In recent years, the thermal insulation of new windows has improved more and more. Ultimately, only a small amount of heat should escape from the windows to keep energy consumption low when heating. Replacing old and leaky windows can therefore have a major effect on improving the energy efficiency of a home. On the other hand, new windows require a non-negligible investment. So in which cases does it make sense to replace old windows and which new ones to install?
WHICH WINDOWS TO REPLACE?
A lot of energy is wasted due to obsolete windows. But how to understand if the windows give off too much heat to the outside? First of all, your window frames must not be damaged. If they are rotten, broken or warped, the windows need to be replaced.
Then your windows should be narrow. The best way to check if your windows still close tightly is to tape a piece of paper between the window frame and the sash. If the card can be pulled out when the window is closed, you know that the window no longer closes tightly. It needs to be repaired or replaced.
Furthermore, the glazing of your windows is an indicator of the possible need for intervention. Single-glazed windows in older buildings have a heat transfer coefficient (U-value) greater than 5 W/m²K. Even double-glazed insulating glass from the 70s and 80s only has half the heat transfer power. However, heat-insulating glass windows are contemporary. In these double or triple glazed windows, the space between the panes is filled with an insulating noble gas, which reduces the U value to 0.5 W/m²K. According to calculations by TÜV Rheinland, 50-70% less heat is lost through these windows than through windows in older buildings.
If your home already has windows with heat-insulating glass, replacing them is not advisable, although today there are certainly windows with even lower U-values. You can tell if your windows have heat-insulating glass simply by holding an open flame in front of the window when it’s dark outside. If heat-insulating glass is installed, one of the front mirror images of the flame must have a different color from the others. This effect is caused by the presence of noble gas in the space between the glasses.
WINDOW REPAIR
In some cases it is not necessary to replace the windows; it is also worth repairing the old ones. Maybe the windows just need to be readjusted so that they close tightly again. And if the old frame is still intact, only the glass can be replaced. This is easier and cheaper than replacing the entire window.
However, the prerequisite for glass replacement is that the hardware and frames of the old windows are sufficiently stable to support the heavier thermally insulating glass. Or new glass is inserted into old window sashes, which is suitable, for example, for old wooden windows that are worth preserving. Or in old frames it is possible to fit new suitable doors with heat-insulating glass.
PAY ATTENTION TO THE INSULATION OF THE FACADE
Normally the U-value of windows should be worse than that of external walls. Windows have the coldest inner surface, and if there is too much humidity in the room, condensation will condense there. Otherwise the humidity in the air will condense on the walls and, in the worst case scenario, mold may form. Therefore, when renovating a house, it is advisable to insulate the external walls before or at the same time as replacing the windows.
In any case, the new narrower windows reduce air exchange throughout the house. This increases humidity and increases the condensation problem mentioned above. This must be counteracted with regular ventilation, which is why when replacing more than a third of the windows in a house it is necessary to come up with a so-called ventilation concept. The ventilation concept determines whether the normal window ventilation is still sufficient after the renovation work has been carried out or whether additional measures such as window fans or a ventilation system are necessary. You can find our tips for correct ventilation here.
Newly installed windows should be part of the insulation layer of the building envelope, which means they must form a continuous layer with the existing facade insulation to avoid thermal bridging. In the case of a single shell external wall this is the position of the window in the middle third of the wall; in the case of double shell wall constructions this is located in the middle third of the separate insulation level. As this is a technically complex solution, the solution is usually to ensure that the new windows are flush with the outer edge of the brickwork and that the frames can simply be insulated without additional fixings.
GLAZING AND FRAME MATERIAL FOR THE NEW WINDOWS
As far as the material of the glass and frames of your new windows is concerned, you are spoiled for choice. As regards glazing, triple glazing with thermal insulation has become established in new buildings. Even today, thermally insulating double-glazed windows are installed in renovation projects. This is due to space reasons or is the result of coordination with other already existing components. In any case, from an energy point of view, the installation of such windows is better than the minimum requirement set by the Building Energy Act (GEG) for the installation of new windows.
The choice of frame material remains. The most common options available are plastic, wood, aluminum or the combination of wood and aluminum materials. Plastic windows make up the majority of windows used in new buildings today. They are easy to maintain, economical and have good thermal insulation properties thanks to their multi-chamber profiles. However, they are difficult to repair.
Wooden windows are still installed in many older homes. With regular care they last a long time and offer a warm look with good insulating properties. Wooden windows are the easiest to repair, but they also cost 20-25% more than plastic windows.
The biggest advantage of aluminum windows is their resistance to atmospheric agents and therefore their very long life. The material allows for narrow, angular frames for a modern, minimalist look. To improve its poor insulating properties, aluminum is often combined with wood inside the frame. The costs for wood-aluminum windows are still 25-30% higher than for purely wood windows.
The replacement of windows is supported by the State through direct subsidies or alternatively through tax breaks. Please note that you must apply for the grant before hiring any commercial services. Your nearest construction partner will be happy to help you.
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