The Glass House, Philip Johnson’s renowned personal residence in New Canaan, Connecticut, recently replaced the oversized glass panes of its iconic exterior after cracking due to thermal stress. The home, part of a 14-building compound on the bucolic 49-acre site, now functions as a historic house museum run by the National Trust for Historic Preservation.
Originally completed in 1949, the Glass House features floor-to-ceiling plate glass exterior walls held in place by steel stops and black-painted steel piers of stock H-beams that expressed the mass-produced, industrial materials employed for its design. In the summer of 2019, one of the home’s 18’-0” x 7’-10” panes of existing glazing cracked due to thermal stress. The stress was caused by temperature differentials and a lack of movement within the original steel frame, said Ashley R. Wilson, FAIA, the Graham Gund Architect at the National Trust for Historic Preservation and the lead architect for the glazing replacement project.
While the replacement of original building fabric is often a contentious topic in the field of historic preservation, Wilson noted that the glass that was replaced was “likely an early-generation replacement” because it was 3/8” annealed glass, meaning that it was heat-strengthened glass—a technology not available yet in the late 1940s. The original glass, Wilson pointed out in conversation, was likely a single pane of 1/4” polished plate glass, per 1948 drawings, and would have been “beautifully clear and flat, but fragile.”
Although the team was not replacing the original glass, the project was not without complexity. The new glazing, provided by Canadian glass manufacturer Agnora, needed to meet ANSI safety standards, accommodate wind load (particularly challenging because of the glass’s large dimensions), avoid overloading of the existing steel supporting rail, and, of course, visually match the original design intent as closely as possible.
To accomplish these goals, the selected glass was slightly thicker, at 9/16” rather than 3/8”, and laminated with an inner layer of PVB for safety, said Wilson. To avoid potential corrosion of the steel and clouding of the glass in case the inner PVB layer gets exposed to moisture, the team added weeps to the glass pocket.
The removal of the existing glazing and install of the new presented a new set of challenges: The old glass was prone to more cracking, requiring extra care in its removal. In order to extract the glass, the steel stops also needed to be removed. The steel frames were cleaned, prepped, and painted before the system was reinstalled. Because construction took place in November 2019 to minimize conflicts with tours and programming, the workspace also needed to be heated and protected from the elements, explains Wilson. What’s more, she noted, “the west wall glass had to use a crane to lift the glass unit over the building,”
Wilson contends that the thermal stress that caused the glass to crack was not due to extreme temperature swings due to climate change but rather to “but an inherent weakness and limitations of 3/8” thick glass at such a large size.” But despite the unique nature of the Glass House—and its oversized glazing units—there are clear lessons to be learned from the project about considerations when replacing glass at midcentury buildings.
“At the Glass House, care was taken by the preservation team to analyze the replacement glass options, thickness, performance, and installation to match the original design while improving performance and complying with current safety codes,” said Wilson. While the replacement project focused more on safety and appearance rather than sustainability goals, it did take advantage of how glass technology has evolved since the 1940s and 1950s. More broadly, new glazing products such as IGUs and more effective, longer-lasting sealants can significantly improve energy efficiency for mid-century buildings, allowing for better buildings that are more highly adapted for what the 21st century will bring—global warming or otherwise.