Getting "Under the Hood" with Energy-Efficient Windows
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It has been a great spring so far for spotting wildlife. A neighbor told me he was shooing a black bear away from his garbage the other day when he saw that he had also frightened off a moose that was also in the neighborhood. Perhaps the moose and... Look at what we ask windows to do. 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. We also 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. Windows do it all And like the porcupine, windows do all that by just sitting there--very few, simple "moving parts," no motors, no pumps, no electricity. How is this possible? The secret is in the engineering. The last 20 years have brought us a whole new generation of high-performance windows that your grandparents wouldn't recognize--double-glazed with low-emissivity (low-e) coatings. Today we are in a midst of the birth of a new generation of windows. Many are now triple-glazed, incorporate multiple low-e coatings, improved frame insulation, and more. It'll take a couple columns to unpack everything about windows that's worth talking about, so consider this a starter. (If you want to cut to the chase, read my full guide to window selection in Environmental Building News.) 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. The number of glazing choices out there can be dizzying. But pay attention and do your homework. It could be easy to get talked into choosing windows that sound really good, but that don't make sense for your climate, your building, or even for the specific wall where you install them. Doubling down on 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 R-0.9 to R-1.75. Double-glazing might have sounded good 10 years ago, but 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). Because heat conduction across the air space in a sealed insulated glass unit (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. Getting argon is a no-brainer, but if it's not specified, you probably aren't getting it--again, ask. Do argon and krypton leak over time? You might think--why invest in argon or krypton, when those gases could leak from the window 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, Robert Clarke of Serious Materials (formerly Alpen Windows) told me 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. A third generation of low-e coatings 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 IGU 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. Asking your dealer or builder what type of low-e coating you're getting, and what attributes it offers, is essential. If you're a builder, think carefully about what you're providing to your client--the choice will matter for decades to come. We'll add more pieces to the window performance puzzle--and begin to put those pieces together--in future columns. Keep your questions, comments, and wildlife stories coming! Tristan Roberts is Editorial Director at BuildingGreen, Inc., in Brattleboro, Vermont, which publishes information on green building solutions. Tristan authored a full guide to choosing high-performance windows available to EBN subscribers. Image: Advanced triple-glazed, low-e Sorpetaler windows from Germany were used in this newly constructed Passive House in Palo Alto, California, designed by Arkin-Tilt Architects and built by Quantum Builders of Berkeley.