The abbreviation ‘B35’ stands for Bolleystrasse 35 – a sloping site, only about 15 minutes walk from the central railway station. Between 1920 and 1990 a partially underground drinking-water reservoir existed here, which, during that period, never was built over.About half of the structure was preserved, integrated into the basement of the new building and installed with new skylights. It now serves as an office and studio. Above the basement is a 1.5-room apartment on the first floor, which occupies only the front part of the building, and two 170 m2 4.5 room apartments with loggia. The uppermost part of the building accommodates a 136 m2 loft apartment which opens onto two corner terraces.
The only load-bearing elements in the upper floors are the solid external walls and the concrete core of the stairwell. All the remaining interior walls are non-load bearing. In many cases they are constructed as built-in cupboard units, with sliding doors connecting to the external walls and the core of the building. Materials which display their industrial manufacturing process dominate the interiors. Bright anhydrite floors and white plastered walls and ceilings provide a contrast to the dark coloured built-in MDF furniture.
The facade was finished in a dark greyish-brown colour, the porous surface of which is reminiscient of tuff stone. It is made from insulation concrete, the surface of which was sandblasted and then glazed. The facade material, where foam-glass serves as aggregate, in order to achieve higher insulation values, was used for the first time as a cavity wall construction with additional core insulation.
Heat energy for the building is supplied by two systems, which can be flexibly combined. Firstly, there is a cascade of three 9 kW heat pumps, connected to two geothermal probes. Secondly, solar hybrid collectors are installed on the roof. Each of the geothermal probes consists of two U-shaped pipes, of which one reaches a depth of approximately 150 metres and the other up to 380 metres into the ground. The two longer U-shaped pipes are insulated in their upper parts (up to 150 metres deep), thereby preventing heat transfer from the pipes to the subsoil. In this way B35 makes use of two different layers of earth: at 12?°C the shallower layer acts as cold accumulator; and with a temperature of at least 18?°C the deeper layer serves as a heat accumulator.
The hybrid collectors consist of commercially available crystal solar cells. Onto the back of the collectors, a system of aluminium tubes is fixed through which water is fed, removing exhaust heat from the PV module. This increases the module efficiency and supplies approximately 30?°C hot water, which can be fed into the low-temperature underfloor heating system. In summer the modules supply excess heat which is fed through the geothermal probes into the sub-soil and is later available for heating in winter.
Two systems are also used to cool the apartments. Making use of the cold from the shallow geothermal probes, the underfloor heating enables ‘free cooling’, and the excess heat from the apartments is fed through the deeper geothermal probes into the subsoil. Higher heat loads can be removed at night by means of inverting the energy flow of the solar collectors. Instead of gaining heat from the sun, they radiate the excess heat from the building into the cool night sky.
In terms of ventilation too, B35 combines two systems: natural ventilation through windows, and so called airboxes. The latter are decentralised air-intake units, which are embedded into the concrete ceilings near the facades and are connected to each other via circular floor ducts. To pre-condition the air supply, each airbox is connected to the underfloor heating circuit. The floor ducts enable the exchange of air between the airboxes. This prevents rooms on the windward side of the building receiving more air than those on the leeward side during windy conditions. Only very small ventilators have been integrated into the airboxes themselves. Primarily, the central exhaust air facility is responsible for the internal air circulation. It is fitted with a heat exchanger intended to minimise ventilation heat loss.
The only load-bearing elements in the upper floors are the solid external walls and the concrete core of the stairwell. All the remaining interior walls are non-load bearing. In many cases they are constructed as built-in cupboard units, with sliding doors connecting to the external walls and the core of the building. Materials which display their industrial manufacturing process dominate the interiors. Bright anhydrite floors and white plastered walls and ceilings provide a contrast to the dark coloured built-in MDF furniture.
The facade was finished in a dark greyish-brown colour, the porous surface of which is reminiscient of tuff stone. It is made from insulation concrete, the surface of which was sandblasted and then glazed. The facade material, where foam-glass serves as aggregate, in order to achieve higher insulation values, was used for the first time as a cavity wall construction with additional core insulation.
Heat energy for the building is supplied by two systems, which can be flexibly combined. Firstly, there is a cascade of three 9 kW heat pumps, connected to two geothermal probes. Secondly, solar hybrid collectors are installed on the roof. Each of the geothermal probes consists of two U-shaped pipes, of which one reaches a depth of approximately 150 metres and the other up to 380 metres into the ground. The two longer U-shaped pipes are insulated in their upper parts (up to 150 metres deep), thereby preventing heat transfer from the pipes to the subsoil. In this way B35 makes use of two different layers of earth: at 12?°C the shallower layer acts as cold accumulator; and with a temperature of at least 18?°C the deeper layer serves as a heat accumulator.
The hybrid collectors consist of commercially available crystal solar cells. Onto the back of the collectors, a system of aluminium tubes is fixed through which water is fed, removing exhaust heat from the PV module. This increases the module efficiency and supplies approximately 30?°C hot water, which can be fed into the low-temperature underfloor heating system. In summer the modules supply excess heat which is fed through the geothermal probes into the sub-soil and is later available for heating in winter.
Two systems are also used to cool the apartments. Making use of the cold from the shallow geothermal probes, the underfloor heating enables ‘free cooling’, and the excess heat from the apartments is fed through the deeper geothermal probes into the subsoil. Higher heat loads can be removed at night by means of inverting the energy flow of the solar collectors. Instead of gaining heat from the sun, they radiate the excess heat from the building into the cool night sky.
In terms of ventilation too, B35 combines two systems: natural ventilation through windows, and so called airboxes. The latter are decentralised air-intake units, which are embedded into the concrete ceilings near the facades and are connected to each other via circular floor ducts. To pre-condition the air supply, each airbox is connected to the underfloor heating circuit. The floor ducts enable the exchange of air between the airboxes. This prevents rooms on the windward side of the building receiving more air than those on the leeward side during windy conditions. Only very small ventilators have been integrated into the airboxes themselves. Primarily, the central exhaust air facility is responsible for the internal air circulation. It is fitted with a heat exchanger intended to minimise ventilation heat loss.