This study examines the effects of the presence of a Venetian blind on the thermal performance of a window. The blind is positioned adjacent to the indoor surface of either a single or double glazed window and the coupled convection and radiation heat transfer problem is solved using a two-dimensional finite volume model. The numerical model is validated with published experimental and numerical results in the literature. The results show that the presence of a Venetian blind significantly improves the energy performance of a single and double glazed window. The blind reduces the overall heat transfer rate through the window by reducing the thermal radiation from the indoor glazing. The improved understanding of the benefits of Venetian blinds may lead to better designs of window/shading systems. (c) 2004 Elsevier B.V. All rights reserved.