Windows are the soul of a building. They let in light and determine how those who live or work in the space see the outside world. Their placement and design determine the balance and beauty of a structure. A historic building with its original windows intact is a gem and can maintain higher property values than one that’s had all its windows replaced. [1]
New Windows Aren’t The Answer
Making a drafty home more energy efficient doesn’t start with replacing windows. The U.S. Department of Energy has found “weatherization to be a more cost-effective option in decreasing energy bills.” [2] And a report by the National Trust for Historic Preservation Green Lab determined original windows can be retrofitted to perform like high-end replacement windows, challenging “the common assumption that replacement windows alone provide the greatest benefit to homeowners.” [3]
Image: window restoration (left) in Frank Lloyd Wright home.
Keeping the original windows also preserves the integrity of historic structures. Since older homes have often settled with age, the windows have settled too and are out-of-square in a way that fits the building. But the reason to keep original windows goes beyond that. Historic windows are made of two extremely rare materials: old-growth lumber and handmade, wavy glass. 
Durable Old Growth Lumber
Old-growth wood originates in unharvested virgin forests and can be 200-300 years old. [4] The trees grew slowly since there was limited light and competition from other trees, making the wood dense and rot resistant. Old growth wood will last indefinitely if properly cared for. They have many compact tree rings compared with newer wood that is second-growth or grew quickly in tree farms.
The Beauty of Handmade Wavy Glass
Today, windows have optically-perfect machine made “float” glass, which was initially manufactured in the late 1950s. [5] Before then, window glass had been made the same way for centuries: it started with glass blown into cylinder form. The “cylinder” process was mechanized after 1900 but still often had waves, ripples and air bubbles or “seeds.”
Restoring Old Windows
Historic wood windows sometimes need to be repaired and restored, which is far better choice than new replacement windows. Depending on the level of repair needed, a homeowner with extra time may be able to do it herself. [6] If the necessary work is more extensive, there are new resources for finding local window preservation experts like the Window Preservation Alliance.
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Bibliography
[1] Simon Aldridge, In: McKeough, Tim, “Market Ready.” New York Times. September 12, 2012. https://www.nytimes.com/2012/09/13/garden/old-windows-worth-keeping-in-a-historic-home-market-ready.html?mcubz=0
[2] Gil Sperling, In: Wald, Matthew L., “Focus on Weatherization is Shift on Energy Costs.” New York Times. December 30, 2008. https://www.nytimes.com/2008/12/30/us/30weatherize.html?mcubz=0
[3] Preservation Green Lab. Saving Windows, Saving Money: Evaluating the Energy Performance of Window Retrofit and Replacement. National Trust for Historic Preservation. 2012. https://forum.savingplaces.org/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=715cccb6-9a30-d72d-e807-39d18f2cf52f&forceDialog=0
[4] Sidler, Scott. “Why Old-Growth Wood is Better.” Web blog post. The Craftsman Blog 1 June, 2015. Web. 6 Sept. 2017 https://thecraftsmanblog.com/why-old-growth-wood-is-better/
[5] Hewitt, Alan M., Bock, Gordon. The Vintage House: A Guide to Successful Renovations and Additions. New York, 2011. Print.
[6] Myers, John H. The Repair of Historic Wooden Windows. U.S Department of the Interior. 1981. Preservation Briefs: 9 https://www.nps.gov/tps/how-to-preserve/preservedocs/preservation-briefs/09Preserve-Brief-Wooden-Windows.pdf
After Restoration: Preserving Windows into the Future
Improving the efficiency of old windows so they perform like new double-panes is not difficult. Indow interior window insert are laser-measured to precisely fit out-of-square windows without a damaging track or magnetic system. They are edged in silicone and press into the interior of the window frame, making them nearly invisible.
A U.S. Department of Energy study found that Indow inserts reduced heating, ventilating and air-conditioning costs in a Seattle home by 20 percent.
