Energy efficient buildings should provide a thermally comfortable and healthy indoor environment. Most indoor airflows involve combined forced and natural convection (mixed convection). This paper used a two-layer model to predict such flows. This model used a one-equation model for near-wall region and the standard k-epsilon model for the outer-wall region. Its validation in six cases shows good agreements between the computed results and measured data. In addition, the computing cost required by the model has been reduced significantly.