A simplified two-dimensional numerical model of a window with a between-panes pleated blind has been developed using commercial computational fluid dynamics software. Knowledge of the effect of blinds on the free convection is important for understanding and predicting the impact of shading devices on the overall thermal performance of a window. Numerical results have been obtained for three fill gases (air, argon and krypton) and several blind geometries over the Rayleigh number range 10³ [less than or equal to]Ra[less than or equal to]10[to the exponent of 5]. The results show that pleated blinds can have a strong effect on window thermal performance. In addition, it has been shown that the data from a convection-only model can be combined with a simplified one-dimensional model to estimate the overall U-value of the enclosure. Using this procedure, the convection data can be applied to a window/blind assembly with arbitrary radiation parameters.