This paper presents a different approach for thermal effective windows, i.e. windows which reduce energy transmitted into or out of the room. The idea is to use a double-sealed glass filled with pcm whose fusion temperature is determined by solar-thermal calculations. The investigation is divided into two main parts: modeling of the heat and radiation transfer through the composite window and the optical investigation of the pcm-filied window. A one-dimensional thermal model for the composite window was developed to predict the resulting effects due to variation of the geometrical and pcm thermal properties. Optical investigations using photo-spectrometry were realized on single glass, double glass filled with air, and double glass filled with pcm, The transmittance and reflectivity tests indicate large reductions in the infrared and ultraviolet radiations while maintaining the good visibility. The effects of thickness of glass and spacing were also analyzed.