By Ashley Wilson and Anna Blyth

Philip Johnson’s Glass House, the National Trust for Historic Preservation Historic Site in New Canaan, Connecticut, spans 49 acres that once belonged to one of the United States’ prominent modernist architects. Johnson used the property as a playground for his imagination, where he created 14 structures that allowed him to experiment with form, material, and their interaction with nature. Like all historic properties, these buildings require continued maintenance to ensure their survival and integrity, when two large panes of glass unexpectedly cracked at the most famous of the site’s buildings, the Glass House, it became apparent that unplanned major maintenance work would be required.

As its name suggests, the house’s most striking feature is the all glass exterior inspired by Mies van der Rohe’s Farnsworth House. In the summer of 2019, two of the largest glass panes cracked within a few months of each other due to differential temperature and corrosion in the glazing pocket. The glass was stabilized in place until the site closed for the season in December and at that time, the staff at the Glass House and the National Trust oversaw the replacement of the damaged glass and the repair of the related window frames.

We spoke with Brendan Tobin (BT) the Buildings and Grounds Director that managed the construction project and Joshua Freedland (JF) of Wiss, Janney, Elster Associates, who is the conservator for the restoration to find out what makes this such a unique preservation project.

What makes this restoration project different from a common window replacement?

BT: Our site is open to the public seasonally from mid-April to mid-December to allow our guests to take advantage of the best weather of the year. To minimize disruption to the Glass House’s programming and events, we scheduled the restoration for December. This introduced considerations for how to best protect the building, workers, and temperature-sensitive materials from the cold. The replacement was much more complex than removing and reinstalling glass as the steel frames and stops were also conserved, creating a few weeks that the windows and frames and exposed to the elements.

JF: Because of the sheer size of these panes and as one of the most dominant elements of the house, extreme care had to be taken in selecting and replacing the glass. The glass panes we replaced are about 8 feet tall and 18 feet long and make up more than a quarter of the facade area.

What considerations went into selecting the appropriate glass?

BT: Of course, we wanted to maintain the architectural intent of house. The original glass from the 1949-1950 construction would have been single pane, polished plate glass, which is beautifully clear and flat but fragile. The glass we replaced was likely an early generation replacement of annealed or heat strengthened single-pane glass. We had to find a glass that was equally clear, without visual distortion and still thin but able to withstand wind and force.

JF: Besides not having roller waves or edge curling that can result from the glass manufacturing process, the glass needed to be able to meet current codes for wind resistance and safety. The new glass is slightly thicker than the replaced cracked glass. The new glass is 9/16” thick and laminated with an inner layer of PVB [polyvinyl butyral] for safety.

What precautions were taken to protect from inclement weather?

BT: A rolling, movable protective room with a cover, almost like a three-sided shed with a roof, was built on the exterior side of each piece of glass that created room for the workers to maneuver and that could be heated. The ‘shed’ needed to seal well enough to maintain heat and keep out snow or rain, but it also had to be easily removable to accommodate the glass installation. The protective covering had to be wheeled back away from the building as the new glass was lifted in.

JF: Plus, there were challenging material considerations. The in-situ broken glass could break further and injure workers so it had to be removed carefully. If the inner PVB layer in the new laminated glass is exposed to water, the glass can become visibly cloudy. We added weeps to the glass pockets to keep water from being trapped in the glazing pocket that could cloud the glass. Care was taken to specify surface preparation and a coating system to protect the steel window frame from corrosion.

Is there more glazing/glass restoration in store for the Glass House?

BT: The building contains 36 pieces of glass—some original and some early replacements—and the likelihood that many of them will be replaced again is high as stresses make each of them reach the end of their useful lifespans. Ongoing maintenance is always happening to keep the Glass House preserved.

JF: The good news is this was a very successful prototype project. Now we have a methodology and specifications for replacing glass if additional pieces break. We know what to do to keep the building consistent and make the installation process run smoothly.

Ashley Wilson is the FAIA Graham Gund Architect, and Anna Blyth, is the Silman Fellow for Preservation Engineering at the National Trust for Historic Preservation.