Marconi Max window is a thermally modified version of Marconi window. The wooden part is 68 mm thick. By introducing a new sash design, we can use 44 mm thick, two-chamber glazing units. This improvement has allowed us to use panes with the heat transfer coefficient Ug = 0.6 W/m2K in standard Marconi Max windows. With this construction we obtain an excellent insulation parameter for the reference window Uw = 0.9 W/m2K. The economic construction of Marconi Max window allows for a very good ratio of price to thermal and functional properties.