Think of the wind whipping around office buildings at street level. Because wind circling a building creates a pressure differential (between the outside air and the inside air) and because that pressure differential increases with height, placing operable windows in tall office buildings is often difficult. The same cross breezes that bring in fresh air also wreak havoc on energy systems and wreck nicely stacked paperwork.
To solve the dilemma of creating an energy efficient building but also allowing people in the offices to open and close windows, the team of Sauerbruch Hutton and Transsolar KlimaEngineering created a double layer, wind pressurized façade for the KfW bank's new headquarters. The modified double envelope system, which the Berlin based architects and Stuttgart engineers call a ‘pressure ring’, regulates air pressure with colorful sensor controlled ventilation flaps.
The traditional double envelope is an efficient system of heating and cooling a building that uses two exterior walls with an air space between to form a super sized insulating cavity wall. But operable windows can upset the system that depends on a tightly sealed building.
On KfW's Westarkade in Frankfurt, the outer glass facade is sawtooth-shaped, where the long sides of the teeth are fixed panes of multi-colored tempered glass and the short sides are mechanically operated vents. A roof mounted weather station monitors wind direction and speed, pressure, temperature, and sunlight. With constant feedback from the weather station and sensors throughout the building, a control system opens and closes the outer facade's vents, kind of like fish gills. The system maintains a ring of positive pressure around the building by continually operating the individual vents.
Regulating the air flow around the building's perimeter allows the inner facade to have operable windows, something a traditional double envelope system does not allow. To manage solar heat gain and excess glare in the all glass tower, automated blinds are also integrated into the cavity between the skins.
Energy efficient buildings with colorful facades are signature design trademarks of Sauerbruch Hutton. The firm also designed Berlin's GSW building, one of the world's first green high-rise office buildings. For the Westarkade, the architects combined red, blue, and green panels that appear colorless when viewed from certain angles. The building's wing-like footprint responds to prevailing wind directions and the sun's path. In addition to the innovative envelope system, the building includes thermally activated slabs, a heat recovery system, and a raised floor ventilation system.
KfW Bankengruppe's investment in their 15 story, $85 million headquarters reflects the sustainable mission of the state-owned development bank. KfW funds national energy conservation programs by administering low interest federal loans for energy efficiency upgrades, including building retrofits and carbon reduction programs. The bank also developed energy standards (KfW-40 and KfW-60) which are used as criteria for credit. Westarkade aims to consume no more than 7 kilowatt-hours (or 24,000 btu) per square foot per year, which is about half the energy used by a conventional European office building and a third of an average US building.