The development of improved windows with respect to energy savings has mainly focused on lowering the U-value of the components glazing and frame. However, the window should be evaluated on its total energy performance, i.e. the total effect of heat loss coefficient, solar energy transmittance and the influence on the heat flow in the surrounding constructions. This paper describes the ideas for a new energy efficient and durable window concept. The window is made as a triple pane glazing with a glass distance of 75-125 mm in which way the window thickness matches the insulation thickness in a well-insulated wall construction. An integrated frame of glass fibre reinforced polyester replaces the traditional spacer in sealed glazing units, and combined with the large glass distance the thermal bridge effect is negligible. As a consequence the total U-value and the centre U-value of the glazing is approximately the same with a value of 1-1.1 W/(m2 K). Due to the large glass distance, the integrated frame and the durability aspect the enclosures between the panes is allowed to “breathe” in order to equalise the internal pressure changes caused by temperature variations. The concept is evaluated theoretically by means of computer simulations of heat flow, internal pressure conditions and the risk of internal condensation. The result is a window with a highly improved durability and a superior total energy performance compared to other existing window products.