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Energy modernisation of windows

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Handwerker beim Einbau eines neuen FenstersSource: Fotolia

Modernisation of windows may be done by replacing the entire window. The energy performance of existing windows worthy of preservation can also be improved by replacing the glazing or by installing additional windows on the outside or the inside of the existing windows.

In doing so, the energy quality of the exterior wall should be taken into consideration. In every case, the U-value of the exterior wall should be lower than the U-value of the windows (i. e. the insulating effect of the wall should be better). Otherwise, there is a risk that moisture condensation occurs on the inside of the exterior wall. Thermal bridges on the reveal must be reduced with particular care in order to prevent mould in these places. A special emphasis should be put on a permanently airtight installation in order to avoid undesirable drafts and structural damages.

The achieved energy cost savings of a modernisation of windows also depend on their initial state.

U-values of selected constructions

The energy quality of windows is dependent on the type of glazing, the edge seals, and the window frame.

The following table shows exemplary U-values of windows in existing buildings. The heat loss through old windows is often very high compared to modern windows.

Window
Construction systemYear of constructionU-Value (W/m²K)
Single-pane windowuntill 19784.70
box-type windowuntill 19782.40
Window with uncoated double pane glazing1979 - 952.70

Source: Verband Fenster + Fassade, "Mehr Energie sparen mit neuen Fenstern", March 2014

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