The study targets the reduction of roof solar heat gain through the use of natural ventilation in a cavity of a factory roof. In the laboratory experiment [1], the average air velocity reached 0.25 m/s. A simulation program was developed to calculate the heat and air flow attained in the experiment. An airflow passage was divided into sections to trace the pattern of the air temperature rise. When the cavity was divided into 20 sections, it was enough to trace the temperature rise pattern, and hence to calculate buoyancy for natural ventilation. Then the simulated air velocities, temperatures and heat transportations were compared with the experimental results. The molecular viscosity and thermal conductivity of the air were modified to adjust the simulation results to the experimental results in a wide range of experimental conditions. When they were multiplied with a magnitude of 30 equally, the least root mean square of the ratio of deviations of the heat transportation was obtained. This simulation could predict the heat transportation as a result of natural ventilation with a root mean square of the deviation of 0.25 in a short calculation time. (C) 2010 Elsevier B.V. All rights reserved.