Thin metal coatings on glass are widely used in modern windows to improve their thermal properties. However, while there are some available papers on the properties of microwave propagation through commercial metallized windows glass below 6 GHz there seems to be a lack of papers regarding metallized multi-glazed windows at higher frequencies. Such data are of the great importance when designing modern microwave communication systems working in urban environments. In this paper, the shielding effectiveness of modern energy-saving glasses and windows were investigated at microwave frequencies from 1 GHz to 20 GHz. Wide measurement range together with proposed fast calculation procedure based on the matrix multiplication method, allowed us to elucidate the reasons responsible for the extremes in dependencies of shielding effectiveness on frequency. It was also shown that by comparing measured and calculated results, the conductivity of the thin metallic layer used in modern energy-saving windows can be determined. During the study, we found that attenuation of microwave radiation passing through modern windows can be as high as 60 dB.