This paper presents the results of a numerical and experimental study on thermally efficient windows. An experimental investigation using spectrophotometry was realized on simple and composite glass windows filled with air or PCM. The transmittance and reflectance tests indicate large reductions in the infrared and ultraviolet radiations while maintaining good visibility. The number of glass sheets, their thickness and the gap between them were also investigated. The numerical model is based upon a one dimensional formulation of the composite window. The program was optimized, and the predicted results were compared with experimental measurements. (C) 2002 Elsevier Science Ltd. All rights reserved.437973993