Curtain Wall Call

Can a 500-ft-tall glass box in Houston possibly be a good idea? The recent expansion of high-rise office buildings in the Central Business District says yes. Spanning a three-block radius in the heart of downtown, three towers — BG Group Place and 609 Main at Texas, both designed by Pickard Chilton, and Capitol Tower by Gensler — are leading a new era of construction in the Bayou City. The Class A office buildings, all of which will be LEED-CS certified, are among the first to incorporate recent innovations in curtain wall technology, leading Houston’s skyline into the 21st century at long last. 

Thanks to a boom in the Texas economy, tower cranes have once again overtaken the city. According to Transwestern’s Trendlines report, in the third quarter of 2014 there were 17.1 million sf of office space under construction in Houston, topping 13 million sf in the San Francisco Bay area, and 9 million sf in New York City. This period of growth comes after a 15-year dearth in downtown development following the recession of the late 1980s’ energy bust. Currently, 23 of Houston’s 25 tallest buildings were built in the 1970s or 1980s, with the remaining two built after 2000 (including BG Group Place, which holds the 15th spot). 

Rather than aim for height records, BG Group Place, 609 Main, and Capitol Tower are making a statement with their all-in embrace of the all-glass curtain wall. The glimmering new facades, composed of increasingly complex curves and angles, stand in contrast to the punched window elevations of their predecessors. 

“The curtain wall is one of the most significant design elements of a building,” said Kristopher Stuart, AIA, of Gensler. “It is the outward expression of the building’s design along with its architectural geometry.” Since its advent in the late 19th century, the curtain wall has juggled a desire for transparency and views with the conflicting realities of thermal performance and comfort. The glass tower has long been the architect’s dream, but a successful building envelope must address a multitude of aesthetic and performative issues: economy of material resources, control of solar heat gain and heat loss, optimization of daylight, minimization of glare, and inspired composition. 

Recently, developments in glazing technology have allowed for maximization of glass in high-performance curtain wall systems. Commonly utilized technologies include multi-pane insulating glass units (IGUs), secondary treatments such as reflective- or low-emissivity (low-e) coatings and application of ceramic fritting, the use of non-glare films, and double-wall construction. While thermal performance and cost of glass may never achieve the level of opaque cladding, the gap is narrowing. It is now possible to design an all-glass curtain wall with previously unheard-of U-values of 0.30 or less, a number that continues to go down.

In addition to improvements in building envelope performance, there have been developments in form-making and fabrication strategies as well. According to Jon Pickard, FAIA, of Pickard Chilton, “The ability of curtain wall manufacturers to more thoroughly consider the design three-dimensionally using the latest software has allowed us to create buildings which are not simply four-sided boxes.” He added, “The continued development of unitized curtain wall systems has allowed for a higher degree of precision for both the assembly and installation of the curtain wall.”

Completed in 2011, BG Group Place was the first high-rise in Texas to earn a LEED-CS Platinum rating. The 46-floor building, located at 811 Main Street, is characterized by a textured, curving tower with a recessed sky garden carved from the upper facade. To protect the building from Houston’s harsh summer sun and intense climate, Pickard Chilton applied two solar shading strategies. Horizontal sunshades of pearlescent white aluminum and ceramic fritted glass protect the gently curving northeast and southwest facades. Equally spaced at two per floor, the sunshades are installed at ceiling and desk height at a slight angle to direct away water and maximize views out. Vertical glass fins spaced 5 ft apart protect the northwestern facade from the low setting sun and extend the entire 630-ft height of the building along Main Street. The sunshades serve an aesthetic purpose as well. “This homogeneous sheath of shading,” said Pickard, “makes the tower appear as a singular form and emphasizes the cut that is created for the sky garden floors. The horizontals sweep the length of the building, emphasizing the curve of the facade.”

Three years after the completion of BG Group Place, construction began on another office building designed by Pickard Chilton for Hines. Located just two blocks east, 609 Main will stand at 48 stories when completed in late 2016. Advancements in BIM software and unitized curtain wall manufacturing allowed for increased geometric complexity. The resulting building design is prismatic, featuring multiple folded faces, a dramatic sloping roof, and column-less glass corners.

Another development present at 609 Main is the lower solar heat gain coefficient of the glass, which allowed for minimization of the external shading strategy for more open views. Here, a lattice of brushed stainless steel tubes along the broad faces and vertical blades on the narrow faces serve a function somewhere between shading and ornament.

Capitol Tower, designed by Gensler for Skanska USA, will be the newest of the three office towers when completed in spring of 2017. The design of the 35-story rectilinear volume of the tower is broken by the insertion of a slender “shard of glass” volume that dissolves the north corner and extends above the roofline. Extruded aluminum blades attached to the vertical mullions provide solar shading at 10-ft intervals on the northeast facade and at 5-ft intervals on the remaining three. Solar modeling showed that deep profiles of the blades would enhance performance. In the interest of conserving material resources and cutting costs, the architects achieved an 11-in depth by setting 8-in-deep blades three inches off the face of the glass. “It’s a very elegant facade that is smart, efficient, and cost-effective,” said Stuart.

The three towers employ similar strategies using low-e double pane IGUs protected by stationary external shading devices. Although these buildings represent the current forefront of curtain wall technology in Houston, there are many exciting new developments to come. Future curtain wall innovations could arise from current development of transparent insulation materials, self-cleaning coatings and thermal films, dynamic facade elements, electrochromic and suspended-particle smart windows, transparent solar cell glazing, phase change materials that absorb and release energy, high-pressure laminate substitutes for glass, and structural glazing that could merge frame and lite into a single component for an entirely transparent facade. 

But must we go all glass, just because we can? Though the notion is dazzling, we need to keep in mind issues of material conservation. Building facades typically have a shelf life of 30 to 40 years, and many current secondary glass processes render the material unrecyclable. Durability and reuse, in addition to development of other exciting facade materials such as ceramics and engineered woods, should be examined in tandem. As cities become increasingly vertical, curtain walls will continue to take on the thrilling task of addressing crucial architectural issues of performance, aesthetics, value, and occupant well-being through the lens of both material efficiency and ingenuity.


Project BG Group Place, Houston
Client Hines
Architect Pickard Chilton (Design Architect) and Kendall/Heaton Associates (Architect of Record)
Design Team Pickard Chilton: Jon Pickard, FAIA; William D, Chilton, FAIA; Brett Spearman, AIA; Yen-Ming Lee; Justin Cochran; Jonathan Aprati; Russell Wilson, AIA; Kendall/Heaton Associates: Laurence C. Burns, Jr., AIA; Thomas Milholland, AIA; Luis Amador; Candy Feuer 
Photographers Peter Aaron/OTTO and Joe Aker

Project 609 Main at Texas, Houston
Client Hines
Architect Pickard Chilton (Design Architect) and Kendall/Heaton Associates (Architect of Record)  
Design Team Pickard Chilton: Jon Pickard, FAIA; William D. Chilton FAIA; Mohamad Hafez; Russell S. Wilson, AIA; Andrew J. Swartzell; Karl B. Hennig; Kathleen L. Maciejko; Taylor Montgomery; Kendall/Heaton Associates: Pat Ankney AIA; Ben Fleener AIA; Matt Upchurch; Warren Carpenter AIA; Raquel Rosas; Lacey Richter
Photographer Joe Aker; Renderer Steelblue

Project Capitol Tower, Houston
Client Skanska USA Commercial Development 
Architect Gensler
Design Team James Furr, FAIA; Craig Taylor, AIA; David Epstein, AIA; Kristopher Stuart, AIA; Rives Taylor, FAIA; Hal Sharp, AIA; Stephanie Burritt; David Alderete; Andres Angel, AIA; Philip Baraldi; Luis Santi-Merayo; Kelly Nicholas; Luis Santi; Suvarna Gupta; Joy Hughes; Andrew Loreman; Jennifer Marshall; Greg Quintero, AIA

Jen Wong is director and curator of the University Co-op Materials Lab at The University of Texas at Austin.

Originally published in the March/April 2015 issue of Texas Architect.