Here’s U.S. architect and sustainability expert Carl Elefante’s opinion on historic windows and sustainability:

“Preservationists familiar with the restoration of traditional wood windows know every trick to restore their operation and material integrity with the most minimal means possible: a segment of rotted wood replaced here, a patch of glazing compound there, replace a broken pane with salvaged glass that matches the characteristics of the original glass. Many of us have experience restoring 100-year-old windows through such straightforward means.

“For preservationists, it is an absolute mystery why so many “high-performance” windows are designed without any consideration for their renewal. Such systems are sold as maintenance-free. In fact, they cannot be repaired. For example, today’s glazing systems are complex, multicomponent assemblies.While their thermal and solar heat gain performance characteristics may be admirable, window assemblies made out of materials that last for hundreds of years (aluminum, glass) are doomed to early retirement due to “differential durability” problems, for example edge seals that fail in a couple of decades. A 20-year guarantee should not mean that a building element is guaranteed to need replacement in 20 years."

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The life-cycle cost analysis of repair versus replacement of historic windows is also addressed by the Canadian heritage experts Craig Sims and Andrew Powter:

“Despite the irreversible impact on the character and authenticity of the building, anticipated energy savings are rarely achieved over the long term. Removing historic windows should be a solution of last resort, not of first resort. In the residential sector the decision to replace is rarely preceded by analysis and serious investigation of the range of alternatives.”

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Here are a few other good resources regarding historic windows:

APT Bulletin: “What replacement windows can’t replace: the real cost of removing historic windows."

Heritage Canada Magazine: “Windows in historic buildings: sustainable, repairable."

U.S. National Park Service Preservation Brief #9: “The repair of historic wooden windows.”

U.S. National Park Service Preservation Brief #13: “The repair and thermal upgrading of historic steel windows.”

Historic Scotland: “Thermal performance of traditional windows.”

Architectural Conservation Notes #11: “Dave’s top five reasons to conserve historic wood windows.”