The Passive House Standard, although increasing in popularity around the world, has until now only been applied to residential and small commercial buildings. For the first time, an office tower in Vienna, the corporate headquarters of Austrian Raiffeisen-Holding, a cooperative conglomerate that runs agricultural and financial-services businesses, has received certification. Set along the bank of the Danube Canal, the Raiffeisenhaus Wien.2 or RHW.2 Office Tower's glazed double-skin facade rises to a height of 240 feet, offering great views over the city and 200,000 square feet of office space. Atelier Hayde Architekten and Architekt Maurer, two Vienna-based architecture firms, designed features to meet the strict Passive House Standard using the sun, water, earth, and air as much as advanced building technologies.
Location Vienna (Danube River watershed)
Gross area 237,139 ft2 (22,031 m2)
Cost $111 million
Date completed December 2012
Annual purchased energy use (based on simulation) 21 kBtu/ft2(233 MJ/m2)
Annual carbon footprint (predicted) 2.9 lb CO2/ft2 (14 kg CO2/m2)
Program Office areas, garage, conference area, restaurant, kindergarten
TEAM & SOURCES
Doors Peneder, Hochgerner
Office furniture Bene, Neudörfler, Hochgerner, Steelcase
The site is tight, flanked on either side by two taller buildings—an existing Raiffeisen-owned building on the north, which is connected to the new building by a bridgelike wing, and an IBM office tower on the south. The new tower's drop-shaped floor plan is the direct result of Austrian codes, which regulate the amount of shadow a new building can cast on adjacent structures. At night the all-glass facade is lit up by 5,500 LED fixtures, which transform the headquarters into a beacon of white light. The $111 million building includes a kindergarten, fitness and medical center, cafe, and bank.
Three main factors allow the building to meet the Passive House Standard: the thermal efficiency of the well-insulated double facade, the use of daylight to reduce electrical lighting requirements, and the advanced mechanical systems. A shading system in the cavity between the facade layers operates automatically but can be over-ridden by the employees, according to their preferences. The tower is only 60 feet deep, allowing natural daylight to penetrate far into the interior from both sides, while the inner layer has operable windows for natural ventilation. Occupancy and brightness sensors in each office reduce the light and cooling loads.
An array of photovoltaics and a biogas-based combined heat and power (CHP) plant supply energy. The hot water for the absorption chiller comes from this CHP. Due to the area utility company's regulations, electric power generated by the CHP may not be fed directly into the municipal low-voltage system but must flow into the public grid by means of a local transformer. If the power taken from the grid is measured against the power fed into the grid, the result is that the CHP plant generates 60 percent of all power required for the building. About 40 percent of the heating needs are delivered from the CHP system, with the rest coming from the waste heat of a neighboring data center.
"With this project, nobody knew whether we'd achieve the Passive House Standard," explains Sarah Richter, project architect with Atelier Hayde Architekten. "It was a challenge for all parties involved, because this is a tall office building with a limit to how much energy it's allowed to consume. We had to recalculate often."
Since the RHW.2 tower is located next to the Danube Canal (an artery of the Danube River), water from the canal is used for cooling the offices in summer. A geothermal pump functions as both a heating and cooling system, but, of the total cooling, only accounts for 8 percent.
While only certified-ecological building materials were used, the embodied energy of the elements is not part of the equation in Passive House requirements. Buildings that follow Passive House Standard are well insulated (both walls and windows have high R-values, low U-values, and no thermal bridges), airtight, and require minimal heating. Most heat comes from the sun, building occupants, and equipment. They may not use more than 1.5 kWh per square foot, per year, in heating. Overall, a Passive House can use only 12 kWh per square foot, per year of energy. All numbers are projected, and a sophisticated monitoring and evaluation system is currently under way. The European Union requires that all new construction be low-energy by 2020.
The RHW.2 Office Tower was certified in accordance with the criteria of the International Passive House Institute in Darmstadt, Germany, a network of stakeholders including building owners, architects, scientists, and manufacturers. Radically increased efficiency in the facade, building component connections, and mechanical systems played a critical role in the certification process. In combination with optimized shading equipment, the heating and cooling load was reduced by 80 percent in comparison to conventional high-rise buildings. Passive House–certified projects are still more expensive up front than conventional buildings. It is estimated that the $3.6 million in extra costs for RHW.2's meeting the Passive House Standard will, through the lower energy costs, have a payback period of 14 years.
"The world's first Passive House office tower is proof that the fossil-fuel era is finally coming to an end," says engineer Günter Lang, the head of the Passive House Standard movement in Austria. "Even skyscrapers can now derive energy from renewable sources in situ. This project is a breakthrough."