Energy performance of windows based on the net energy gain

The paper presents a new method to set up energy performance requirements and energy classes for windows of all dimensions and configurations. The net energy gain of windows is the solar gain minus the heat loss integrated over the heating season. The net energy gain can be calculated for one orientation or averaged over different orientations.

The averaged value may be used for energy labeling of windows of standard size. Requirements in building codes may also be based on the net energy gain instead of the thermal transmittance of the window. The size and the configuration of the window, i.e. number of glazing units, have a very large effect on the net energy gain.

Therefore the energy labeling or the requirements based on the standard size may not give valid information on the energy performance of windows of non-standard size. The paper presents a method to set up requirements and classes for energy performance based on the net energy gain that includes the effect of window size and configuration.

The net energy gain of windows can be divided into the net energy gain of the glazing unit and the heat losses of the frame and the assembly of the glazing unit and the frame. The glazing unit contributes proportionally to its area. The frame also contributes proportionally to its area, but as the area of the frame is the width times the length, the heat losses of the frame and the assembly both contribute proportionally to the length of the frame.

Accordingly, the net energy gain of the window can be expressed as a function of two parameters representing the energy performance and two parameters representing the geometry of the window. The two energy performance parameters are the net energy gain per area of the glazing unit and the sum of the heat losses through the frame and the assembly per length of the frame.

The two geometry numbers are the area of the glazing unit relative to the window area and the length of the frame profiles relative to the window area. Requirements and classes for the energy performance of the window can be given by assigning values to the two energy performance parameters. In this way the requirements and classes for the energy performance of the window become a function of the geometry of the window.

Therefore the effect of the dimension and the configuration of the actual window are taken into account and make it possible to express the energy performance of all windows in a correct manner. The method is useful in relation to comparison of window products for replacement of existing windows, but may also be useful for handling the energy performance of windows in a general but realistic way in the early phase of the design of new buildings.