Refurbish box windows, seal, improve U-value

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Refurbish box windows, seal, improve U-value

The building sector causes about 40% of CO2 emissions in Germany, so the most important measure for the environment is the energetic improvement of existing buildings.
This also helps the homeowner: Studies by the German Energy Agency showed that the energy consumption of existing buildings can be reduced by up to 85%. This applies particularly to the replacement of energy-obsolete windows and glazing.
Preserved box windows

  • Historic wooden box windows from the Wilhelminian era can be found in many old houses, for example in the big cities such as Leipzig and Berlin. They inspire with many forms, the quality of the execution also testifies to the high design standards and the high craftsmanship of the generations before us. Often, the uncritical replacement of the old wooden box windows is observed against new, insulating glazed single windows. In the best case, new wooden windows or plastic windows are installed, which at least externally imitate the historical appearance. This, too, hurts aesthetes, but at least still hints at the past. Often, however, is completely replaced without style and consideration and so the entire facade image destroyed.
  • Window manufacturers and many craftsmen also advertise the advantages of modern materials and technologies, such as plastic frames and double-glazing, with a view to their own earning potential. The Kreditanstalt für Wiederaufbau (Reconstruction Loan Corporation) is also more bank than monument protection authority and only promotes the commercial installation of newly purchased windows.
  • From the conservation advocates get the
    Owners of old box windows less support, in the maintenance work often earn small craft businesses that can not afford a large marketing machine, from the field of economics come here only partial services we new glass. The homeowner is here largely dependent on obtaining his own information.
  • Box windows are not only worth preserving for aesthetic reasons: There are also technical arguments in favor of this type of building. Retreaded box windows often overshadow industrially manufactured plastic windows with insulating glass panes in terms of insulating properties and the danger of mold infestation. Not for nothing there is a DIN standard, which prescribes a surface temperature of about 13 degrees Celsius on the window sill, the wall is colder, is reflected at this point dew down. The walls around box windows are often even up to two degrees above this value, mold does not form under these conditions. But this comes quickly when the box windows are replaced by thick insulating glass windows that move the coldest spot to the window sill. A decades-long practice of false ventilation, which was distorted by the box window, now leads to condensation glass window to condensation on the wall and as a result to mold. Nevertheless, the energetically unfavorable box windows must be rehabilitated.

Old box window with new values

  • Old box windows are among the largest energy wasters in Germany, currently it is believed that there are more than 25 million simply glazed windows in Germany
    gives. The traditional box window has a U-value (heat transfer coefficient) of 2.8 to 3.3, about twice as high as the value prescribed by the Energy Saving Ordinance. This does not have to stay that way: if an old box window gets new glass, gaskets and a thorough paint job, it can already reach the EnEV standard with a U-value of 1.3.
  • Such retreading may include, for example, the following steps: The old paint is completely removed, which is often necessary because synthetic resin paints have been applied in the meantime, which, in contrast to the previously used oil paints, become brittle and can permanently damage the building fabric. The glass is removed, then all frames and fittings are revised by the carpenter, the wood parts are repaired, the fittings made feasible. An elastic seal is incorporated, new glass is introduced, then repainted. This creates a window construction with
    improved thermal insulation values, sound insulation and weather protection values, the high aesthetic value is maintained even with a new U-value from 0.95 to 1.3.
    • In the rarest cases a retreading is not possible, then modern heat-insulating windows can be made, which are the same as the original. Or composite windows are used which consist of two wings and open and close like a single window.

    New developments

    • Difficulties often make the glass thickness in these renovations.If old single glazing is to be replaced by multi-pane glazing, most modern insulating glazing is not thin enough to fit in the often very narrow original wood frames. Newer energy-saving windows with a wider frame, for example, can not be replaced by the monument protection authority. The complete replication of the original windows with today's technical possibilities would be very expensive.
    • Although it is usually possible to gain a few millimeters during the revision of the old frames, it is still necessary to install an optically unconvincing glazing bead on the inside of the window when using standard insulating glass, so that the glass can be used.
    • For this reason, box-window enthusiasts are relying on new vacuum glass, which is already available in a glass thickness of 6.2 millimeters. Soon, thicknesses of 4 millimeters will also be available on the market.

    Editorial tips

    1. It should be noted that the U-value of the windows must be kept lower than the U-value of the wall. If this is not the case, the moisture from inside does not diffuse out through the windows, but chooses its way through the masonry, which can result in damp walls and mold.
    2. Many cities and municipalities do not want the destruction of the old buildings and provide advisers willing to refurbish, but ask for it.
    3. Incidentally, even ordinary old wooden windows can be rehabilitated and dammed much more often than the quick advice suggests to exchange. This can also be cheaper than the exchange (even if there is no promotion).

    Video Board: Replacing Thermal Glass.

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