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On route to a low-energy urban district
Redeveloping an urban district represents not just an architectural challenge – the tenants behind the facades must also be taken into account. Those involved in the “Weingarten 2020” research project are aiming to meet both demands. With the involvement of the Fraunhofer Institute for Solar Energy Systems ISE, they have created the first passive high-rise building in a district of Freiburg. A further important milestone was the optimisation of the energy supplies.
With its 16 storeys stretching across 45 metres, the first fully refurbished building at Bugginger Strasse 50 certainly deserves the title “flagship” project. It is one of four “research buildings” in the Weingarten-West urban district in which refurbishment measures and innovative technologies are being implemented as pilot projects. For the existing high-rise building owned by the Freiburger Stadtbau company, this specifically meant renewing windows, doors, building services equipment, floor structures and the entire interior fittings and bringing them up to a passive house standard. Only the building’s shell remained intact.
© Markus Löffelhardt
© Fraunhofer ISE
© Fraunhofer ISE
As part of this energy-oriented refurbishment, the most serious thermal bridges were the long, adjoining balconies. The project managers decided to integrate the balcony areas into the interior space and thus within the thermal envelope. This provides advantages not just in energy terms but also economically: it created additional living space and allowed 139 smaller apartments in different sizes to be created from the previous 96 apartments. This also enabled their floor plans to be reconfigured so that three-room apartments now cover the space previously used by just two-room apartments. In addition new, thermally separated balconies were installed. This increased the total living space after the refurbishment from 7,100 to 8,100 square metres.
The facade was clad with a 20-cm-thick thermal insulation composite system. Depending on the existing wall structures, this enables U-values between 0.11 and 0.13 W/m2K to be achieved. Aerogel was used for the first time as thermal insulation behind the roller shutter casing, at the transition from one window to the next, with the steel structures and in other places where there was little space for thermal insulation. With a Lambda value of 0.013 W/mK, this material has a low thermal conductivity. Since the hot water is provided centrally with circulation, the distribution losses are relatively high. The project participants attempted to counter this by providing better insulation for the circulation pipes. However, the losses were only reduced by a marginal amount. The controlled living room ventilation with heat recovery is achieved using central air intake and air extraction systems. “Because of the size and dimensions of the highrise building, we decided to use ventilation equipment that is normally deployed in office buildings,” explains Florian Kagerer, scientific project head at Fraunhofer ISE. “This uses a highly efficient cross-flow double heat exchanger and fans.” The central plant is placed in a dedicated plant equipment floor that is located on the roof as a thermally separated steel structure. This facilitates the servicing and also provides economic advantages. The construction cost around 13.4 million euros (for the cost groups 200 to 700 according to DIN 276 Buildings Costs). This sum corresponds to 1,680 euros per square metre, of which approximately 600 euros are for the energy-oriented refurbishment and 240 euros for other modernisation measures.
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