The aim of the project has been to investigate and to develop thermally improved windows based on an evaluation of the energy balance of the window, i.e. the total influence of the window on the energy consumption for space heating. The energy balance is the net heat flow per window area which means that both energy losses and transmittance of solar radiation is considered.The final goal of the project was to improve the energy balance of a window with at least 30%. As reference is chosen a common low energy glazing mounted in a wooden frame construction measuring 1188 × 1188 mm2. The transparent area forms about 68% of the total window area and the total U-value of the window is about 1.4 W/(m2 K).The energy balance depends strongly of the window orientation and the thermal mass of the room behind. In order to characterize the energy balance of a window with a single value a method has been developed, which combines the results from several different building types and building orientations. The energy balance of the reference window has been calculated to -50 kWh/m2 window area, i.e. the refence window accounts for a net energy consumption for space heating of 50 kWh/m2 window area. A 30% improvement of the energy balance then corresponds to an reduction in net energy loss of 17 kWh/m2 window area.The frame costruction and the joint between glazing and frame is the thermally weak part of modern windows compared to centre values of the new super insulating glazings. As a result the main emphasis has been put on improvement of the frame construction and the interaction between frame and glazing. Several theoretical analyses have been carried out and a prototype construction has been made, that meets the goal of a 30% improvement of the energy balance.The prototype has been tested in the guarded hotbox facility at the institute and a total U-value of about 1.2 W/(m2 K) have been measured, which is a reduction compared with the reference window of 0.2 W/(m2 K). The improvements have primarily been obtained by replacing parts of the frame construction with polystyrene foam insulation. Other frame solutions have been described based on a combined effect of reducing the frame area and improve the insulating properties of the frame construction.A new window concept with increased glass distance and part of the frame integrated in the glazing has been described as a result of the analyses carried out in the project. The large glass distance helps to reduce the traditional thermal bridge effect of the spacer and the integrated frame leads to an increase in transmitted solar energy. Furthermore, a controlled air exchange in case of pressure differences between the enclosures in the glazing and the surroundings is included in the concept which strongly improves the life time of the windows. The thermal performance will be better than traditional windows with super insulating glazings.
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Department of Buildings and Energy, Technical University of Denmark, 1998