Making Windows Work Better
By Paula Melton and Peter Yost
From curtains to motorized shutters, window attachments help you get the most from your windows without having to buy replacements.
Let there be light, but no glare. Let there be a breeze in summer, but no winter air leaks. Let there be home security, but also quick emergency exits.
In a perfect world, our windows would do all this at once. Since we don’t live in a perfect world, we have created any number of window attachments—everything from interior drapes and blinds to exterior storm windows, awnings, and roll-down metal shutters. Many attachments offer fairly low-tech solutions that allow on-the-spot comfort control and seasonal flexibility. The cost of most attachments is quite low compared with the benefits, which can include significant energy savings.
Choosing and using window attachments isn’t always straightforward, though. To perform as designed, most of these attachments require proper installation and use. But once they’re installed, it is easy to ignore them, forget to use them, or deploy them in useless or counterproductive ways.
That brings us to the other challenge: hard numbers on the energy performance of window attachments have been hard to come by, leaving us at the mercy of overstatements by manufacturers. With objective information on attachments scarce and key tax incentives rewarding window replacement, many windows that could be improved end up in landfills instead, and many simple window-performance problems remain unsolved or inadequately managed.
In this article, we’ll walk you through a wide variety of window attachments and discuss how to get the best performance out of each type; the ten tips at the end of this article will help you make the most of this information. We’ll also take a sneak peek at ongoing research the Lawrence Berkeley National Laboratory (LBNL) is conducting on window attachments in partnership with BuildingGreen, Inc., publisher of EBN. As in our recent article on windows (EBN Feb. 2011), we will focus on residential strategies—but most of the discussion applies to commercial buildings as well.
Holes in the Wall
Windows offer us benefits we can’t do without, but there is no getting around the fact that they are holes in the wall, reducing overall R-value and creating opportunities for air leakage and unwanted solar heat gain. Over the last 30 years, the window industry has worked hard to mitigate these problems without resorting to attachments. Features like low-emissivity (low-e) coatings and sealed double- or triple-pane assemblies have created windows that perform very well from an energy perspective—and can in some circumstances rival the thermal benefits of an opaque wall.
The National Fenestration Ratings Council (NFRC) rates the thermal and visual performance of windows on their U-factor (rate of heat loss through the whole window assembly, and mathematically the inverse of R-value), solar heat gain coefficient (SHGC, the measure of how much heat gets into the building compared with how much sunlight is striking the window assembly), and visible transmittance (VT, how much visible light comes through—and how well we can see out). All of these measurements must be reported on a special window label. An air leakage (AL) rating exists but is optional.
The challenge of rating attachments
What doesn’t yet exist is an NFRC rating system for window attachments, even though most are designed to alter the U-factor, SHGC, VT, or AL. NFRC does rate surface-applied tints and films, and is currently developing standards for more window attachments. According to Jim Benney, the CEO of NFRC, consumers will probably start seeing an NFRC label on a number of window attachments in 2012.
Technical ratings have already been established for many attachments, such as blinds and storm windows, Benney told EBN. However, labeling window attachments for casual consumers in a way that is “fair, credible, and accurate” as well as understandable has been a challenge, because window attachment performance sometimes depends on interactions between windows and attachments, as well as on climate, window orientation, indoor and outdoor temperature, and other factors.
Also, window attachments are specifically designed to counteract problems with windows—so they often function quite differently, and established fenestration ratings simply don’t apply. Speaking about NFRC’s efforts, John Gant, sustainable development manager for Glen Raven, maker of Sunbrella exterior shading products, said, “They have their hands full with their own products.” He is a member of several committees trying to develop NFRC ratings for attachments and is also involved in the LBNL research. “Shades are not really any more complicated than windows,” Gant claimed, but the complexities that get factored out to establish window ratings are different from those that need to be factored out for shades and other attachments.
In part to solve some of these problems with both metrics and communication to consumers, says Benney, the new labels will use bars, graphs, or stars to give people an at-a-glance sense of how well the product compares with others of the same type. EBN is interested to see how accurately such a system can characterize energy performance, and whether the new ratings will create market demand for higher performance. A second phase of the NFRC program will likely involve online tools allowing consumers to research how their particular window type will perform with different attachments in use, and this may prove more useful. “This is a challenge, but it is going to save energy,” Benney said. “There are amazing products out there.” He added, “There are also some claims out there that we want to make sure are validated.”
While NFRC develops ratings, LBNL continues independent research on window attachment performance. Based on field and lab testing, computer modeling, and data gathered in prior studies, BuildingGreen is working with the U.S. Department of Energy (DOE), LBNL and other partners to present the results to consumers. Much of the information—including some important new cautions about mixing and matching windows and attachments—is collected here for the first time.
Keeping Heat Out
Excess solar heat gain, along with glare and lack of privacy, is one of the main complaints with windows; solar heat and privacy issues are the main reasons that people have been resorting to low-tech attachments like curtains and shades for years. These attachments often introduce compromises, though—tradeoffs that some higher-tech versions of conventional shading devices try to address.
While we often think of curtains and drapes as “window dressing,” their primary function, like that of conventional roller blinds and louvered blinds, is to prevent glare and provide privacy. They can also save quite a lot of energy during the cooling season. According to LBNL research, when installed over clear (not low-e) glass, these attachments alone can block 20%–60% of solar gain (depending on material and color), reducing or preventing the need for air conditioning. For comparison, the highest-performing low-solar-gain windows on the market have an SHGC of 0.20 or lower. While that means that those windows block 80% of solar gain while still permitting a somewhat darkened view, they don’t provide much privacy, so many people will still use curtains or blinds.
Combining the two is a good bet: the curtains offer privacy, while the low-SHGC windows block the sunlight before it gets into the house, which is much more effective (more on this in the discussion of awnings below). One drawback of using curtains or drapes is that you may end up with dark rooms and need to turn on lights, which can cut into energy savings. Louvered blinds can be adjusted at the top to let some daylight reflect off the ceiling, but this light will bring some heat with it—especially if you have clear glass rather than low-e. Another option may be a solar screen that filters sunlight and prevents solar gain but still permits a view—although curtains may still be needed for nighttime privacy.
Outside help: Exterior shading
Like interior window coverings, awnings and exterior shades can prevent glare and solar heat gain while also improving aesthetics. Most can be operated to offer flexibility.
Curtains and blinds are effective at blocking solar heat and glare, but most of these products don’t allow you to look out the window while they are in use. Additionally, even the most effective interior shade blocks the radiant heat after it comes through the window, which means some heat will be trapped in the house even if you install a shade with a high-reflectance color or a metallic exterior face. Awnings solve both of these problems, reducing glare without affecting views too much, and blocking up to 90% of solar heat gain on south-facing windows—before the heat can come through the window. Awnings also block direct ultraviolet (UV) radiation that can damage upholstery and other items, and they protect the window assembly from the weather.
According to John Gant, awnings “have a different SHGC depending on indoor and outdoor temperature and the position of the sun” as well as fabric weight and color. In general, a heavier fabric in a darker color will provide more shading. However, Gant cautions that “products from different manufacturers may look the same but perform differently due to the quality of pigments and materials and fabric construction.”
Stationary metal awnings are available for homes, but be aware that a fixed exterior attachment works best if orientation, climate, and other factors are taken into account using sophisticated computer modeling—a service used routinely for commercial buildings but almost never for homes.
Exterior shades and shutters
Like awnings, exterior roller shades, made of vinyl-coated polyester or fiberglass fabric, block solar heat gain while still allowing a filtered view through the window from the inside. These work much the same way that interior shades do—rolling down in front of the window—but go on the outside of the house. Depending on their openness factor (how much light goes through the weave, anywhere from 3%–30%), some of these shades simultaneously provide daytime privacy by blocking views from the outside—although this effect doesn’t work with lights on at night!
Exterior shades are generally more effective than interior shades at preventing solar heat gain, providing up to 85% blockage, and tend to offer better ventilation. Similarly, hinged exterior shutters (unlike the common decorative kind, which cannot be closed) shade the window while allowing ventilation as well as daylight; some hinged shutters can be manipulated with adjustable louvers. Hinged shutters generally block views from the interior, though, and are more popular for weather protection. They may also be more accepted in historic areas and condominium communities, where many new-fangled exterior attachments are not allowed.
One such new-fangled device is an exterior roller shutter. Roller shutters are a multipurpose attachment with hollow or insulated slats that fold neatly when not in use but put the home in thermal, visual, storm, and security lockdown when deployed. Is there anything exterior roller shutters don’t do? Unfortunately, yes: the aesthetics leave much to be desired, from most homeowners’ point of view. They look fairly industrial from the outside and completely block light and views from within. Though popular in some European countries, exterior roller shutters have only caught on in North America in hurricane and wildfire zones. However, they can be a good solution in extreme climates where air-conditioning loads are very high and shading doesn’t go far enough—particularly if home security is also a high priority.
If you have windows that are in pretty good shape but don’t have a low-e coating, or if you have a great view that you don’t want to mar with blinds or awnings, you might consider a solar-control window film for blocking solar heat gain, reducing glare, or increasing privacy. Even if you have low-e windows already, some films are designed to provide other benefits. This industry is going through a period of rapid growth and innovation, so new options are appearing on the market frequently. Surface-applied films are rated for solar heat gain and visible transmittance, and NFRC publishes an online list of certified products, making it easier for consumers to make choices.also provides helpful guidance on the wide variety of films available.
Keeping Heat In
Solar gain can be a serious problem in the summer (or year-round in cooling-dominated climates), but during the short winter days, we welcome the free heat. Winter window attachments are designed to keep heat in rather than keeping it out by insulating the window assembly and improving airtightness.
If you’re stuck with single-pane or other poorly insulated windows, there is hope: you can boost their R-value and slow down air leakage by covering up the whole window assembly with another layer of plastic or glass. The best products perform almost as well as high-end replacement windows and tend to have a lower price tag.
Seasonal window film
A box of clear plastic and double-sided tape is probably the least expensive and most widely available window attachment—and the performance isn’t too shabby, given the cost. These kits, commonly found at hardware stores, provide modest insulation (adding an R-value of about 1), significantly reduce air leakage, and improve comfort by adding a layer between cold glass and warm rooms. They also help prevent condensation, which can rot wooden window frames, and both renters and homeowners can install them cost-effectively.
For windows that are very leaky, combine these kits with seasonal rope caulk or weatherstripping for better results. According to LBNL project manager Charlie Curcija, Ph.D., this film performs about as well as an interior “storm” panel with no low-e coating, although many people don’t like the aesthetics of the film kits, and some types of tape can pull off paint or finish—especially if not removed immediately when the film is taken off in spring.
Cellular shades and window quilts
Conventional fabric window coverings and interior shades have modest built-in thermal benefits: they increase comfort by adding a warmer layer between the cold window glass and the heated room. Insulated cellular shades (which many people use just like a conventional window shade, for reducing solar heat gain) and window quilts (a quilted cloth roll-down shade popular in colder climates) increase this benefit while still offering glare control, privacy, and protection from solar heat gain. Cellular shades and window quilts that fit snugly to the window trim with side tracks or other fasteners offer better thermal benefits than products without these features. (Conventional insulated shades improve R-value by 1–2, while higher-performing shades with side tracks improve R-value by up to 4.) In winter, many people open the shades fully or partially during the day to take advantage of daylighting and solar heat gain (some types can be opened at the top or bottom, which gives maximum flexibility), then close them after the sun goes down. The shades need to be completely closed in order to provide the full thermal benefit.
Insulated shades work well in heating-dominated climates but can remain on windows during the summer and be used just like conventional shades. Homeowners should be very cautious, though, about combining insulated shades or quilts with double-glazed windows. Windows with low-e coatings present the primary concern, but some higher-performing uncoated windows may be an issue as well. Preliminary research suggests that this combination, particularly on south- and west-facing windows, may overheat during the summer even in northern climates if insulated cellular shades are deployed on a hot, sunny day; some modeled temperatures have gone above 200° F (93° C), which could damage seals and shorten the expected window life. Finding other ways to control glare and solar gain on south-facing low-e windows, such as awnings or conventional curtains, is probably a good idea until more is known.
Interior “storm” windows
Fixed interior window panels (widely called “interior storm windows” even though they do not protect windows from the weather) offer unobtrusive high performance starting at a relatively low price and are especially popular in places where exterior storm windows won’t work—in historic homes, condominiums, long-term rental properties, and on dangerously high windows. They are made of plastic or glass—sometimes even low-e glass, which costs more—and come with different framing materials (most commonly aluminum). Some have operable parts and can remain in place year-round, but most have to be removed seasonally; this is the main drawback of interior panels, as they can block emergency exits through the window and also require space for safe storage. Plexiglas or Lexan panels can also scratch or fog more easily than glass, although manufacturers have largely addressed these problems—if you follow their cleaning instructions.
According to LBNL research, low-e interior window panels offer a number of benefits when properly installed. They improve R-value by about 2, increase comfort by creating a barrier between the cold window glass and the room, and do a pretty good job of sealing air leaks; the tight seal on the interior also helps reduce condensation by preventing warm, moist indoor air from entering the window assembly and being chilled by the air near the cold glass. These panels also block outdoor noise very effectively, if they are airtight. Low-e versions have a radiant barrier that further reduces heat loss, bringing the entire window assembly almost up to par with low-e double-pane replacement windows.
Weathering the storm
Exterior storm windows generally perform about as well as interior panels, with fixed, low-e storms offering the best insulation and air sealing. The typical triple-track storm windows, with two moveable glass sashes and an insect screen, are more flexible and don’t require seasonal switch-outs and storage, but they tend to result in greater air leakage. A double-track system with just one moveable glass sash can offer the best of both worlds.
Unlike interior panels, exterior storm windows offer weather protection, helping with durability and maintenance of frames, sills, trim, and finishes. The tradeoffs are a generally higher price tag and greater potential for condensation and frost on the exterior storm panel. Weatherstripping can help, and some exterior storms manage condensation with weep holes at the sill, which reduce condensation potential without much of an energy penalty in terms of airtightness.
High-performance storm windows provide a good companion to leaky or single-pane clear windows that are otherwise in good shape—but be very careful about attaching low-e storm windows to double-glazed, low-e windows. Modeling performed for LBNL by sustainability consultant Thomas Culp, Ph.D. has uncovered the potential for serious overheating problems when low-e storms are added to low-e windows: in hot weather, in direct sunlight, temperatures up to 185° F (85° C) may be reached. That kind of heat can cause premature aging or failure of the insulated glazing unit’s seals. Further testing will yield a better understanding of the exact conditions under which this can occur and possible solutions, but it would be wise to avoid using low-e storms in combination with low-e double-pane windows until more is known.
Slapping on an extra layer of plastic or glass won’t solve every problem, particularly if windows are broken or very leaky. An air-sealing upgrade and repair package for the existing window can be the place to start. Professional air sealing and repair may involve replacing seals and gaskets throughout the window assembly, bringing window frames back into square, and repairing sash frames or the glass itself. While these repairs can be fairly inexpensive, they do not add much R-value to the window—though replacing a sash pocket pulley system with a spring system and filling the air space with insulation will provide marginal improvements.
Air sealing does save energy as well as improve thermal comfort by closing off air leaks and reducing drafts, and it can be combined with high-performing storm windows or insulated shades to get up to four times better thermal performance without a pricey window replacement—a great option if you need shades for glare or privacy anyway, or if maintaining the look and feel of original windows is important. Improved operability can also save energy by allowing windows to be opened in the summer for ventilation. If windows are in terrible shape, have lead paint or asbestos, or require special handling due to historic value, the price tag for this service can go up considerably—but restoration may still be worth it.
“The only reason to replace a window is complete deterioration,” says Jean Carroon, FAIA, an architect with Goody Clancy. Carroon specializes in historic preservation, and her work often leads to extensive restoration and repair of historic windows as well as the addition of attachments like interior panels—or even replacing window panes with insulated glass units in the historic window frame. Carroon is distressed by the rate at which windows are replaced, especially since most of today’s double-glazed, low-e windows cannot be repaired. Installing non-repairable windows leads to a “cycle of replacement” and is “a symbol of a non-sustainable world,” she says. While restoring windows can be labor-intensive (and thus expensive), “you are almost always pouring the dollars into the local economy,” not sending it to far-away manufacturers, Carroon notes.
While we use many of these same window attachments in commercial and multi-family buildings, and many of the same principles apply, there are a few additional options and slightly different considerations on the commercial side.
Overall, according to John Carmody, director of the Center for Sustainable Building Research at the University of Minnesota, the larger amount of glass in commercial buildings means that solar gain is a much more important issue than heat loss—even in northern climates. This gets complicated in office buildings, because shading devices “may mean that energy is saved for cooling but increased for electric lighting,” Carmody told EBN. Shading the windows without blocking daylight—which is important not only for energy performance but also for indoor air quality and employee productivity in many commercial buildings—can pay off if done properly, though. “It is usually true that existing buildings will not have high-performance glazings that include technologies such as low-solar-gain low-e coatings,” said Carmody. “This means that the impact of solar control attachments is likely to be greater on existing buildings than on newer ones with high-performance glass.”
In addition to the attachments used in homes, commercial buildings may incorporate other devices, such as overhangs, light shelves, and vertical fins. Automation is also more common in commercial settings, where motorized interior shades as well as exterior shutters, louvers, or awnings may work on a timer or even respond automatically to lighting conditions, offering “a higher level of energy and peak-demand reduction since it is not dependent on operation by occupants,” according to Carmody.
While window attachments for commercial applications tend to be complex, commercial building retrofits often involve professional energy modeling. LBNL and other partners are planning window attachment research for commercial buildings that should provide further guidance to both building owners and professional designers.
As we’ve discussed, window attachments tend to be low-tech and are usually less expensive than window replacements. But that doesn’t mean buying and using them is a simple matter: we hope this article has shown how complex these decisions can be. And yet many people don’t really know the full range of attachment options; they often pick them mainly for aesthetic reasons without thinking through other considerations. Before you go out and buy one-size-fits-all attachments—and definitely before you start shopping for replacement windows—use these ten tips to ensure you get the most from your windows and whatever you choose to put on them.
1.Replace existing windows only if they have failed or are in poor shape (see “Do You Need New Windows?”). Almost all windows need attachments, and opting for a higher-performance attachment, rather than a window replacement plus a new conventional attachment, may be a better solution.
2.Use the summary table of window attachment options and attributes to get a handle on the entire spectrum of possibilities, and then prioritize your needs.
3.Using these priorities, compare how well conventional window treatments meet these needs in comparison with more expensive but higher-performing window attachments.
4.Use the individual window attachment fact sheets (at) to gain a complete understanding of each option.
5.Look for and use credible resources—not manufacturer or sales claims—to support your window attachment decision-making. (See ”The Best Tools for Making Window Decisions”)
6.Select an attachment with multiple attributes; sometimes one attachment can solve multiple problems.
7.Nevertheless, optimal manage-ment of heat loss and gain may require two window attachments—an interior one for the former and an exterior one for the latter.
8.Take care in combining double-glazed low-e windows with either low-e exterior storm windows or high-performance insulating interior attachments; deployment of either in managing solar gain during the summer may result in damage to the insulated glazing unit seals. (See “Weathering the storm” and “Cellular shades and window quilts,” above.)
9.Establish routines for best performance of adjustable window attachments. Flexibility is a good thing, but it also means that adjustment is required for superior performance. Advise clients about proper operation of attachments. Consider automated (motorized and sensor-driven) window attachments to achieve optimal performance without continuous manual adjustment.
10.Prioritize energy performance in your window attachment decisions, but don’t forget to seek out sustainable and nontoxic materials. While some window attachments typically come in only one form, the market is expanding rapidly. Recycled content, low-emitting materials, non-treated fabrics, and other green options may already be available.
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