Building envelopes act as the thermal interfaces between the indoor and outdoor environments, thus can greatly influence the indoor thermal condition and the energy consumption of air-conditioning systems. The development of high-performance exterior envelopes is anticipated to be the most effective way to guarantee both low energy consumption and high indoor thermal comfort for a building. Recently, designing and structuring intermediate enclosed air layers have become a popular way to improve the thermal insulation property of building envelopes. Based on the establishment of a dimensionless model, this study numerically investigates the flow and heat transfer characteristics of the insulation air layers with different geometrical sizes and temperature boundary conditions. By analyzing the variation tendencies of the streamlines, isotherms and temperature profiles, a simplified Rayleigh number (Ra) based judgment basis is summarized for the heat transfer pattern of the insulation air layers. Simultaneously, the critical thicknesses of the heat transfer pattern are determined under different temperature boundary conditions. Furthermore, the coupled convective and radiative heat transfer characteristics and the influencing factors of the heat transfer through the air layer are examined. Finally, two measures are proposed to enhance the air layer's thermal insulation performance. The optimal air layer thickness is determined to be 20-30 mm depending on the temperature boundary conditions. Reducing the surface emissivity enjoys a great potential for the thermal performance improvement of insulation air layers. When the emissivity decreases from 0.95 to 0.2, the thermal resistance of the air layer can be improved by 87.15-172.73%. A case study indicates that using the air layer as insulation helps to reduce the annual heat transfer through the building envelopes by 10.54-39.23% depending on the climate condition.