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    Monday, October 22, 2012


    Saving Windows, Saving Money: Evaluating the Energy Performance of Window Retrofit and Replacement

    September 2012 (Preservation Green Lab)

    Executive Summary


    Homeowners and design professionals seeking to upgrade the performance and efficiency of existing windows are faced with many choices—from simple, low cost, do-it-yourself solutions such as window films and weather stripping to replacing older windows with new ones that require investments costing tens of thousands of dollars. Often these decisions are made without a clear under¬standing of the range of options available, an evaluation of the ability of these options to provide energy and cost savings, or proper consideration for the historic character of the existing windows. This study builds on previous research and examines multiple window improve¬ment options, comparing the relative energy, carbon, and cost savings of vari¬ous choices across multiple climate regions. Results of this analysis demonstrate that a number of existing window retrofit strategies come very close to the energy performance of high-performance replacement windows at a fraction of the cost.



    There are readily-available retrofit measures that can achieve energy savings within the range of savings expected from new, high performance replacement windows. This challenges the common assumption that replacement windows alone provide the greatest benefit to homeowners.

    The figure on the previous page shows that for all cities, at least one and often two of the selected measures can achieve energy savings within the range of savings expected from new, high performance replacement windows. Specifi¬cally, interior window panels, exterior storm windows combined with cellular blinds, and in some cases even exterior storm windows alone fall within the range of performance for replacement windows.


    Energy savings alone should not influence decisions to upgrade windows without consideration of initial investment. For all climates, the cost analysis shows that new, high performance windows are by far the most costly measure, averaging approximately $30,000 for materials, installation, and general construction commonly required for an existing home. In cold climates, all other retrofit measures, with the exception of weather stripping and heat reducing surface films, offer a higher average return on investment when compared to new, efficient replacement windows. In hot climates, all of the study retrofit measures offer a better average return on investment than new windows, with the exception of weather stripping.


    In recent years, awareness around energy use and its financial and environ¬mental impacts have placed buildings in the spotlight. Residential buildings alone are responsible for approximately 20 percent of total U.S. energy use and carbon dioxide emissions. The vast majority of these buildings are single-family homes where heating and cooling represent the largest use of energy. Windows are one important aspect of how heat loss (and gain) affects a home’s opera¬tional efficiency and cumulatively represent over $17 billion in annual U.S. house¬hold expenditures on heating and cooling.

    In this study, computer simulation is used to model energy use in a typical, prototype home both before and after window improvements. Several com¬mercially available window improvement options were analyzed ranging from simple, low cost applications to more expensive options representing the high¬est energy performance on the market.

    The study analyzed energy, cost, and carbon savings for seven selected mea¬sures: weather stripping existing windows; interior window panels; exterior storm windows; insulating cellular shades; a combination of exterior storm win¬dows and insulating cellular shades; interior-applied surface films; and new, high performance replacement windows.

    Variations in climate and regional energy grids were addressed by evaluating the home’s performance in five U.S. cities—Boston, Atlanta, Chicago, Phoenix, and Portland. A thorough cost analysis allowed for the comparison of average return on investment for each window option in each of the cities.


    Findings from this study demonstrate that upgrading windows (specifically older, single-pane models) 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. Due to the cost and complexity of upgrading windows, however, these options are not likely to be the first intervention that homeowners undertake. For many older homes, non-window-related interventions—including air sealing, adding insula¬tion, and upgrading heating and cooling systems—offer easier and lower cost solutions to reducing energy bills.

    In addition to providing insights into the energy performance and investment costs of window options, the study’s findings reinforce several additional ben¬efits in choosing to retrofit existing windows rather than replace them. Ret¬rofits extend the life of existing windows, avoid production of new materials, and reduce waste. Additionally, wood windows are often a character defining feature of older homes, and conserving them helps to preserve the historic integrity of a home. The Secretary of the Interior’s Standards for the Treatment of Historic Properties and The Secretary of the Interior’s Illustrated Guidelines on Sustainability for Rehabilitating Historic Buildings offer guidance on how best to approach the preservation of windows in historically designated homes, or homes that may be eligible for listing.

    Selecting the most appropriate measure for upgrading windows requires a detailed understanding of climate and energy costs in addition to window per¬formance and installation costs. This study provides a valuable analysis of these variables that can be used to help inform the decision to improve the energy performance of and reduce the carbon dioxide emissions from older and his¬toric single-family homes.


    This report presents computer-simulated results of estimated energy use that indicate the value of individual retrofit measures relative to each other in a variety of climates. The energy savings noted (whether as a percentage or as an annual estimate of energy cost or CO2 savings) should only be used to compare options, not to predict the final savings that a retrofit will achieve. In reality, sav¬ings will vary widely depending on the actual house retrofitted (size, condition, number of windows, construction characteristics, etc.) and occupant behavior (windows/doors left open, temperature set points, nighttime setbacks for HVAC systems, etc.). Nonetheless, this study offers useful guidance for homeowners and industry professionals choosing among window retrofit or replacement options.

    The following recommendations set out best practices for selecting window retrofit and replacement options.

    1. Don’t start with windows: Tackle other energy-efficiency measures first.

    As discussed in Section 4 of this study, whole-house air sealing, improving insulation, and upgrading HVAC systems are often suggested as first measures homeowners should consider from a cost-effectiveness and energy efficiency perspective. Although investigating the sequence of all the possible energy ret¬rofits in an existing house was outside the scope of this study, Figure 12, which compares the savings from the minimum-efficiency and high-efficiency HVAC systems, reinforces the importance of considering window interventions within the context of other possible energy-efficiency measures. Homeowners who desire to maximize return on investment should consult an experienced energy professional, a house designer or architect, and a contractor who is familiar with energy saving retrofits to help evaluate applicable energy-saving solutions, proper sequencing, and estimated construction costs for a specific house.

    The Pettygrove Residence modeled in this study was assumed to have already performed many common energy retrofits, including insulation, air sealing, and an upgraded HVAC system. Because the prototype had already substantially reduced its total energy consumption through these strategies, window inter¬ventions made a greater percentage impact in both cost and CO2 savings than if the house had not already completed the other energy efficiency measures. While window retrofits and replacement typically should not be the first inter¬vention considered by homeowners, they do offer efficiency gains and energy savings, and are a significant part of a whole-house approach to achieving energy efficiency.

    2. Choose window retrofits over replacements.

    Window retrofits can achieve comparable energy savings at a much lower cost. Many homeowners may be surprised to learn that enhancing the performance of existing windows can offer nearly the same energy performance improvement as replacement windows.

    For all cities studied, at least one and often two of the improvements to the existing windows can achieve energy savings within the range of savings expected from new, high performance replacement windows.

    The results of this study show that interior window panels, exterior storm win¬dows combined with cellular blinds, and in some cases even exterior storm windows alone fall within the range of performance for replacement windows. Importantly, not all retrofit/replacement window options are equal: To achieve the highest total energy performance for a window retrofit, use a product and installation method that is at the highest performance end of the range for that measure (lowest U-factor, most appropriate SHGC for the climate condition, and lowest air leakage rate).

    Furthermore, retrofitting existing windows is far less costly than installing high performance replacement windows. Figures 9, 10, and 11 demonstrate that replacement windows have comparatively low returns on investment for homeowners. While replacement windows may offer high energy performance improvement, the upfront costs are substantial and are not rapidly recovered through savings in energy bills. Installing cellular shades typically offers the highest return on investment, while the use of storm windows and/or the use of storm windows with insulating shades also offers a solid return on investment. Interior storm windows offer other advantages as well, including reduced poten¬tial exposure to lead-based paint, while exterior storm windows help extend the useful life of historic windows by offering protection from the elements.

    Saving existing windows avoids the environmental impacts associated with new windows production.

    Reusing existing windows has other advantages beyond operational energy and cost savings. Keeping existing windows saves the energy and resources that would be needed to create a new window. Like any product, the production of replacement windows requires materials, and these materials generate CO2 and other environmental hazards from the extraction, manufacture, transport, and disposal processes. Retrofit measures also require materials, but are often less materials intensive and have less of an environmental impact than an entire window replacement.

    A full life cycle assessment was outside the scope of this report, and is needed to further evaluate this issue. In the absence of such analysis, high performance green building standards such the Living Building Challenge can also serve as a useful guide for material selection for homeowners, providing stringent standards for eliminating “Red-List” materials and chemicals found in build¬ing materials (such as PVC, a common material used in window products, both replacements and retrofits), using only sustainably sourced wood products, and selecting locally manufactured materials to reduce transportation energy and support regional economies.

    Finally, anticipated lifespan is also an important consideration when selecting materials.

    Many old windows are made from old growth wood, an increasingly scarce resource, which is extremely durable and easily repaired. Replacement windows do not offer such durability or reparability. To extend the life of the existing window, other upgrade measures should be considered when address¬ing the performance of the existing window, regardless of the energy savings produced. These include general sash and frame repairs such as replacing and rebalancing the counter-weight system, adjusting the stops, checking that the sash lock is drawing the meeting rails tight, and repairing failed glazing.

    Saving windows preserves a home’s character.

    Historic windows were custom fit to their original openings and often have sizes, shapes, and muntin patterns not found today. Replacing them often requires changing the size and/or shape of the opening. Standard-sized new windows, with or without applied muntins, might save on operational costs but will com¬promise the character and historic integrity of a home. For this reason, repairing existing windows and/or choosing attachments to improve their thermal perfor¬mance and occupant comfort is generally less expensive than custom replace¬ments and preserves the character of the home.

    Retrofits extend the life of existing windows, avoid production of new materi¬als, and reduce waste. Additionally, wood windows are often a character defin¬ing feature of older homes, and conserving them helps to preserve the historic integrity of a home. The Secretary of the Interior’s Standards for the Treatment of Historic Properties and The Secretary of the Interior’s Illustrated Guidelines on Sustainability for Rehabilitating Historic Buildings offer guidance on how best to approach the preservation of windows in historically designated homes, or homes that may be eligible for listing.

    3. Take climate into consideration.

    The best retrofit option for Phoenix may not be right for Chicago. The results from both the energy simulation and the investment analysis show that for all climates and cities studied, interior window panels and exterior storm windows are recommended options for reducing the energy loss from existing single-pane windows. In many cases, these two storm window measures have comparable energy performance to new, high performance replacement win¬dows at a fraction of the cost.

    In heating-dominated climates, insulating cellular shades helped reduce heat loss, especially when using a side track and in conjunction with exterior storm windows. As the need for winter heating decreases and summer cooling increases, the ben¬efits of insulating cellular shades decline.

    Interior surface films that reduce solar heat gain produced the best savings and greatest return on investment in cooling-dominated climates. Further, the applica¬tion of low-e coatings to exterior storm windows substantially improved simulated energy performance for cooling-dominated climates. However, in heating-dom¬inated climates the energy simulation showed an increase in energy used due to beneficial solar energy being reflected away from the house during the heating season. Thus, interior surface films or low-e coatings should be selected for these climates that simultaneously maintain a medium-to-high solar heat gain coefficient and a low U-factor. In climates with no summer cooling, such as Portland, facades that face the sun during the winter may maximize beneficial solar gain by using clear glass without any film or low-e coating.

    Homeowners should consult with an energy consultant familiar with passive solar design during the design phase of a project to make sure that the complex interaction between the sun and a home’s heating and cooling needs is considered. An important climatic consideration when selecting window enhancements is whether existing exterior shading from overhangs, trees, or other nearby buildings will reduce the impact of installing an upgrade measure with a low SHGC in cool¬ing climates. If windows are already shaded by exterior elements, or if windows are not oriented toward the sun, they will receive minimal or no cooling benefit from the addition of a low SHGC retrofit.

    4. Take matters into your own hands.

    Perform high-return, do-it-yourself installations first, where possible.

    Weather stripping and interior surface film generate immediate, low-cost savings and don’t preclude future installations of other window measures that may pro¬duce additional savings. However, expected returns from weather stripping are highest where the windows are old and drafty, so focus on those first for immediate energy savings. Inte¬rior surface films are an excellent option for homeowner installation, especially for homes with big cool¬ing bills in hot climates. Use care in applying films or low-e coatings to windows in colder climates, con¬sulting a designer or energy pro¬fessional to assist with the proper selection of materials and window locations that may produce the best year-round savings.

    While not directly related to energy savings, a comprehensive window renova¬tion that includes repairing the counterbalance mechanism, adjusting for proper fit, and repairing weather-damaged window components can substantially extend the life of the window and improve window tightness. Care should be taken to properly assess and abate lead-based paint during any window repair activities. As resources allow, simple enhancements such as cellular shades, especially those with side tracks to reduce air infiltration, can substantially improve the energy performance of windows over time. A combination of mea¬sures such as cellular shades with exterior storm windows in a cooler climate or interior surface film in a warmer climate can produce dramatic energy savings.

    Taking a phased approach to window upgrades, focusing on the highest returns first and using savings to pay for future improvements, can eventually lead to long-term savings of money, energy, and carbon emissions for older homes, even for households that are on a tight budget.

    CONCLUSIONS AND FUTURE RESEARCH The results of this study show that window retrofit and replacement options have the potential to significantly improve the energy efficiency of a home with existing leaky, single-pane windows. How much varies substantially among retrofit options, energy costs, and climate variations. Several retrofit options fall into the range of expected performance that a replacement window might achieve (specifically exterior and interior storm windows, especially when com¬bined with cellular shades), showing that retrofit options should be a first con¬sideration before replacements. This study identified a number of future research opportunities that could pro¬vide a more comprehensive understanding of window retrofit and replacement options for older leaky, single-pane windows. These include:


    This study evaluated only the energy savings of various test conditions and did not address impacts to the environment or to human health associated with material production, transportation, maintenance, replacement, or disposal over the anticipated life span of the retrofitted or replacement windows. Further research is needed to understand how each test condition compares based on these impacts. Due to the wide range of material choices that exist for window retrofit/replacement measures, this type of analysis was outside the scope of this current study. However, the energy results from this analysis could provide a basis for a more comprehensive study on life cycle impacts in the future.


    This study was limited to an evaluation of a baseline home assumed to be served by a natural gas-powered furnace and electrical window/wall air conditioning units. Variations in the type and efficiency of the heating/cooling system as well as the fuel type could potentially change the results of this study. More research is needed to understand how these variables affect the decision to replace or retrofit windows in different climate regions.


    In many cases, choosing to retrofit or replace windows may not be the most cost-effective or efficient way to improve the energy performance of an older home. A much more detailed analysis is needed to evaluate how to prioritize window upgrades in the context of other energy-efficiency measures such as adding insulation, whole-house air sealing, and upgrading existing heating and cooling equipment.


    The energy simulations for this study used assumptions for window perfor¬mance that were assembled from a meta-review of past windows reports. The selections of U-factor, SHGC, and air infiltration characteristics were based upon previously tested or modeled conditions for actual assemblies. Low and high performance assumptions did not reflect exact climate conditions in the five cities selected. A follow-up study is needed to provide guidance about how to properly select low-e coatings, films, and glazing for the different window retrofit options presented, ideally for each of the climate zones identified in the International Energy Conservation Code.


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