Concentration of CO2 in the Atmosphere

OpenSash: Retrofitting Existing Windows for the Many Since 2010

: Glazing a window at the Vermont College of Fine Arts in 2014. Photo: Jeb Wallace-Brodeur.

Christopher Pratt

I began repairing windows in Portland Oregon in 2007. It was good work, but climate change was steering people toward energy efficient replacement windows that didn’t require storm windows. I was about to give up, until I discovered a method of adding a single pane of low-emittance (low-e) glass to the outside of the upper and lower sash of existing wood windows. The glass pane was simple to replace, affordable to apply, and didn’t change the look or function of the windows from the original. It was based on the Marvin Window Energy Panel system that was introduced in the mid-50s and continues to be sold today. Being able to add glass panels to old windows in this way makes it possible to weatherize old windows to an efficiency level that compares well to modern windows with typical insulated glass

How it Compares

The National Fenestration Rating Council (NFRC ) test conducted on the Marvin low-e energy panel (EP) windows gives it a U-factor of 0.35. Modern argon-filled low-coated insulated glass units have U-factors that range from 0.23 to 0.30. To put this in context, the u-value of single pane glass is 1.0. Single pane glass with a low-e storm window is around 0.5. What is more tangible is how much warmer the inside surface of the glass is when the temperature is -20º F outside. Inside temperature is zero for single pane, 46 for the low-e EP, and 52 for a LoE2 272 argon filled insulated glass unit. Another major component to weatherization is air infiltration. It is hard for me to get laboratory professional quality air-change numbers, but this problem is certainly one that good seals, window sizing and good execution can solve.

A window retrofit in 2016 at the Vermont College of Fine Arts in Montpelier, VT. Image: Christopher Pratt.

Cost is Key

These numbers are good but serving the needs of the many and reducing our carbon emissions for the many means making the OpenSash retrofit system affordable. After ten years of being in business, I have managed to keep the cost at about $500-600 for a standard sized window. The cost of adding the glass runs about $200 per window and the seals are another $100. The rest of the cost is in restoration and repair.

This fall I participated in the most recent NESEA virtual trade show as one of the sponsors with my own booth. The 2020 theme was how to conserve carbon in the retrofitting of existing buildings, a natural fit for OpenSash. I made a lot of good connections; I was not selling new high-performance products or services. I was the only sponsor in the repair business, and I was alone in my service. We did not share a common definition for retrofit. In the commercial window world, a window retrofitmeans throwing the old windows out and putting new ones in. We dont save carbon if the new affordable replacement window cannot be repaired and has life span of 10-15 years.

The Keynote speaker at the NESEA conference, Rev. Mariama White-Hammond, was enthusiastically applauded for an address that put this simple question to us, Who do you serve?How do you answer that question when you are making a new high performance product that is unaffordable to most Americans? I do my best to targeted middle- to low-income people because that is where the greatest weatherization needs are and the greatest carbon savings are. I am less likely to appeal to wealthy clients who demand the highest performance modern replacement windows on the market.

Climate change mitigation requires that achieving window efficiency be durable and accessible. Smaller, locally-owned businesses could be doing the job of window repairs, retrofits and weatherization for existing windows by aiming for a middle path in energy efficiency. The new green economy reduces material use, increases local employment, and makes quality repairable products. I have a great business plan for anyone who wants to get serious about window retrofit could do and be part of the climate solution.

Christopher Pratt owns OpenSash, a window retrofit company. He lives in East Montpelier, VT. More information on OpenSash can be found at

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