Abstract: The present invention is directed to an insulated window assembly including a fixed frame adapted to be installed in a building structure and a movable, pane carrying operating sash mounted on the fixed frame. The invention is characterized by the provision of a novel seal structure defined between the fixed frame and outer pane member, said seal being formed laterally interiorly of the glazed connection between the pane and sash whereby leakage between the fixed and moving frames is effectively blocked, the seal also protecting the said glazed connection. Where used in conjunction with a metal framed window device, the assembly provides protection against heat loss comparable to that provided by conventional thermal break windows, without the attendant disadvantages thereof.