Window Energy Rating System and Calculation of Energy Performance of Windows

The goal of reducing the energy consumption in buildings is the background for the introduction of an energy rating system of fenestration products in Denmark. The energy rating system requires that producers declare, among other things, the heat loss coefficient, U, and the total solar energy transmittance, g, for the panes and whole windows. The energy balance of a window can be described by the net energy gain, which is the solar gain minus the heat loss during the heating season. The net energy gain is a suitable quantity for evaluating the energy performance of windows in a simple and direct way, and for reference condi-tions it is used for classification of panes. In actual situations the net energy gain is dependent on window orientation, climate and shadows. Therefore a simple calculation program is made that takes these circumstances into account. The program is combined with a database of Danish fenestration products. Using the program makes it possible to calculate the total net energy gain to a specific building through different windows in a simple way and from this select the optimal windows. The program facilitates a direct and more accurate product comparison and will also be helpful in product development, e.g. when the resulting effects of a reduced frame area are evaluated.