The filler structure is one of the most critical parts of the building's envelope so great emphasis should be placed on its proper design. In addition to the number of panes in the glass system, material of the spacer, the type and the concentration of the filler gas in the cavity between glasses of the insulated system as well as the size of the cavity are also important. The paper focuses on the assessment of the influence of filler gas in the glass system cavity to the energy features of the glass structure. The reason for the assessment is the key influence of the inert gas and the glass cavity dimension on the energy properties of the glass system and the fact that its gradually decreasing concentration greatly affects the properties of the glass system.