Blog Post

Getting "Under the Hood" with Energy-Efficient Windows

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

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 bear are rehearsing for a new wildlife buddy movie?

I spotted a porcupine crossing the driveway last week. I love that fact that porcupines don't seem to give a damn. I'm sure that they are capable of running, but unlike a lot of animals that run when encountered, porcupines seem to just keep on going wherever they were going, at the same measured pace, with the same loopy sort of lope. When worst comes to worst, they can simply sit there, knowing that only a handful of predators can get around those quills.

What's the highest-tech part of your home?

What's the highest-tech piece of engineering that's a part of your home or workplace? I would suggest that by some measure, it's not your furnace, your fridge, your dishwasher, your hot water heater, or even your ground-source heat pump and radiant heat, if you have that. It's your windows. 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. 

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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!

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.

Published May 23, 2011

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Comments

May 24, 2011 - 5:56 am

Thanks for your comments, Mike. When I was shopping for windows for my "Passive-like" house I had difficulty finding triple pane windows with reasonable SHGCs. What are typical numbers for SHGCs for newer triple pane windows?
As for the shutter suggestion, I realize that an ideal window captures solar radiation during the day without loosing heat at night. But since that ideal cannot be reached, insulating shutters on low U-value windows should make such a window-shutter system approach that ideal. A good shutter design would also be able to minimize window frame conduction losses. Shutters need not be ugly or non-conventional in appearance. I could not find any real insulating shutter designs either. Do they even exist? I certainly can envision designs for such shutters. Maybe I should start a shutter business.

May 24, 2011 - 7:32 pm

You don't need to build to extreme PassiveHaus standards to get a net heat gain from south glazing. Even clear double-glazed windows have been giving a net energy benefit in cold climate passive solar homes for more than the 30 years I've been building them. And even without internal insulating shades or exterior shutters (which then makes it other than truly passive).

In fact, given the difficulty and expense of securing high SHGC triple-glazed windows in the States, I have found that a good-quality insulating frame with lowE double-glazed argon filled IGU is the most cost effective option, at least for the south facade. Fortunately, Pella now offers a 0.50 SHGC "Natural Sun" double glazing in their very cost-competitive Proline (aluminum clad wood) and Impervia (insulated fiberglass) window lines.

May 23, 2011 - 10:20 pm

wolfger,

it can get a little more complicated when talking about heat loss and high performance windows...

true, you can have windows that, as a component, have a lower U-value than adjacent construction. however, high performance windows in a Passivhaus can be energy positive, meaning they let in significantly more energy that they lose.

in several projects we've been looking at, high SHGC triple pane windows - even in cloudier environs - "produce" twice as much energy as they lose.

then the only shutters you need are external and to prevent overheating in summer/shoulder - which operate and look a heck of a lot better than insulated shutters....

May 23, 2011 - 6:27 pm

Even the best windows still have R values much lower than one can easily design and place into the walls, ceilings, and floors and thus represent a large heat loss percentage for the total house. It would seem to me that the time has come to reconsider external shutters not just the old fashioned type but some good high-tech insulating and airtight shutters. Is there any activity in this area? Will you touch on this at all?

May 26, 2011 - 10:46 am

Wolfger,
Along the southern and gulf coasts we often use roll down coiling hurricane shutters.
Primarily for hurricane protection but they also work well to keep heat in at night.
They can be lowered to varying levels to control day time heat and glare.
You can put them all the way down which is almost airtight or down just enough so each slat slightly open. The slats jambs are perforated which allows some light, views and airflow with the window open.
These shutters also provide a level of security and rain protection so you can leave the open whenever you want natural ventilation.

The use of these shutters eliminates the need for expensive impact glazing.

Both the shutter and the impact glazing would need to be replaced if they encountered a direct hit during a storm but hopefully the structure would remain which is the point and very sustainable.

Unfortunately, shutter manufacturers are probably in the process of testing their products for tornadoes, due to recent storms.

May 27, 2011 - 4:58 am

Thanks for that information, Linda. I'm quite familiar with roller shutters from Germany where they are integrated by design into the wall-window system to be invisible in the withdrawn state. I have not found such an approach here, though I admit I haven't searched lately.