With a great advantage of presenting physical phenomenon in a three-dimensional vision, the technology of computational fluid dynamics was utilized in the current study for simulating and revealing heat transfer in the air gap/layer between human body and clothing. Five clothing configurations including one with realistic heterogeneous air layer and four simplified ones with homogeneous air layers with air gap thickness (AGT) varying between 4 and 16.34 mm were built. All clothing configurations were simulated under environmental air speeds of 0.2-2.5 m/s. It was found the thermal plume, airflow through clothing openings and pathway at armhole have resulted in an uneven distribution of heat transfer underneath clothing, even for clothing with homogeneous air layers. Simply averaging AGT blurs the heterogeneity of heat transfer over the whole body. The threshold of AGT for arising free convection in the air gap is within 12-16.34 mm at the air speed of 0.2 m/s and 8-12 mm at 2.5 m/s, indicating a substantial influence of environmental airflow on thermal flow in the air gap. Besides, increase of air speed would enhance local heat exchange by entering the air gap through clothing openings, except at the arms, where the heat flux fluctuates due to the direction of airflow in sleeves changing from upwards to downwards. Results of the current study have offered a comprehensive insight into the heat transfer in the air gap and can contribute to improving the accuracy of analytical human - clothing - environment models in clothing thermal engineering.