Abstract: A window unit is provided for reducing heat loss through the window while at the same time providing a desirable inflow of prewarmed ventilation air to maintain interior air quality and thermal comfort. The window unit comprises an outer window frame and a sash frame designed to support an inner dual pane sealed window unit together with a separate outer single pane thus providing a triple pane action. The sash frame is also modified to provide a slot at the lower edge for entry of exterior air into a channel defined between the outer pane and the outer surface of the sealed window unit. The building is maintained at reduced pressure to cause air to flow in a laminar action along the channel toward a slot above the top of the sealed window unit so that heat escaping from the outer pane of the sealed window unit is carried by the laminar air flow back through the upper slot and into the interior of the building.