Double Duty: A two-skinned façade combats intense heat

 

Chicago Architect

09.01.2008

Double Duty: A two-skinned façade combats intense heat

By Travis Soberg, AIA, LEED AP

Environmentally responsive exterior walls are key elements in the design of modern, energy-efficient office buildings. This is especially true when designing for regions subjected to extreme conditions like the United Arab Emirates, where daytime summer temperatures can reach upward of 115ºF. Here, building facades not only must mitigate a 40ºF interior/exterior temperature differential, they must also be able to protect themselves from intense sand storms and the constant corrosive mist of the neighboring Gulf coast.

In Abu Dhabi, Goettsch Partners has designed a unique response to these climatic conditions as part ofSowwah Square, a five-building, 3 million-square-foot development.  Using a combination of a mechanically ventilated cavity and a double-skin façade system over large portions of the building, Goettsch Partners provided a solution that both buffered the temperature differential and protected the more delicate components of the façade.

A double skin for a desert climate is fundamentally different than the version more commonly implemented in cooler, northern regions. While both systems rely on creating a cavity of air that cushions the interior from the exterior, a cool-region system is designed to minimize the heat loss from the interior spaces and relies on the increased temperature of the cavity air to do so. In the UAE, however, the opposite is true and the air cavity is called upon to minimize the radiant heat gain from the exterior to the interior occupied zones.

This is important not only from an energy-saving perspective but also as it relates to the occupants’ comfort level. The ability to deliver the appropriate air temperature is only half the battle on occupant comfort. Another 50 percent of an occupant’s thermal comfort can be attributed to the temperature of any nearby radiant source, such as a glass facade.  This is easy to understand if you imagine standing in front of a window on a cold winter night, feeling the chill of the darkness in front of you. Despite the temperature of the air around you, the temperature of the radiant source that your body “sees” provides an equally strong sense of your thermal comfort.

At Sowwah Square, the double-skin cavities run uninterrupted along the entire height of the 31- and 37-story buildings (two of each height), starting from the fourth floor and extending to the penthouse mechanical floors. Within these cavities, active solar shades continuously track and adjust for the sun angle in order to provide optimal shading to the building’s interior. While the shades themselves are robust enough to withstand the expected exterior conditions, the gears that operate them are not. As happens with most gears, corrosive and gritty particles are detrimental to their long-term operation and will cause them to bind, locking the shades into a fixed position. So it was imperative that the double-skin cavities be sealed from the outdoor environment, isolating the shades from airborne particles such as sand and sea salt, which would have compromised their ability to follow the arcing sun.

To meet this need for a sealed cavity, a technique was required that would prevent the cavity from heating up like a greenhouse and increasing the internal radiant temperature. The first step was to minimize the amount of solar energy penetrating the outer layer of the double-skin system. Utilizing an outboard lite with a very high shading coefficient, the design team was able to effectively block 76 percent of the solar energy from ever entering the air cavity. The remaining energy was then blocked from reaching the inner façade by the active shading, although its presence contributed to an elevated air temperature within the double-skin cavity.

To alleviate the accelerated temperature and achieve the moderating air buffer, the warm cavity air needed to be flushed out using an air source cooler than the natural air temperature. The solution Goettsch Partners developed was to collect the exhaust air from the tower offices and, instead of allowing it to escape into the atmosphere, redirect it back down the double-skin cavities, where it is exhausted at the fourth floor mechanical level. Sensors within the cavities will modulate dampers at the top of the building, directing the air to the optimal zones of the cavity depending on the time of day and outdoor temperature. Additional dampers will allow filtered exterior air to enter directly into the cavity during economizing periods, such as night and winter, when the outdoor air is lower in temperature than the collected exhaust air.

Through these efforts, the design team expects the double-skin cavity to be an average temperature of 89º F when the exterior temperature reaches 115º F. This will allow the high U-value of the insulated inner glazing to more easily block the air cavity’s radiating energy.

Most importantly, calculations estimate that the double-skin system designed for Sowwah Square will generate a savings of 7200 kwh of electricity per day (for all four buildings) and provide a more comfortable thermal environment near the perimeter wall, all while protecting itself from the harsh external elements. For Goettsch Partners, these are the results that make environmentally responsive facades such a key element of modern office design.

Travis Soberg is an associate principal and director of sustainable design at Goettsch Partners.

http://www.gpchicago.com/users/news_view.asp?FolderID=1849&NewsID=73

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