This paper presents a different approach for thermal effective windows, i.e. windows that reduce the energy transmitted into or out of a room. The idea is to use a double sealed glass filled with a phase change medium (PCM) whose fusion temperature is determined by solar-thermal calculations. The PCM used is polypropylene glycol. The investigation includes modelling of the heat and radiation transfer through a composite window and optical investigation of conventional and PCM filled windows, testing of the window and comparison with numerical simulations. A onedimensional model for the composite glass window is developed to predict the thermal performance as a function of the geometrical parameters of the panel and the PCM used. Optical measurements were realized using photo-spectrometry to determine the transmittance, reflectance and absorptance. The specimens used include single glass of different thicknesses, double glass of different gap spacing and thicknesses filled with air or PCM, and finally coloured PCM. The results indicate big reductions in the energy transmitted, specially in the infra-red and ultraviolet regions, while maintaining a good visibility.