A numerical model has been developed of the conjugate heat transfer by conduction, convection and radiation for two triple-glazed windows with moderate and large glazing distance. Existing numerical studies on heat transfer in window constructions treat the conditions in the glazing and the frame in a separate or sequential method. The presented model treats the heat transfer in the glazing and frame construction in one integrated calculation. The integrated finite element model is compared to calculations of U-value and temperature distribution using simpler calculation methods e.g. according to EN ISO 10077-1/2. The comparison illustrates the need to employ more advanced calculation methods for advanced low-energy solutions for window constructions.