Storm Windows Save Heating Energy

A version of this article appears in the May/June 2013 issue of Home Energy Magazine.

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Homeowners looking to upgrade the energy efficiency of windows have a number of options short of replacement. The obvious first steps are to make any necessary repairs and to seal air leaks. Then there are numerous interior and exterior window attachments to consider. This article will focus on three of those attachments: plastic film applied to the interior side of a single-glazed window, exterior clear-glass storm windows, and exterior storm windows with a low-e coating.

Common Alternatives

Plastic film, glass, and rigid plastic are the materials most often installed over a primary window to reduce heat loss from infiltration and conduction. Simple, off-the-shelf window film kits are widely available and can be installed by tenants or homeowners on a tight budget. Rigid, clear sheets of polymethylmethacrylate —commonly known as acrylic or Plexiglas—with a metal or vinyl border are often installed on the interior side of primary windows. They are held in place by a frame consisting of integral magnetic strips. These Plexiglas attachments are more expensive than plastic films, but they are also much more durable.

Probably the most common window attachment used over single-glazed primary windows is a so-called triple-track storm window. This is a single-glazed, double-hung storm window with an operable top and bottom sash, a perimeter aluminum frame with three built-in tracks, and an insect screen. A slightly different version consists of an aluminum-framed, single-glazed, single-hung storm window with an operable bottom sash. The top sash is sealed in place. The benefit of this configuration is that it reduces the area of the window subject to infiltration. Both kinds of storm window may be glazed with either glass or acrylic. Hard-coat low-e coatings are available for both glass and acrylic storm windows; they are also available for interior window attachments. In this article, we will examine the evidence as to which retrofit options for existing single-glazed windows best reduce space conditioning costs. We will also see whether the evidence supports DOE’s statement that “If you can afford exterior storm windows, you can probably afford some newer, more energy-efficient windows, which will be a better investment.” (See the end of the article, under “learn more,” for the source of the quotation.)

To answer these questions, we began by conducting an extensive review of the literature that compares heat loss through various types of windows, and through windows with different types of secondary attachments. The literature turned out to consist of four articles. Three articles described laboratory experiments that examined these issues. The fourth article described one field evaluation of low-e storms over single-glazed wood-framed windows. The four articles and their respective findings are summarized below. See “learn more” for the full reference citations.

Oak Ridge Study

In a study by Oak Ridge National Laboratories (ORNL), the authors selected two older double-hung single-glazed wood-framed windows from homes that were to be demolished (Desjarlais, Childs, and Christian, 1999). They removed both the window and 6 inches of the wall surrounding the window. Their purpose in doing this was to control any thermal effects attributable to the connection between the window frame and the rough window opening in the wall. These wall sections with windows intact were brought to a laboratory and installed in a guarded hot box, where technicians could control temperature and pressure differences across the windows and monitor the results.

The authors first determined baseline heat loss through the primary windows alone. Simple aluminum-framed storm windows were then installed over the primary windows and the result was retested in the guarded hot box. The findings are summarized in Figure 1. It is not clear what form of heat transfer the term excess represents. The authors define it only as the heat flow not accounted for by conduction or infiltration. Perhaps excess represents radiant heat flow, but we are not certain.

Klems Study

In a study completed in 2002 and reported in a paper delivered at a meeting of ASHRAE (Klems, 2003), the author used a calorimeter to compare energy flows through a single-hung, low-e, argon-filled, double-glazed window in a “well-weather-stripped vinyl frame” to energy flows through a double-hung, single-glazed, wood-framed window. This comparison served as a baseline from which to compare the efficacy of three different window attachments placed over the single-glazed wood-framed window to the efficacy of the low-e, vinyl-framed, double-glazed replacement window. The three different window attachments were

• a low-e exterior storm installed over the primary single-glazed wood-framed window;
• a single-glazed, clear-glass storm installed over the primary single-glazed, wood-framed window; and
• a low-e interior secondary window installed over the primary single-glazed, wood-framed window.

One of the most interesting findings from this study concerned the impact of solar heat gain. The peaks in solar heat gain occur during the warmest part of the day, and the troughs occur during the coldest part of the night. As the author points out, the single-glazed, wood-framed window does not lose much more heat than the low-e, vinyl-framed replacement window during the middle of the day. This is true even though the windows face north and are never exposed to direct sunlight.

The other notable finding is the performance of the single-glazed, wood-framed window with the low-e storm compared to that of the low-e, vinyl-framed, double-glazed replacement window. And the low-e interior secondary window over the single-glazed, wood-framed window performed a bit better than the exterior low-e storm.

The performance of the various window attachments was detailed in the Klems report (see “learn more” at the end of the article for a link to the Klems and other research reports). There is very little difference between the heat loss of the single-glazed, wood-framed window with the exterior low-e storm and the heat loss of the low-e, vinyl-framed, double-glazed replacement window. Klems states that the performance of the low-e storm with the single-glazed, wood-framed window was very close to that of the efficient low-e, vinyl-framed, double-glazed replacement window, and differences were apparent only after long-term averaging.

New Zealand Study

In New Zealand, Nick Smith and Nigel Isaacs (2009) used a guarded hot box to compare “thermal transmittance measurements” of a “typical single-glazed aluminum window to four different attachments added to this window.” The four attachments were

• thin plastic film;
• an acrylic sheet held in place with magnetic strips on the interior side of the primary window;
• aluminum-framed storm; and
• aluminum-framed, low-e storm.

The authors conducted a review of the international literature to determine the best methods for comparing thermal transmittance through each of these four window attachments and a typical single-glazed aluminum window. The results of the review indicated that a guarded hot box was the most accurate method for measuring thermal transmittance through all four configurations. They set up one side of the hot box at 77°F and the other at 64.4°F. The configured window was installed in the hot box so that one side was exposed to the cooler temperature, and the other to the warmer temperature. The hot box was closed, and the temperatures on each side of the window were allowed to stabilize. The hot box was then allowed to run overnight while data were collected and exported to a spreadsheet. The results are shown in Table 1. Note that the aluminum storm with Low-e glass had the highest improvement, followed by the aluminum storm with clear glass, the magnetic acrylic interior attachment, and the thin plastic applied to the interior.

Field Evaluation

During the winter of 2005–06, three researchers conducted a case study to “quantify installed costs and energy savings of clear and low-e storm windows in a cold climate and provide guidance to home energy efficiency raters wishing to analyze storm window performance with energy simulation software” (Drumheller, Köhler, and Minen, 2007). Six occupied single-family homes in Chicago were selected for this study; the participants were recruited by the Cook County Weatherization program. All of the homes were built between 1920 and 1970 and still had their original single-glazed, wood-framed windows. All storm windows on the six homes were removed at the start of the study to obtain a baseline measurement of the heat loss for each home with the original windows only. Occupants were asked not to change the setting of the heating system thermostat for the duration of the study. Baseline data were collected from late October 2005 to late January 2006. At that point, low-e e storm windows were installed on four of the six homes and clear-glass storms were installed on the other two homes. All storm windows were aluminum-framed, double-track units. One of the tracks held the bottom (operable) storm, and the other track held an insect screen. The inoperable top glass sash was sealed. Data were then collected from the homes from the time the storms were installed until mid-April 2006.

Before the storms were installed, researchers collected data to obtain baseline measurements, as described above. Furnace run time, indoor temperature, relative humidity, and surface temperatures of the primary windows were captured with data-logging equipment. In addition, a blower door test was conducted on each house before and after the storms were installed to determine the impact of the storms on infiltration. The addition of storm windows showed an average reduction in infiltration of 7%; range was from 232 to 335 CFM when the house was at a pressure difference of 50 pascal in reference to outside.

Due to data correlation problems, two of the six houses had to be eliminated from the final energy analysis. Final comparisons were conducted between two homes with low-e storm windows and two homes with clear-glass storm windows. The results are shown in Tables 2 and 3.

Implications

Our review of these four studies indicates that any type of storm window can reduce heating costs. For example, the New Zealand study shows a significant reduction in heat loss with the addition of a simple and temporary clear-plastic covering. In fact, this plastic covering was nearly as effective at reducing heat loss as the much more durable (but more expensive) clear-glass storm window. The three studies that included low-e storms found a significant reduction in window heat loss when low-e glass is used in storm window applications. And the Field Evaluation study found that low-e storm windows can be installed on an entire home for a fraction of the cost of a complete window replacement.

The Klems study found that most heat loss through windows occurs at night. This raises a question about the impact that interior insulated thermal window treatments, such as insulated shades or drapes, might have on heat loss. Could they be a less expensive, but equally effective alternative to storm windows? This option was not investigated in any of these studies, nor did we locate any study that addressed the question. The Klems and Field Evaluation studies certainly raise questions about the efficacy of replacing original single-glazed wood-framed windows in older homes with double-glazed, argon-filled, low-e, well-sealed replacement windows. But the fact that the Klems study was conducted nearly 11 years ago calls into question the validity of those findings, given present window technology. Klems calculated a U-factor of 0.43 for the energy-efficient replacement window. But this represents the U-factor of a typical energy-efficient window in 2002. Currently double-glazed energy-efficient replacement windows can be purchased with a U-factor of 0.3. If the Klems study were repeated now, using current energy-efficient window technology, would a low-e storm compare as well as it did in the 2002 test? In addition, triple-glazed replacement windows are becoming available at prices not significantly above those of double-glazed energy-efficient windows. How might those compare with low-e storms?

The Field Evaluation study indicates that low-e storms can significantly reduce heat loss through single-glazed windows. And retrofitting low-e storms on an entire home is relatively affordable compared to the cost of purchasing high-quality replacement windows and having them professionally installed. This study did not compare a home fitted with low-e storm windows to a home retrofitted with energy-efficient replacement windows, so we cannot determine exactly how well low-estorms would compare with energy-efficient replacement windows. However, a complete window retrofit of an existing home costs several thousand dollars, with a simple payback of at least 40 years. The Field Evaluation study showed that low-e storms, purchased and installed at a fraction of the cost of energy-efficient replacement windows, had a simple payback period of just 5.1 years. At the current cost of high-quality, energy-efficient replacement windows and professional installation, it seems unlikely that these would be a better option than low-e exterior storm windows. The Field Evaluation study also raises another question: Would potential savings warrant replacing existing clear-glass storm windows with low-e storm windows?

Given that we have reviewed just four studies, how confident can we be in generalizing the findings? From a purely statistical perspective, we’re not very confident. Just two of the four studies used the same measuring device. And none of the studies compared exactly the same windows with identical window attachments. However, the four studies conducted actual measurements that compared heat flow rates through various types of window and window-storm window combinations. That makes their findings extremely valuable. While many computer software programs can be used to model heat flow through various window configurations, these programs greatly simplify the complexities that occur in real situations. Therefore, we believe the data generated by these studies can help inform decisions about window retrofit choices in residential structures. We can say with great certainty that much more research is needed in this area. While these improvements are typically done for the purpose of saving money, it is also important to consider human comfort. When people are near a cold window, they lose heat through radiation to the cold surface of that window. This can make them feel uncomfortably cold. Similarly, the person can feel cold air that enters the home through cracks around a single-glazed window, because the body is losing heat to the cold drafts. Storm windows can effectively prevent these uncomfortable conditions by reducing radiant and convective heat losses and thereby make a home more comfortable.

Mark Pierce is an extension associate and Joseph Laquatra is a professor in the Department of Design and Environmental Analysis at Cornell University.

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