while limiting draught risk and cooling energy. The framework provides an evidence-based basis for ventilation design balancing health, comfort and energy in hot-humid educational buildings.