Combined suspended particle device (SPD)-vacuum glazing is a potential adaptive glazing for low energy building application. Glazing transmission is an essential. parameter to determine indoor comfort level of building due to glazing. In this work, above 0.5 clearness index, a strong correlation between glazing transmission and clearness index (Atmospheric transmission) has been evaluated for south facing vertical plane glazing. Below 0.5 clearness index, isotropic diffuse transmittance was dominant and one single value of glazing transmission was found which is suitable for building energy calculation throughout the year. Below 0.5 clearness index, for south facing vertical plane SPD-vacuum glazing transmission was 17% and 1.1% for transparent and opaque states respectively.