Renovate an old box window

Especially in the German cities with old buildings such as Berlin, Hamburg, Leipzig and Wiesbaden, there are a large number of box windows. They no longer meet the legal requirements with regard to heat and sound insulation in force today. When renovating some conversions can bring the desired success.


Box windows are among the worst insulated windows in old buildings. In almost all cases they consist of single-layer window glass types. The seals of the wooden frame usually have no sealing profile.

Before redeveloping box windows, the potential weak points of the design must be located and analyzed:

  • Composite between frame and soffit
  • -Zr Holzrustand deahmen
  • Condition of the sash frame
  • Existing glazing
  • Tightness of the frame profiles
  • Mechanical closure
  • Thickness or volume between the discs
  • putty

Outer frame and glazing

After sealing and modernization of the foaming and the installation of window sealing tape between soffit and window frame, the window repair of the moving parts must be done.

If the glazing has become leaky due to putty that has become porous, the window putty needs to be repaired or replaced.

Exchange window sash

In order to increase the heat protection of the window, with single windows the replacement of only the inner window can be sufficient to achieve sufficient insulation values.

When replacing the window sash, the problem often arises that the thicker double glazing does not fit into the existing old frame. An alternative may be single-layered insulating glass windows. In this case, however, often outer and inner wings must be replaced.

Insulation direction and condensation

Basically, the renovation of old box windows always follows the principle that the sealing from outside to inside becomes stronger. If different glazings are used, the better insulated pane should always be fixed to the inner window.

The frame on the exterior window must not be completely hermetically sealed even with old windows. After the renovation, the air must be able to escape to the outside between the windows of the box window. Too much tightness causes condensation on the inside of the outer pane.

Tips & Tricks

If you want to renovate old box windows, you must follow the U-values ​​laid down in the Energy Saving Ordinance (EnEV). In many cases, funding from KfW is possible.


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