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.

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

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 2: The charts show the difference between the installation value calculated using the precise frame geometry and the installation value determined using the simplified model by subtracting the latter from the former. The type of frame structure is indicated on the left-hand side of each chart. To serve as a comparison, each chart shows the installation value predefined in the PHPP: 0.040 W/(mK).

The results suggest that the values for installation positions where the insulation covering has a very positive effect (e.g. timber and PVC frames with connections at the top) deviate more significantly from those obtained using the precise model. This means that the simplified model underestimates the thermal bridge. The values for aluminium and timber-aluminium frames are most consistent. It should be noted that the installation details assume that the window has been optimally positioned in the insulation layer. The few additional comparative calculations with window frames in the insulation layer indicate that the interactions of the materials and geometries at the weak points will become more significant and cannot be depicted as well using the simplified model.

This method provides a practical and fast way of calculating thermal bridges. The results collected for aluminium frames or frames with aluminium shells match those collected using the precise calculation method while the deviations in the calculations made for timber and PVC frames also have some measurement uncertainty (around 0.005 W/(mK)). Nevertheless, it seem safe to say that the simplified method can achieve greater accuracy than mere estimates or than using the starting value of 0.04 W/(mK) predefined in the PHPP. Using the simplified thermal bridge calculation model offers a straightforward means of better determining energy balances according to PHPP, even during the early stages of the planning process. The model should, however, always be created with building physics in mind. Precise installation values for certified frames can be found on the certificates.

The installation of various frame types was investigated by performing comparative calculations using a simplified model and a model with a precise frame geometry. Using the simplified thermal bridge calculation model offers a straightforward means of better determining energy balances according to PHPP, even during the early stages of the planning process.

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