Window installations have a considerable impact on the heating energy consumption of Passive Houses. The differences in heating demand may be in the region of 1 kWh/(m²a). While estimating the thermal bridge coefficient of installations is inaccurate, it takes too long to calculate it accurately during the planning stage. This is because doing so requires the precise geometries of the frames as well as all their material properties to be researched.

This is presumably why projects are regularly found to have unsuitable, simplified thermal bridge calculations, in which a panel and its Uw value are used as opposed to the window itself. This calculation is completely meaningless! When this method is applied, the results depend on the choice of glazing. In reality, however, glazing has no influence on the installation position. To help overcome this obstacle to the proper planning of energy-efficient window connections, this report investigates a simplified model for various frame types and compares the calculations with those made using models with precise frame geometries.

The simplified model depicts the frame as a panel which varies in terms of its materials and Uf value. Three different methods for various frame types are outlined in Figure 1. In this figure, aluminium shells with excellent thermal conductivity are shown as a separate layer. When calculating the equivalent thermal conductivity, the aluminium layers can be left out.

To check the simplified method, comparative calculations were performed and the results from the models with precise frame geometries were compared with those from the models with simplified panels. Calculations were made for the installation positions of three different timber, aluminium and timber-aluminium frames respectively as well as four different PVC frames. Both the frames and wall structures are of Passive House quality.

The following charts present the differences between the results obtained using the precise calculation method and the simplified calculation model. A positive value indicates that the result calculated using the simplified model is higher than that recorded using the precise calculation method, and that using the former to determine the energy balance generally has a low level of measurement uncertainty.

Figure 1: The simplified thermal bridge calculation model differs depending on the frame type.