Geothermal System Improves Facility Performance While Preserving Historic Character

With its soaring spires in the heart of Manhattan, St. Patrick’s Cathedral is among the world’s most iconic worship spaces, welcoming nearly 5 million visitors a year. Originally designed in the 1870s by James Renwick Jr. and last renovated in 1949, the cathedral’s recent five-year-long, $177 million renovation combines stringent conservation methods with the city’s largest-ever geothermal system to enhance worship and functionality, as well as improve sustainability and resiliency.

Led by MBB (Murphy Burnham & Buttrick Architects), New York, the comprehensive effort impacts the entire cityblock- sized campus, achieving a 29 percent reduction in annual energy use while stabilizing significant historic fabric. Recognized with a 2019 National COTE Top Ten Award from the Washington, D.C.-based American Institute of Architects, the project offers valuable lessons for making significant historic buildings effective, relevant and resilient, as well as provides a compelling case study about creating sustainable building systems with a focus on long-term, flexible solutions.

Cathedral Upgrades

A designated city, state and federal landmark, the cathedral is one of New York’s most visited destinations. Working closely with the Trustees of St. Patrick’s Cathedral and clergy leadership, MBB approached the renovation with a focus on stabilizing the cathedral structure and providing a better experience for clergy, visitors and worshippers while respecting the character of this sacred and iconic building. The project scope included preservation of exterior and interior surfaces—marble, slate, metalwork, ornamental plasterwork, decorative woodwork, cast stone and stained glass. New architectural interventions include mechanized glass entry doors that reduce air infiltration and a glass wall that provides acoustic isolation for the Lady Chapel worship space while life-safety upgrades include the installation of a mist fire-suppression system in the attic and improving egress by upgrading exit paths and instituting a fire marshal program.

The extensive renovation also presented an opportunity to address the cathedral’s energy usage in a comprehensive and holistic way. From the project’s inception in 2006, the design team was charged by the Trustees of St. Patrick’s Cathedral to evaluate the existing mechanical systems as part of a needs and conditions assessment of the cathedral campus. It was clear the air-conditioning system was overdue for an upgrade; much of the HVAC equipment dated back to the 1960s. Leaders sought a permanent solution with a high level of functionality that was also in keeping with how the cathedral building is used. Having a minimal impact on the historic fabric was important. As the renovation gathered steam, the project team assessed geothermal technology as a potential means to meet the trustees’ objectives.

At the time, there were few examples in New York City of large, established institutions deploying this technology. The project team initially considered a design for a conventional system using a large fan wall adjacent to the Cardinal’s Residence near the southeast corner of the block. Because of the required size of the plant, mechanical elements would have been visible; modifications to the cathedral architecture would have been required, along with extensive and costly rock excavation; and the fan wall would have generated noise—a series of modifications that would not only impact the city’s Landmarks Preservation Commission approval process, but also the ongoing experience of cathedral visitors and clergy residents.

By contrast, a quiet, low-impact geothermal system obviated the need for noisy, obtrusive mechanical equipment and promoted efficient use of space. Conducted in collaboration with construction manager Structure Tone Inc., New York, the project team’s feasibility study examined upfront costs; long-term costs, including equipment replacement, operation and maintenance; and qualitative and quantitative criteria, such as impact on the cathedral’s fabric and the duration of the overall approvals process. The geothermal plant option brought the best long-term benefit in terms of cost, energy efficiency and minimal impact to people. Just as important, this proposal engendered the support of community groups and local officials, including the Landmarks Preservation Commission.