- Choosing a Frame Material
- Glazing Options
- Adding it Up: Performance Metrics
Choosing Windows: Looking Through the Options
By Tristan Roberts and Alex Wilson
When we poke a hole in the wall and stick a window in it, we strike a high-stakes bargain. We want a visual connection to the outdoors that lets in daylight and that is itself pleasant to look at, both from the inside and the outside. We expect windows to provide fresh air and cooling breezes at times, but at other times we expect them to be completely airtight and provide good thermal insulation. Insects should be kept out; children and pets in. In heating climates, we want to get solar heat gain from windows, but not too much, and in all climates we don’t want glare.
Along with these key functions, we need windows to be durable in every way: resistant to condensation, wind, driving rain and ice, as well as the occasional baseball from over the neighbor’s fence or hurricane-driven debris. Windows must operate easily and accommodate attachments like curtains, awnings, and other devices. We want windows that are quick to install, that integrate with the rest of the building envelope, and that won’t break the bank. Given that they are a big investment, they should last a long time—several decades at least. We want windows to not cause undue environmental harm during their life cycle, whether from material extraction, manufacture, disposal, or as a hazard to birds.
In this article we’ll try to untangle some of the many threads that go into choosing windows for our homes, offices, and other buildings. This article will be most relevant to light construction, but many of the concepts and technologies extend to commercial glazing and curtainwall systems. EBN will also cover window attachments and retrofit options later in 2011 as we learn from research we’re doing as part of a Lawrence Berkeley National Laboratory (LBNL) project.
The good news for anyone buying windows today and in the next ten years is the ongoing technical and market progress. Speaking about overall trends in the window market, John Carmody of the University of Minnesota told EBN, “Over the last 20 years or so, we’ve been through a generation of double-glazed low-e [low-emissivity] windows penetrating the market.” Now, said Carmody, the Energy Star program, codes, and other factors are moving us “toward the next generation of higher performance” with innovations in glazing, coatings, frames, and more.
The window frame material and appearance often get the most attention from occupants, so we’ll start there. Once that foundation is in place, we’ll look at glazing options.
Choosing a Frame Material
Window frame materials are chosen first for structural characteristics, but the material should ideally provide good thermal insulation. Durability, maintenance requirements, and cost also factor in to frame selection.
The share of the window frame and sash market owned by PVC, or vinyl, was only 5% in 1984 but now dominates with 66% (see Frame Materials table). In replacement windows, vinyl has an even bigger edge with 70% of the market. Most vinyl window frames are hollow extrusions but vinyl is also common as a cladding over wood windows.
Topping the list of vinyl’s advantages are its low cost and minimal maintenance requirements. Vinyl windows generally do not need painting, although some consumers complain about warping, cracking, fading, and yellowing that occur over time. Major downsides include their aesthetics, which many people consider inferior, and the environmental costs of PVC production. However, both in terms of structural qualities and energy performance, vinyl is comparable to wood. When the cavities in extruded vinyl are filled with polyurethane foam, energy performance exceeds that of wood.
Vinyl’s Achilles’ heel in practice is its very high coefficient of thermal expansion. Over time, expansion and contraction from temperature changes can loosen seals, and cause cracks at corners and on flanges.
Aluminum: Losing market share
Due to its structural strength in window frames, aluminum (along with steel) has been used in great quantity but is gradually losing market share to vinyl. Aluminum is also used as a cladding for wood windows. Aluminum can have high recycled content and is readily recyclable, but manufacturing of the raw materials is very energy-intensive.
Metal window frames are very thermally conductive, and so they impose a big energy penalty on whole-window performance and can cause condensation. In its 2007 report comparing vinyl to other common building materials on a life-cycle basis, the U.S. Green Building Council (USGBC) found that aluminum windows performed much worse than either vinyl or wood windows, due to poor energy efficiency coupled with its energy-intensive production. To avoid energy penalties, metal window frames must incorporate thermal breaks.
Wood was the first material used in window frame and sash construction, and it remains popular for homes, although it continues to lose market share to vinyl. Wood is attractive, it is a natural and renewable product, it has a warm feel, and it is relatively energy-efficient. With proper care it should be durable, although paints or stains will need to be reapplied over the lifetime of the windows.
The need for strength, dimensional stability, and durability means that only top-quality, knot-free wood is used for windows. Fortunately, the premium price brought by top-quality millwork has helped make wood from certified forests a more common option—although not nearly common enough. Marvin, for example, began in 2010 to offer Forest Stewardship Council (FSC) certification as an option for all its wood products, following Loewen, which made the same move in 2007. EBN doesn’t know of any wood window manufacturers using certified wood as a standard, however.
Although it commands only a fraction of the market, fiberglass has emerged as the fourth-most-popular frame material, and is often found in the highest-performing windows. Fiberglass is durable and strong. Its coefficient of thermal expansion is far lower than that of vinyl or aluminum and much closer to that of glass—which makes it ideal in window frames. According to Robert Clarke, marketing director at Serious Materials, a maker of fiberglass and vinyl windows, the pultrusion process used to make fiberglass frames is relatively slow, and machining the fiberglass requires very hard tools. These factors are likely to keep it at a price premium compared to vinyl.
Fiberglass appears to have an open-ended lifespan, making it more durable than vinyl or wood. According to Clarke, the core material in fiberglass is unstable under UV radiation from sunlight, but “its finishing will protect it forever.” On the down side environmentally, fiberglass is difficult to recycle into anything other than aggregate. Use of recycled material in the windows may not be possible, and air pollution emissions from fiberglass can be significant. The curing of fiberglass resin during production emits pollutants including styrene and volatile organic compounds (VOCs), and fiberglass plants are regulated by the U.S. Environmental Protection Agency (EPA).
Despite the broad market-share statistics quoted above, window frames aren’t so easily categorized into single materials. It is common for combinations of materials to be featured prominently in window frames and interiors, providing benefits representing the best of each material.
Comparing the materials
According to Carmody, existing life-cycle assessment data for window frames is old. If anything, he said, it points to wood as the best material, but he hopes a project currently starting at the University of Minnesota will lead to more up-to-date data. In the USGBC study mentioned earlier, which relied on older data, neither wood nor vinyl emerged as a clear winner, although major concerns were raised about health impacts from vinyl manufacturing. Vinyl is often chosen based on cost, but if choosing on performance and environmental impact, then wood is a better choice, and possibly fiberglass. One of the most exciting reasons to update our life-cycle data will be to evaluate the performance of fiberglass, which is particularly common in super-energy-efficient windows.
As mentioned, aluminum was a clear loser. Aluminum or steel frames should be considered only in climates with little heating or cooling, or when their strength is essential. Even then, thermal breaks should be specified.
In most cases, energy performance will determine the environmental impact of windows over their lifetime, and with most current windows on the market, that will be determined by glazing choices. To understand what goes into high-performance insulated glazing units (IGUs) here’s an overview of key window performance technologies.
Double- and triple-glazing
With storm windows dating back 200 years and sealed double-glazing units dating to the 1930s, adding a second layer of glazing has long been the first step for window manufacturers toward improving energy performance. A second layer of glazing—or a third in the case of triple-glazed windows—improves window insulation by trapping dead air. For example, going from one layer of glass to two with a ¼" (6 mm) air space increases the center-of-glass insulating value from U-1.11 to U-0.57 (R-0.9 to R-1.75).
Double-glazing has become ubiquitous over the last couple of decades, and triple-glazing is now becoming more common in both residential and commercial windows. Triple-glazing is provided through a third layer—either a third pane of glass between the two outside panes or, less commonly, a plastic film suspended between the two panes of glass (Heat Mirror is the best-known film product).
A thicker air space between the panes of glass reduces conductivity of heat through the gas in the space. For example, doing nothing other than increasing the air space from ¼" (6 mm) to ½" (13 mm) increases the center-of-glass insulating value from U-0.57 to U-0.49 (R-1.75 to R-2.04). If the air space gets too wide, however, convection loops form, increasing convective heat loss through the window. With air or argon gas fill (see next section), the optimal thickness is ½" (13 mm); the ideal space for krypton gas is 3/8" (10 mm) and ¼" (6 mm) for xenon.
Low-conductivity gas fill
Because heat conduction across the air space in a sealed IGU contributes to heat loss, we can improve performance by replacing the air with a lower-conductivity gas. The most commonly used gas fill is 90% argon, which is plentiful, inexpensive, and inert. With low-e glass in an IGU with ½" (13 mm) spacing, argon boosts the center-of-glass insulating value from U-0.29 to U-0.23 (R-3.45 to R-4.35). More expensive gases like krypton perform even better and can be found in the highest-performing windows or where a thinner profile is desired.
Although safe, these inert “noble” gases are very buoyant and difficult to contain, and they will leak from an IGU over time. As a technical bulletin from Cardinal Glass states, “No organic seal ultimately can prevent the internal atmosphere of an IGU from becoming the same as the ambient atmosphere over time.” Since the “ambient atmosphere” we breathe contains just 1% argon and much more nitrogen and other gases, that argon or krypton will eventually escape. Accelerated-weathering tests used by major manufacturers require 80% argon to remain after testing; this suggests that after years of service, most of the argon will remain. Citing German research, Clarke told EBN that a loss of 1% of the gas per year is expected. While the benefits of gas fills may not be permanent, they are substantial and long-lasting, and the incremental price premium is easily justified.
First introduced in 1979, low-e glazings have grown tremendously in popularity. A thin transparent coating of silver or tin oxide on the glass surface or on a suspended plastic film with such a coating allows short-wavelength sunlight to pass through but blocks long-wavelength heat radiation. Low-e coatings are not one-size-fits-all: both the type and the placement of these coatings contribute to wide variations in performance metrics.
The most common type of low-e coating is called soft, or sputtered, coat. Thin layers of silver and anti-reflective coatings are applied to the glass surface through a vacuum deposition process. Because the coating is delicate, it must be protected within the IGU.
Pyrolytic or hard-coat low-e glazings have a thin layer of tin oxide incorporated into the surface of the glass during manufacture when the glass is still hot. Hard-coat low-e glazings are durable and can be used in single-glazed windows or storm panels, but their emissivity is not as low as that of soft-coat low-e glazings. Hard-coat glazings generally offer weaker insulating value compared with soft-coat glazings but have higher solar-heat-gain values.
Low-e technology has changed tremendously since single low-e coatings first became common in the 1980s. In the 1990s, a double (layered) low-e coating came along, dubbed “low-e2” or “low-e squared.” According to Clarke, the evolution was due to a market demand for cooler glass, with lower solar-heat-gain. Particularly given that demand, the market also shifted to favor soft coats. Adding standard soft-coat low-e2 glazing to an IG unit with a ½" air space increases the center-of-glass insulating value from U-0.49 to U-0.29 (R-2.04 to R-3.45).
The 2000s have seen “low-e3” (or “low-e cubed”) glazing take hold, with yet another layered low-e coating. Clarke told EBN that again a demand for reduced heat gain, particularly for cooling-dominated office buildings, has driven this shift, along with technical advances allowing coatings that cut out the low and high infrared light while leaving more of the visible light to pass through. Today, low-e, low-e2, and low-e3 coatings are all available, with single low-e coatings making a comeback for heating-dominated climates, and improved hard coatings also available for applications favoring solar heat gain.
Warm-edge glazing spacers
IGUs are sealed around the perimeter by spacers that maintain the distance between the panes of glass and help seal in any gas fills being used. Aluminum has been the most common material for glazing spacers, but it is very thermally conductive. Warm-edge spacers using rubber, foam, silicone, thermally broken steel, and other materials have become common in high-performance windows, and drastically reduce heat loss or gain at the edges of IGUs. Warm-edge spacers with integrated desiccant beads also reduce the risk of fogging within the IGU.
Fixed windows that don’t open are less expensive and, due to simpler construction, more durable than operable units. They are also more airtight, offering an important energy benefit. While operable windows provide ventilation and a better connection to the outdoors, it’s worth considering for each window location whether it should be fixed or operable.
Operable units use weather-stripping to ensure airtightness. As a general rule, hinged casement or awning windows that open out use a compression-type, synthetic weather-stripping gasket that offers a tighter, more durable seal than a seal used on a sliding-sash window. However, there are significant differences among manufacturers and products, so it always makes sense to examine product labels carefully.
Glazing dimensions and lites
Because high-performance glazings generally lose more heat at the edges, the larger the glazing-area-to-perimeter ratio the better the overall window energy performance. Divided lites, in which the sash is divided into multiple individual panes separated by muntins, offer a more traditional look, but they result in reduced glazing area and thus lower overall (unit) insulation value. (These windows also tend to cost more due to a more complicated assembly.)
Windows with larger glazing areas and no subdivided lites, or with simulated lites (through a combination of applied grilles, grilles inside the IGU, or both), offer better energy performance.
Heat loss through windows, solar heat gain during heating periods, and avoidance of solar gain during cooling periods should all factor into window selection. To enable apples-to-apples comparisons among windows, industry leaders formed the National Fenestration Rating Council (NFRC) in 1989 to create standard metrics for comparing performance of windows.
The NFRC procedure for U-factor accounts for the U-factors of the window frame, muntins, and glazing, each of which has different characteristics. It is useful to consider lower center-of-glass U-factors along with whole-window U-factors, but be careful not to confuse the two. U-factors are calculated using a prescribed computer program, and then those values are verified for a small number of products within a product line through actual testing by an accredited testing laboratory. See Choosing Windows table for guidance on what U-factors to aim for.
The solar heat gain coefficient (SHGC) is the ratio (between 0 and 1) of the solar heat gain coming through the window to the incident solar energy striking the window. The value includes both radiant solar gain and solar heat conducted through the window. SHGC replaces an older, confusing, “shading coefficient” metric that sometimes still appears in product literature. Recommended SHGC values for heating and cooling climates are listed in the table.
We install windows to let in light and enjoy the outdoors, so it’s important that we don’t lose sight of this in our search for highly insulating windows. Visible transmittance (VT) is a value from 0 to 1 that indicates the amount of visible light transmitted, taking into consideration light blocked by the frame and muntins. Most double- and triple-glazed windows have values between 0.30 and 0.70. Higher VT is desirable, but lower VT values are often delivered along with lower U-factors. According to Clarke, people start to notice a gray appearance to windows with a VT less than 0.40.
Special considerations in heating climates
Lower U-factors are desirable in both heating and cooling climates, but SHGC introduces complications in heating climates. In climates with significant heating seasons, passive solar heat gain from south-facing windows is desirable. (Not much solar heat gain is available from the north, and overheating is a risk with western windows in winter or summer. Some experts say that solar heat gain is also desirable from the east on winter mornings, but that needs to be balanced against risks of summer overheating.)
The need for passive-solar heat gain is met with higher SHGC values. Unfortunately, due to the different low-e coatings used, higher SHGC values generally also come with somewhat higher U-factors, meaning reduced insulation value. Experts EBN spoke with agree that due to this tradeoff, those higher SHGC/higher U-factor windows should only be sought for the south and possibly east, and other orientations should get the lowest U-factor possible. For locations where summer overheating is a danger, overhangs or other types of shading for high-angle sun should be installed on these passive-solar windows.
For buyers in the northern U.S., however, getting high-performance windows with high SHGC and low U-factor can be tricky. Standard window options from U.S. manufacturers are typically tuned to southern needs. Although Pella, Marvin, and other major U.S. companies are increasingly offering products sensitive to northern buyers, Canadian companies are generally ahead of the game.
Minute cracks in the window assembly can add up to significant heat loss and gain, so it’s worth looking for air leakage numbers (typically lower in fixed or casement windows—see above). NFRC labels sometimes provide an air leakage rating (AL), although it is not required, and manufacturers often omit it. AL is expressed as cubic feet of air passing through a square foot of window area (CFM/ft2). The lower the AL, the less air will pass through cracks in the window assembly. Select windows with an AL of 0.30 or less.
“Superwindows”—At a Cost
An increasing number of window manufacturers are combining multiple layers of glazing, multiple low-e coatings, and very-low-conductivity gases such as krypton to create super-high-performance windows, or “superwindows,” a term coined by Dariush Arasteh, a staff scientist at Lawrence Berkeley National Laboratory. In the early 1990s, Arasteh predicted that advances in technology could make all windows, even north-facing windows in northern climates, net-energy-gainers. Whether or not that day has arrived is a matter of debate, but there’s no doubt that the advances since the 1990s, when window buyers were dreaming of U-0.05 (R-20) windows (and window makers were making tantalizing demonstrations), have been astounding.
Today, that race has cooled off to some extent in favor of climate-specific solutions. As Stephen Thwaites of Thermotech in Canada told EBN, “A window doesn’t have to be R-20 to be as energy-efficient as the wall around it,” due to the ability of a window to gain solar heat and provide ventilation. “A home with no windows will use more energy than a properly designed home with R-5 windows,” he said. That puts the emphasis on proper design by orientation, shading, and window-to-wall ratios, and on buying the best windows for each application according to the budget.
It’s still exciting to dream of what we’ll see in the next 30 years. For example, vacuum glazing, in which most of the air is evacuated from the space between panes, reduces thermal conduction and convection to nearly zero (leaving radiation as the primary means of heat transfer), and can currently offer U-0.08 (R-12) with double-glazing and one advanced low-e coating. Steve Selkowitz of LBNL cautions that vacuum glazing is still largely in research and development, however, telling EBN, “The key issue is the seal, which is a much tougher problem than a normal IGU.” Because the vacuum between the glazing tends to pull the panes of glass together, tiny glass or ceramic spacers are also needed in a grid pattern to hold the glass apart—a technical and aesthetic complication.
Other technical wonders are electrochromic glazing, a technology available now in some commercial and residential windows in which the glazing darkens at the push of a button (seeEBN June 2006) and photochromic glazing (darkening triggered by light) or thermochromic glazing (darkening triggered by temperature), both of which are under research. For windows where light but not views are wanted, glazing filled with the translucent material aerogel can offer good thermal performance—up to about R-8 per inch.
The biggest limit on energy performance is and may continue to be the wallet of the buyer. Windows imported from Germany meeting the Passivhaus standard, for example, offer U-factors under 0.14—at a cost of over $90/ft2 of window area. Triple-glazed Canadian windows typically cost $40–$50/ft2, in contrast with a price range for more conventional double-glazed windows of $30–$35.
Choosing your windows
There are a lot of window options; making a choice should include environmental and energy considerations (see Choosing Windows table), but in the end can come down to subjective factors such as aesthetics or how well the sales rep sells the durability of the showroom model.
Whatever you choose, don’t be too cheap, said Selkowitz. “People make most of the decisions about a home based on preferences and amenities.” The same homeowner who doesn’t think twice about a granite countertop might balk at the cost of a higher-performing window, but Selkowitz said it’s an important investment that offers more “payback” than the countertop ever will.
To get the best performance for your money, you may also need to go your own way. In southern Vermont, Kirstin Edelglass invested countless hours in coming to a decision on windows and doors for her family’s home, looking at seven manufacturers before settling on a combination of three of them. In the process, she up-ended several applecarts, as she related to EBN.
Edelglass cut her costs by over 30%, largely by substituting fixed windows for operable (keeping enough operable glazing for natural ventilation). She used the company recommended by her homebuilder for only the basement windows after deciding at the showroom that those less-expensive models weren’t built as well. She rejected one company because it was sacrificing performance for her climate by putting the low-e coating on the wrong glazing surface, and with the company she used for the bulk of her windows, she insisted on a glazing different than usual for the company for higher SHGC values. She also asked her builder to read the installation guide provided by the manufacturer, which led to a fruitful conversation about best practices in window installation.