Early Modern Walls
In the mid-20th century, with the advent of centrally controlled HVAC systems, predominantly of the variable air volume (VAV) type, and with the requisite over or under pressure conditions to drive return air, operable windows were felt to be unnecessary, even detrimental to system performance. The building skin became a hermetically sealed membrane, absolutely decoupled from the HVAC system.
The wall did retain influence in that a better thermal barrier at the wall would allow for smaller and less complicated HVAC systems. In the 1970s through the 90s, most designers felt that a balance between HVAC system costs and wall design would allow 40 to 60 percent vision glass. Spandrel glass, stone, or other facing materials made up the remainder of the wall surface.
Beginning in the early 80s, improved energy efficiency in vision glass allowed its use as a more predominant component of the building skin. Highly selective low-e coatings, gas fillings, and better thermal characteristics of components allowed architects to design walls nearly completely of glass.
One example was the Reedy Creek Improvement District (RCID) building, the facilities department for Disney grounds and properties in Orlando, Florida designed by Murphy/Jahn. Despite the hot climate, a building of extreme transparency and luminosity was feasible, and 100 percent of the glazing is vision glass.
RCID represented a culmination of a pursuit toward visual transparency, but as is often the case, success exposed the limitations of that goal. While visually transparent, the building remained a sealed container, entirely dependent on environmental control systems.
We realized that our goal of re-energizing the development of curtain wall design was in pursuit of a broader concept: "environmental transparency." Visual transparency was not enough; we felt that wall systems should engage all positive aspects of the environment.
Decentralization and Integration
With the goal of environmental transparency in mind, Murphy/Jahn designed the Bayer Headquarters in Leverkusen, Germany with an active, environmentally responsive facade. A double skin creates environmental buffers and zones of control to address occupant comfort.
To provide solar control, 12-inch- (30-centimeter) wide perforated aluminum louvers were located between the two glass walls, exterior to the occupied space. The perforation pattern was carefully designed to maintain visual connectivity to the exterior while minimizing glare and insolation. The ends of the building, without double glass walls, are shaded by an exterior metal mesh. In both cases, direct solar gain is blocked so as not to produce a secondary radiant heat load in the occupied space.
Between the two glass walls of the double skin develops a laminar air flow that further controls heat transfer. The louvers separate two layers of air. The outermost layer is heated by sunlight on the exterior glazing and the blinds. This heated air moves up due to a natural chimney effect and exits through outlets at the roof. This convection is fed by fresh air entering via inlets at the underside of the the glass "shingles."
The inner layer of air is cooler, introduced through ground ducts to the interior side of the blinds. This configuration draws air into the occupied space across tempering fin-tube convectors, creating a comfortable temperature-controlled natural ventilation. Radiant coils within the exposed concrete ceiling are the main source of temperature control, reducing the role of air as a temperature-exchange medium.
The lower air volumes required result in lower velocities, allowing natural stratification to separate the heated air for extraction at grills placed high in the room. An under-pressure air system or fan box draws the ventilation air over the fin tubes at each module. A low-volume distribution system transmits the required hygienic ventilation air at a base level temperature, allowing the tempered exterior air to be controlled by individual occupants, according to their choice for both air volume and temperature.
The inner wall contains operable windows, which may be controlled by the occupants. These allow for the influx of large quantities of fresh air when exterior conditions allow. The venting of the buffer zone can be modulated to extend the period throughout the year when exterior or buffered exterior air is suitable for natural ventilation.
Murphy/Jahn extended the principles established in the four-story Bayer Headquarters to the 40-story Deutsche Post highrise in Bonn, Germany, where a fan-powered slab-edge fin tube convector was put into full production. There are no interior shafts or air handling equipment.
Scale Establishes Concepts
The next step in the evolution of these double-skin walls was to integrate such decentralized environmental control systems within the standard curtain wall thickness. In the MAX Project in Frankfurt and the Hochhaus Ensemble Am Munchner Tor, in Munich, the physical properties of the 6-foot- (1.8- meter-) thick composite walls of Bayer and Deutsche Post are accommodated in 10 inches (26 centimeters).
For solar control in these newer buildings, an exterior floating plane of laminated heat-absorbing glass shields the fixed, insulated vision unit. To the side, a perforated metal panel shields the operable vent window. Glare control can be provided by normal interior blinds without a thermal penalty because the solar heat load is stopped at the exterior glass plane.
The floating plane of heat-absorbing glass is 6 inches (15 centimeters) from the insulated glass unit, with an open vent slot at the top and bottom. The "waste" heat converted by the glass is expelled by natural convection before reaching the interior spaces. Visibility is not impaired.
The horizontal slots in the metal panel provide air directly to the slab edge convector unit as at Bayer and Deutsche Post. Fresh air enters directly from the exterior, and internal baffles control excess pressures, noise, and pollution.
An operable window is located behind the perforated metal panel. The extent and size of these openings in the metal exterior shield will be adjusted during design to balance the air volumes that enter the tenant space. For example, at the top of the 660-foot- (200-meter-) high MAX, high wind speeds dictate more restricted airflow through the metal, thus narrower units or denser panels. This panelized wall system can be considered an "appliance" that integrates heating, cooling, ventilation and daylighting.
Adoption of these technologies in the United States has lagged behind European applications because of more restrictive temperature design ranges and a preference for deeper, more open office spaces. But as the comfort levels of an environmentally transparent workspace become known, and as these control technologies develop, their global acceptance seems inevitable.
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John Durbrow, AIA worked on these projects while senior vice president/ principal architect at Murphy/Jahn in Chicago. He is currently assistant professor of architecture at the Illinios Institute of Technology in Chicago and is principal of Bluework Design. Matthias Schuler of Transsolar was EnergieKonceptor for all projects but RCID.