Energy efficient buildings should provide a thermally comfortable and healthy indoor environment. Most indoor airflows involve combined forced and natural convection (mixed convection). In order to simulate the flows accurately and efficiently, this paper proposes a two-layer turbulence model for predicting forced, natural and mixed convection. The model combines a near-wall one-equation model [J. Fluid Eng. 115 (1993) 196] and a near-wall natural convection model [Int. J. Heat Mass Transfer 41 (1998) 3161] with the aid of direct numerical simulation (DNS) data [Int. J. Heat Fluid Flow 18 (1997) 88].