An ordinary house has a dozen windows, a large multifamily house often has several hundred windows and the 449-meter-high Empire State Building in New York City has 6,514 windows. The purpose of windows is of course to provide natural light to the rooms, but they are also important components in the building’s appearance and energy balance. Old windows were often built of the best, most resistant and affordable timber that could be found. Many older windows were very beautiful, but they did not insulate very well against cold and noise.

Therefore, replacing old windows with new ones with better performance should be obvious. This is done in many houses and buildings at a considerable cost to tenants, and is sometimes worse for the building’s appearance. Disruptions to the tenants during the conversion can also be significant.

Energy consumption is 15 to 20 percent lower with insulated glass.

“We have been renovating and painting windows for more than 25 years. We realized that there must be better solutions than throwing out the old windows and replacing them with new ones,” says Conny Grundel, CEO of Grundel Window System.

“In the early 90s, we had jobs to improve the noise-insulating ability of windows. At the same time, energy issues became increasingly important for property owners. Our solution was to install double-glazing on the inside of the existing windows, and I think we are quite alone in the world on this particular technology. Our double-glazing is patented and manufactured in Oxelösund,” continues Grundel. “The cost of installing double-glazing is about a quarter compared to buying new windows.”

The glazing in place

“Working in a building does not disturb tenants. The existing interior glass is cleaned thoroughly and then we mount a steel section with energy glass on the inside of the window. The new inner glass is double sealed with silicon and a silicon decorative strip so that it blends in with the window’s appearance,” explains Grundel. “We expect that the energy consumption is 15 to 20 percent lower and inconvenience from cold drafts decreases, resulting in improved living comfort. It will also be much quieter inside. The design has a 10-year warranty and we expect that the investment will pay for itself within 7-12 years.”

Grundel uses a proven model that includes a number of preparatory steps, such as assessing the quality of the existing windows and seals, as well as measuring the window surfaces. The glazing is mounted in a three-step process. The process takes about 30 minutes and insulating glass mounted on an existing 2-glass windows has an energy value of 1.0 to 1.4 ° CW/m2.

So here’s the deal:

  • A frame of galvanized steel is mounted on the existing and clean window.
  • The frame is heated and vulcanized with insulating glass.
  • A decorative strip conceals the joint between the insulation frame and the old window. The three steps do not give rise to any construction dust or demolition materials and the premises can be quickly reused.

So far, nearly half a million double-glazed windows have been installed in commercial and residential properties.

Energy insulation of Årsta Castle

Årsta castle in Södertörn in Södermanland, Sweden is a beautiful building dating back to the 1600s. The building has over the last 400 years been restored a few times but the exterior has remained relatively intact. The castle houses a restaurant and a museum and has been used in various film and television productions to create historical environments.

Grundel installed double-glazing on all windows on the Årsta Castle.


One problem for the staff, are the drafts caused by leaky and poorly insulated windows. The castle’s comfort and energy efficiency needed solving and the measures included the installation of a geothermal heat pump and a window restoration using Grundel’s glazing.

“Årsta Castle is a building with a look that is essential to maintain and therefore Grundel’s insulation was a good solution for us,” says Kaj Nilsson, energy management engineer at the castle. “Grundel in the autumn installed double glazing on all windows of the castle. I now look forward to positive results on both the indoor climate and our energy consumption at the castle.”

The glazing can be installed on all types of windows without affecting the appearance of the building. This is something that is perceived as beneficial to buildings where the need for energy savings and improved indoor climate should not impinge on the classic look.

Another example is Westmanska Palace in Norrmalm in Stockholm, where the 200-year old building’s rustic windows retain their character by the existing windows complemented by double-glazing.

The article was published in September 2013