The average life span of windows is 40 years. They should then be replaced if this has not already been done. However, there can be good reasons for replacing windows before this time has elapsed. Today, however, technical progress and also changed requirements can make windows that are only 20 years old appear old. As windows are structurally relevant components, when replacing them it is important to consider the entire building and its properties.
Greater comfort, lower energy costs
If you consider the heat losses of a house, old windows are often the greatest weak point. Replacing windows that are 15–20 years old can reduce the heat loss by between 15% and more than 40%. Therefore, for most renovations, replacing old windows is the top priority. It is important to create a renovation concept, as noticeable improvements only occur when individual measures supplement and complement each other. When renovating windows, it often makes sense to think about insulating the outer shell to avoid heat losses at the connections and to prevent any formation of mould. Whether or not renovating windows pays off and over what period can be calculated by performing a detailed analysis of the existing windows and the heat losses that occur. The use of a thermal imaging camera can reveal specific weak points.
No side issues
In existing buildings, a side window reveal often takes the form of a masonry stop. If external insulation is applied but the windows are not replaced, in most cases it is not possible from a construction perspective to also insulate the reveal retrospectively, which creates a thermal bridge with high heat losses. In cold weather, the reveal and frame can cool down so severely that condensation and mould are almost unavoidable. One solution is to break off the masonry stop and to insulate the reveal up to the window frame with a thickness of at least 3 cm.
If the outside wall is insulated and new windows are also installed at the same time, in this case the windows can be finished flush with the masonry and the insulation applied over the frame. If an existing masonry stop is to be retained, the opening dimension of the window in the building shell is reduced, thus decreasing the pane area, as space is lost due to corresponding insulation thicknesses.
The ideal situation is the installation of new windows in the insulating level. However, windows can then only be subsequently removed with a great deal of effort and damage to the façade. This connection construction is recommended for insulation thicknesses from approximately 14 cm and requires more detailed planning.
Creating no bridges for the heat
Compared with old windows, new windows have significantly lower joint permeability with better structural properties. This can shift the dew point with a subsequent formation of wet mould. In existing buildings with no insulation, the temperature of the connection components can fall below 10°C in some areas, meaning in particular that condensation can form in the connection joint. If this condensation cannot disperse, mould forms. When forming joints, it must therefore be ensured that the water vapour diffusion permeability resistance (as per DIN 4108, Part 1) of all components reduces from the inside to the outside. Care must also be taken to avoid thermal bridges in the reveal (see above). An isothermal representation can provide information about the correct position of the window in the masonry.