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HOK's Circadian Curtain Wall Connects Office Workers to Outdoors, Reduces Energy

View of a proposed Brooklyn high-rise sheathed in HOK’s Circadian Curtain Wall concept.

A finalist in Metals in Construction’s 2018 Design Challenge, HOK’s Circadian Curtain Wall concept draws on biophilic design to offer building occupants abundant natural light while minimizing solar heat gain.

How can facade systems play a more important role in improving employee health and well-being? That was the crux of Metals in Construction’s 2018 Design Challenge, and it got HOK facade specialist John Neary, AIA, LEED AP, thinking about an idea.

neary-150For the past two years Neary and his colleagues in HOK’s New York studio had been working on a concept for a load-bearing facade for high-rise buildings. That design, Structural eXterior Enclosure, replaced much of the aluminum found in modern curtain walls with steel, giving the facade additional strength to serve as part of the building’s overall framing and, because steel requires one-third the amount of carbon to produce as aluminum, reducing its embodied energy.

More recently, Neary had been toying with a further evolution of the concept: What if curved glass could also reduce the use of aluminum while giving the building skin more strength and wind resistance?

“I was playing with this idea of curved bay windows and putting them into clumps where you have curve atop of curve in a kind of fractalian pattern,” says Neary. “And it struck me that such a curved glass facade also spoke to Metal in Construction’s design brief. It extends the indoors outside and creates an office environment with elements from nature that we know contribute to health and wellness.”

Thus was born the Circadian Curtain Wall.

Geometry Inspired By Nature  

Interior view of the Circadian Curtain Wall looking west toward Manhattan.

As its name suggests, the Circadian Curtain Wall draws on the very real connection humans—and all living organisms—have with the daily cycle of daylight and darkness. The facade’s bubbled glass offers wide-angled views to the outdoors and brings natural light deep into building, keeping occupants synced to the circadian rhythm of night and day. Natural light and views are also key components of biophilic design that have been shown to boost performance and general happiness, notes Neary.

ccw-circadian-clock-300Biophilia plays into the design in other ways, too.

The design competition challenged participants to imagine a facade for a proposed 30-story office building on a waterfront site in Brooklyn. HOK’s concept, submitted with assistance from WSP Built Ecology, imagined the building with an ovoid floor plate that, with the addition of the convex curtain wall glass, would create a flower-like organic geometry.


The ovoid floor plate of the proposed building is accented by the curved glass of the curtain wall and north and south atriums (green) that could include plant life and access to fresh air.

“The enclosure and building form are shaped in response to the path of the sun both discreetly at each window and overall across the oval floor plate,” says Neary. “You can follow the sun around the footprint of the building somewhat like the hand of a clock. The circular geometry of the windows and the floor plan follows the cycle of the circadian rhythm.”

Within office areas, the facade’s bay windows serve to extend the interior out past the building’s main footprint, allowing office workers to surround themselves with views and natural light. Dual atriums on the north and south of the building further connect building occupants to nature. In contrast to the office bays, these spaces bring the outdoors into the building, playing host to distinct plant life in a semi-conditioned space. Flora in the north atrium include species that thrive in low light, whereas plants in the south atrium feature full-sun varieties.

Passive and Automated Shading

The Circadian Curtain Wall’s bulged windows provide external shading even on sides of the building exposed to full sun. At any given moment the tower to the right is 75 percent self-shaded, the one on the left 50 percent.

As the sun moves around the building, the clusters of curved window bays limit the surface area exposed to direct sunlight and provide shading to adjacent areas of the facade. This design allows more natural light to enter the building and reduces the reliance on view-limiting window shades to control glare. Modeling analysis found the Circadian Curtain Wall’s natural shading also significantly reduces solar heat load when compared to a traditional building with a rectangular floor plate and flat facade.

“The oval floor plate exaggerates this phenomenon, but I think the Circadian Curtain Wall could provide similar benefits if used on a rectangular building,” says Neary. “The curvature of the glass means the sun is only hitting the windows at a right angle in a fairly limited area, and the windows’ convex shape is providing some shading elsewhere.”

Even with natural shading, building occupants will require additional sun filters. To address this, the design team proposed automated shades in a ventilated six-inch cavity between an exterior piece of glass and insulated glazing along the interior of the building.

Circadian Curtain Wall’s shades deploy automatically depending on the location and strength of the sun. Here fewer than half of the shades are deployed.

The shades protect each bay window from direct solar gain and glare at all times and automatically open and close in response to the sun’s location and intensity. Housing the shades within a vented cavity supplied with filtered air keeps them clean and reduces maintenance costs, while automating them increases the amount of natural lighting within the building and reduces energy use. In making them mechanical, the design also eliminates human error, i.e. people’s tendency to draw the shades and then forget to open them when the sun is no longer a factor.

A Peek Behind the Curtain

A cavity between the facade’s exterior and interior glazing pulls heat off the building and provides a clean, maintenance-free housing for the automatic shades.

The Circadian Curtain Wall offers several other advantages over traditional building envelopes. The concept’s double skin facade, for example, provides the building with an extra level of thermal and sound insulation. Additionally, the cavity between the outer and inner glass could be used to create a Trombe Wall effect where solar-warmed air within the cavity is used to heat the building during the winter or siphoned off to reduce cooling load during warmer months.

“Trombe Walls were popular in the 1970s when a lot of hippies—including my parents—were building solar houses in places like New Mexico where I grew up,” says Neary. “That background notwithstanding, they are a simple and effective form of passive solar energy that could be used more widely for heating a building in cooler months and shedding heat or storing it for nighttime in the summer.”


The curtain wall’s design limits the exposure to direct sunlight, allowing the automated shades to remain open in parts of each window bay. Air in the cavity space for the automated shades is circulated to aid heating and cooling of the building’s perimeter.

In using curved glass, the Circadian Curtain Wall also has much greater inherent strength and stiffness than it would with traditional flat-panel glass (though the glass is not safe to augment the building’s primary bracing). Wind-load analyses (below) suggest that the curved glass acts as a beam spanning floor to floor and could eliminate the need for supportive mullions every five feet as in most conventional curtain walls.


Convex glass panels measuring 10-by-15 feet could be connected with structural silicone with only a slight aluminum frame needed to house the internal shading units. In eliminating the supportive mullions, Neary’s team estimates that a 30-story building like the one used in the design competition could save 300 to 400 tons of aluminum in 300,000 square feet of skin compared to a conventional curtain wall.

Promising Results

Circadian Curtain Wall grew from an abstract idea to a detailed concept in just a couple months. Though the design team is now refining some of the facade’s initial assumptions and ideas, the initial analysis of the curtain wall has been encouraging.


Daylighting and energy model simulations (above) conducted by WSP Built Ecology found that the system could reduce energy use by 16 percent, peak cooling load by 24 percent and peak heating load by 27 percent in a 30-story building.

“Those are significant amounts,” notes Neary. “Intuitive belief, knowledge about people’s innate need to connect to nature and results from early modeling suggest that Circadian Curtain Wall is a concept worthy of further exploration.”

In addition to Neary, Circadian Curtain Wall’s design team included input and expertise from Michael Miller, Zhenhuan Xu, Blake Kurasek, Marie Achalabun, Mario Claussnitzer, Apoorv Goyal and Matt Breidenthal from HOK’s New York office and Matthew Payne and Elliot Glassman from WSP Built Ecology.

Related: An In-Depth Look at HOK’s Load-Bearing Curtain Wall Concept - Structural eXterior Enclosure