The Value and Care of Historic Windows: Studies + Research

The retention of original windows has always been a high priority in preservation. A renewed emphasis on saving energy is shaping stringent new policies and mandating prescriptive building codes while voluntary ratings are incentivizing tight building envelopes and efficient appliances. Original windows remaining in historic buildings whose maintenance has been neglected over time are first on the list to be replaced.

Preservationists know that low-tech historic windows that were made to be durable and easily maintained represent real energy and resource conservation over time. But where is the quantification of this knowledge to supply the forms required to meet current rating and code thresholds?

Existing window efficiency and repair effectiveness studies have been performed for decades and continue today. Many studies objectively demonstrate the viability of original window repair. The effort to quantify restored window thermal performance and reduce material and manufacturing impacts to the environment has made a difference in how the building industry has responded to existing window repair. Retention of historic windows have become incorporated into efficiency ratings and building codes. Existing areas of qualified historic buildings to remain are exempt from energy codes. Building codes include a "performance" method for analysis of the overall thermal performance of the building envelope, as opposed to the "prescription" method that requires the performance of each individual part to meet specific standards.

All the studies listted here examine the test results of original window repair and other conservation strategies with  new window replacement to compare energy efficiency and impacts on the environment.

13 Things to Know About Retrofitting Historic Windows

Julia Rocchi discusses how historic windows often get the blame for a building’s energy loss in this article from the National Trust. In it, Ms. Rocchi identifies issues and goals that consumers should consider when determining how to repair therir windows. The article includes a slide show, which is shared here.

Ms. Rocchi also promotes the National Trust report from the Preservation Green Lab in Seattle, "Saving Windows, Saving Money: Evaluating the Energy Performance of Window Retrofit and Replacement" (reviewed below) which is also useful when planning historic window repair.


National Trust for Historic Preservation Window Evaluation

A report produced by the National Trust for Historic Preservation Green Lab provides cost guidance for homeowners weighing the financial and energy tradeoffs between replacing or repairing older, less efficient windows. This report, "Saving Windows, Saving Money: Evaluating the Energy Performance of Window Retrofit and Replacement", builds on previous research by examining multiple window improvement options, comparing them to replacement windows across multiple climate regions.

Report cover of Saving Windows, Saving MoneyKey findings offer homeowners, contractors, architects and others with compelling evidence of the merits of retrofitting windows as opposed to outright replacement:

  • Retrofit measures can achieve performance results comparable to new replacement windows. This study shows that there are readily available retrofit measures that can achieve energy savings close to new, high performance replacement windows when the performance for each upgrade option is taken into account.

  • Almost every retrofit option Offers a better Return on Investment (ROI) than replacement windows. Findings from the cost analysis showed that new, high performance windows are by far the most expensive measure, costing at least double that of common retrofit options when considering materials, installation and general construction commonly required for an existing home. In all climate zones analyzed, cellular shades, interior storm panels and various exterior storm window configurations offer a higher average return on investment compared to new, efficient replacement windows.

  • The Bottom Line. Retrofitting windows with high performance enhancements can result in substantial energy savings across a variety of climate zones. Selecting options that retain and retrofit existing windows are the most cost effective way to achieve these energy savings and to lower a home’s carbon footprint. Retrofits extend the life of existing windows, avoid production of new materials, reduce waste and preserve a home’s character.


Window Preservation Standards Collaborative

In the Fall of 2010, the top preservation trades people in North America and Europe met for the International Preservation Trades Workshop in Frankfort, Kentucky. Five window experts, Duffy Hoffman, John Leeke, Jim Turner, David Gibney and Bob Yapp met and decided it was time to create national standards for the repair and weatherization of old and historic windows. The WINDOW PRESERVATION STANDARDS COLLABORATIVE was conceived.

Using cutting-edge window testing technology, the Collaborative's tests are verifying "what most people in historic preservation have known for years, old and historic windows can cost effectively be made as or more energy efficient than new, disposable replacement windows." The Collaborative recently published a manual for the assessment and preservation of historic wood windows, reviewed below.

"Window Preservation Standards" Available

The national Window Preservation Standards book catalogs specific methods for the assessment, maintenance, repair, preservation and weatherization of older and historic wooden windows. Many detailed methods, procedures and materials are included, as well as basic strategies for saving older and historic windows.

Window Preservation Standards book coverThe Standards were developed and written by more than 100 window specialists who collaborated from all across the United States and Canada.

Topics include the Window Preservation Standards Collaborative, guiding principles for window preservation work, window part names and definitions, organizing and planning window preservation projects, and how to use the Standards.

Methods include surveying and assessing conditions, glazing, painting, wood repairs, weatherization updates, storm windows and maintenance with many more. Each treatment standard has a brief description of a specific method with appropriate use, a step-by-step procedure that shows what is done and materials are listed. A special section describes how to judge the character of the completed work with easy observations and simple tests to assure the best quality work.

The results of energy performance testing done at the 2011 National Window Preservation Summit are included. It confirms what is already known:  Save the primary sash and frame, add weatherstripping and a storm to make the window meet or exceed current performance standards for air infiltration. All the details are in the book, including which energy upgrade methods worked best.

The Window Preservation Standards book is a non-profit project done in partnership with the Preservation Trades Network, Kentucky Heritage Council and Kentucky Preservation. Ordering information can be found here.

Window Repair and Retrofit Studies

"Window Repair, Rehabilitation and Replacement" , Peter Baker, P.E.

This report was prepared for Building America, Building Technologies  Program, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy to evaluate advanced retrofit measures.  A balanced approach is presented to guide contractors and homeowners to decide whether to repair or replace considering many factors, one of which is historic preservation. November 2011

"A Comparative Study of the Cumulative Energy Use of Historical Versus Contemporary Windows"  By Frank Shirley, AIA, Fred Gamble, PhD, Jarod Galvin, RA, LEED AP

This study compares the life-cycle costs of two residential window systems in a pre-1940 house in Boston, Massachusetts. One is an original double-hung window with a new triple-track storm unit. The other is a new, vinyl, double-hung replacement window. Results are obtained from an algorithm that yields the total present value of all costs associated with a window system over its entire life, including acquisition, installation, maintenance, and energy.  December 2010

"Field Evaluation of Low-E Storm Windows"  By S. Craig Drumheller- NAHB Research Center, Christian Kohler- Lawrence Berkeley National Laboratory, Stefanie Minen-Utilivate Technologies

 A field evaluation comparing the performance of low emittance (low-e) storm windows with both standard clear storm windows and no storm windows was performed in a cold climate. Six homes with single pane windows were monitored over the period of one heating season. The homes were monitored with no storm windows and with new storm windows. Simple paybacks for the addition of the storm windows were 10 years for the clear glass and 4.5 years for the low-e storm windows.  December 2007

"The Effects of Energy Efficiency Treatments on Historic Windows"   by Larry Kinney and Amy Ellsworth  January 2011

This study focused on empirical testing of the energy efficiency and  economy of a range of options for upgrading the energy performance of  historic windows. The study involved retrofitting windows in a test home in a  historic district in Boulder, Colorado. It included testing in a window  laboratory facility developed for the study.

"Thermal Performance of Traditional Windows" by Dr. Paul Baker for Historic Scotland Revised September 2010

Technical Paper 1 tested the use of curtains, shutters, blinds, and secondary glazing. All were shown to reduce the heat loss through the glazing to varying degrees. Secondary glazing was found to be the most effective overall option, as it reduced heat loss through the window by 63%. Timber shutters are the most effective option of the traditional methods, reducing heat loss by 51%; curtains reduced heat loss by 14%; a Victorian roller blind reduced heat loss by 28%; a modern roller blind reduced heat loss by 22%. The greatest reductions in heat loss came from combining these measures (i.e. blinds, shutters and curtains all closed) and by adding extra insulation to these options.

"Slim-profile double-glazing in listed buildings: Re-measuring the thermal performance" by Nicholas Heath & Paul Baker, Historic Scotland  2013

Technical Paper 20 remeasures the efficiency of previously installed slim-profile double glazing in existing historic windows.

"Thermal Assessment of internal Shutters and Window Film Applied to Traditional Single Glazed Sash and Case Windows" by John Currie, Julio Bros Williamson, Jon Stinson & Marie Jonnard, Historic Scotland

Technical Report 23 assesses the effectiveness of two inexpensive and minimally invasive methods for improving the thermal performance of single glazed windows.  This technical paper demonstrates that a range of options, including minimally invasive and inexpensive methods, can play a worthwhile role in the overall thermal improvement of buildings.

"Of Paint and Windows - Replace or Repair" by Bob Yapp

Bob Yap Blogs Master craftsman Bob Yapp writes about all things restoring windows.

"Thermal Performance of Historic Windows" by Chris Wood, (England)

“Building Regulations and Historic Buildings: balancing the needs for energy conservation with those of building conservation” The English Heritage Interim Guidance article touches on all parts of preservation and conservation of power and fuel, and the chapter on windows is very relevant.

“Repair or Replace Windows in Historic Buildings: Arriving at a Sustainable Solution” The Heritage Canada file contains two articles, one from Andrew Powter and Craig Sims discussing how to arrive at a decision to replace or repair original windows, and Susan Turner explains the sustainable nature of window repair rather than replacement.

"The Effects of Energy Efficiency Treatments on Historic Windows" is an empirical study recently completed by the Center for Resource Conservation in Boulder Colorado. The study involved retrofitting windows in a test home in a historic district and investigated and then compared the energy efficiency and economy of eleven different preservation treatment options with that of new vinyl windows. Most of the proposed treatments were able to outperform a new vinyl window.

“Life Cycle Of Window Materials - A Comparative Assessment” by Asif, Davidson and Muneer. A comparative life cycle assessment of the environmental impact of different window materials is included for its interesting materials energy cost analysis.

“Domestic Retrofitting Strategies in the UK: Effectiveness vs. Affordability” is an interesting presentation of the effectiveness of different energy retrofitting strategies, including shutters.

“What Replacement Windows Can’t Replace: The Real Cost of Replacing Historic Windows” Walter Sedovic and Jill Gotthelf provide an excellent discussion of the comparative value of window replacement versus repair. Many aspects of sustainability are considered.

“Testing the energy performance of wood windows in Cold Climates” by Brad James, Andrew Shapiro, Steve Flanders and Dr. David Hemenway is a thoroughly researched paper that concludes that historic windows should not be replaced for energy considerations alone, as retrofitted and replacement windows perform similarly.

“Lincoln Hall Windows Research Report: A Case Study of Options for Treatment for Windows at Lincoln Hall, University Of Illinois, Urbana Champaign” This report provides empirical data to assess window repair or replacement options for a proposed LEED Gold project, addressing the existing windows in terms of energy consumption.

Window Replacements in the Media


"Storm Windows Save Energy" Home Energy Magazine, 1/28/14

"Old Windows Find a Following" by Katie Zezima. New York Times 7/27/2011.

“Windows Aren't Always A Clear Path for Savings: In Quest to Boost Energy-Efficiency, Small Changes Might Have Bigger Payoff by Terri Rupar. Washington Post 5/16/2009.

Thinking Of New Windows? Save Your Energy National Public Radio (NPR) 10/11/2009. 

"What Should I Do About My Windows?" by Bill Mattinson, Ross DePaola, and Dariush Arasteh for Home Energy Magazine.  August 2002