A Window Through Time

Franz Freundorfer applies his knowledge to the development of energy-efficient windows for heritage buildings

Going from modern energy efficient houses to historic buildings must have been a big leap for you. What were your initial thoughts when the building conservationists knocked at your door and asked you to create a ‘historic’ but energy-efficient window for the Waaghaus in Bolzano, one of the case studies of the 3ENCULT project?
I first saw it as a possibility to increase the range of my work in the field. I thought that the biggest challenge would first be to understand the frame of mind of a heritage building conservationist. Then, the second challenge would be to develop an energy-efficient window that respects the aesthetics of a historic building.

There are many windows currently available on the market. Why do we need to develop a new one for historic buildings?
I found out the answer to this question from Waltraud Kofler-Engl, the curator of the Waaghaus (Public Weighing House), after she took a group of us on a stroll through the Old Town to look at the historic buildings, during a workshop in Bolzano. I then realised that today’s windows are massive, often with very wide frames, and they don’t fit the proportions of a historic facade. In addition, contrary to historic windows, modern insulated glazing maintains a constant pressure inside the window, which causes the glass to bow a little to the outside or the inside depending on the external air pressure. Before this project, I had never noticed how dramatic this effect could be. But now it’s clear to me and I can understand better the concerns of building conservationists.

What did you take into consideration when solving the problem?
We agreed relatively quickly that we needed to use a box-style window with two different types of glass. To respect the visual integrity of the historic building, we used single glazing on the outside and an insulating triple glazing on the inside. Our ‘SmartWin Historic Window’ is the equivalent of a ‘countersash’ window, in which two windows are installed close to each other. The next step was to find the right historic profile for the design of the window.

So you not only had to work as a window developer, but also collaborate with the building’s conservationists as a historian. How did you come up with the design idea for your window in the Waaghaus in Bolzano?
Collaboration with the conservationists was essential. And it was fun to search through the drawings—some of which were several hundred years old—and to find out that people back then had many of the same preoccupations as we do now. Of course they had poorer quality glass but, like us, they wanted to get the maximum amount of light into their buildings. They had already thought through the problem and developed solutions that have somehow gotten lost over the years! One of their innovations that I found in a historical drawing was the ‘Geißfuß’, a very smart geometrical arrangement that allows the window sash to be tucked horizontally in behind the blind frame. It was by reusing this idea that we were able to insert the countersash window into an energy efficient frame.

So your window design is a combination of old and new technology—of high-tech materials and a historic narrow frame.
That’s true. You lose much less heat with a narrow frame because a windowpane has far better energy efficiency than a window frame: the narrower the frame, the lower the energy loss. At the same time, more glass gives you greater exposure to the sun. In fact, from this perspective, a window cannot be considered energetically well-engineered until the frame width is zero. And this, in the end, was our aim: to depart from conventional wisdom and aim to save every millimetre on the frame-width we could achieve.

What are the further plans regarding the ‘SmartWin Historic’ window prototype?
Soon we would like to start using a thin layer of glazing for the historic look of the inner window and in a few years we might also try using vacuum glazing. Already at this stage, we’ve received Smart Window certification together with our French manufacturer “André Menuiserie”, and it is ready to be used in other historic buildings as well.

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