An in-depth look at the fascinating world of low-conductivity gas-fill for high-performance windows.

Over the last two weeks I've covered the major strategies for improving the energy performance of windows: adding extra layers of glass, increasing the thickness of the airspace between the layers of glass, and adding low-emissivity coatings. Another important strategy is to use a low-conductivity gas instead of air in the space between the layers of glass. Most commonly argon is used, though krypton is available for the highest-performance windows, and xenon is occasionally used.

Low-conductivity gas-fills don't make as much difference as adding an additional layer of glazing, increasing the spacing between the layers of glass from a quarter- to a half-inch, for adding a low-e coating, but they are nonetheless significant--and definitely worth it when choosing new windows that have low-e coatings. Adding argon is the most cost-effective improvement you can make to a window. But what are these gas-fills and how do they work?

Why low-conductivity gases make sense To understand how argon works, we have to go back to how heat moves through windows. There are three modes of heat transfer: conduction, convection, and radiation. With clear (non-low-e) double-glazed windows, radiation accounts for about half of the heat transfer, with conduction and convection each accounting for about 25%. When a low-e coating is added to a window (see last week's blog), the radiant component of that heat loss is significantly reduced, and as a result the conductive and convective portions become much more significant. As the name implies, low-conductivity gas fills reduce conductive heat flow. Most of us think of conduction, which is the transfer of kinetic energy from molecule to molecule, as occurring only through solids--think of a hot cast-iron skillet handle--but conduction also occurs across gases. Sometimes we refer to this as gas-phase conductivity.

The Noble gases

Air has a thermal conductivity of 0.014 Btus per square foot per hour for every degree Fahrenheit difference in temperature (don't worry about those units) at room temperatures. If we can replace that air with a lower-conductivity gas, we can slow the heat loss through windows. Argon is a great option. It has a conductivity of 0.0092--34% lower than that of air--and it is, by far, the most common low-conductivity gas for windows.

Some of the highest-performing windows use the more exotic gas, krypton, in the space between the glass. Krypton has a conductivity of 0.0051, which is 63% lower than that of air. Xenon, an even rarer gas, has a conductivity that's 79% lower. These gases--found in the far-right-hand column of the Periodic Table--are all highly stable and unreactive, an attribute that earned them the moniker of "noble gases" (so named because, like nobility, they don't interact with commoners).

All of these gases are components of the air we breathe. Argon makes up a little less than one percent of our atmosphere (third after nitrogen and oxygen) and is produced quite inexpensively as a byproduct of extracting oxygen out of the air. Krypton is present in air at a concentration of about one part per million (one ten-thousandths of a percent), and xenon is present at an even smaller concentration. As a result, these exotic gases are far more expensive to extract.

Buying a window filled with krypton instead of argon adds about $100 to the price, according to a Marvin Windows and Doors rep I spoke with recently, while there is little if any additional cost for argon. At a manufacturing cost of only about 10¢ per window, it's one of the best deals around, according to Randi Ernst, president of FRD Design, Inc., which sells gas-filling equipment to the window industry.

The benefit of low-conductivity gases

Adding argon to a double-glazed window reduces the U-factor by about 0.05 (reducing the U-factor means reducing heat flow). With non-low-e glass, adding argon drops the U-factor from 0.50 to 0.45, a 10% reduction in heat loss (assuming optimal spacing for the glass). When there's a low-e coating, that same argon improves the U-factor from 0.30 to 0.25--a much more respectable 17% improvement in performance. Using krypton with an optimal spacing drops the U-factor by another 0.025, so the total improvement over air is 25%.

The optimal thickness for gas-fill

With an insulating glass unit (IGU), there is an optimal thickness that varies according to the gas fill. With a thicker air space there's less conductive heat loss, but if the spacing gets too deep convective loops form that begin increasing heat loss (see my blog two weeks ago). With air, the optimal thickness for the air space is about a half-inch--assuming the standardized temperature conditions used for modeling window performance in this country. Argon is about the same--just a few millimeters thinner.

Significantly, if we assumed a lower difference in temperature (delta-T) between the indoors and outdoors, as they assume in Europe, there would be less convection between the glass and the optimal thickness would be greater--as we find on European windows. Because most of the U.S. actually experiences a significantly lower delta-T than the 70°F assumed in U.S. standards, a thicker glazing spacing actually makes sense.

With krypton, though, the optimal thickness is significantly less: about 5/16th of an inch (with U.S. delta-T assumptions). This is because krypton is more slippery than air or argon. It forms convective loops more easily, which increases that convective component of heat flow.  

Do we really want radioactive windows?

It is a relatively little-known fact that krypton is somewhat radioactive. There are a lot of isotopes of krypton; krypton-85 with a half-life of 10.8 years, is the one that raises concern. Krypton-85 is produced by the fission of uranium and plutonium, and it gets released in the atmosphere through nuclear bomb testing, releases by nuclear power plants, and by the reprocessing of spent nuclear fuel.

The latter source is the most significant, and a majority of that comes from the French reprocessing plant, Cogema La Hague, which has been operating since 1976. The concentration of krypton-85 in the atmosphere has increased several-hundred-fold since the early 1940s, and some of that krypton-85 ends up in the krypton we extract from the atmosphere. As a result, canisters of krypton gas have measurable levels of radioactivity.

Is this significant for us, though? Probably not very. In most areas, the radioactivity from krypton in our windows will be lower than background radiation. If we're willing to live with other sources of radiation in buildings, such as concrete foundations and granite countertops, we probably shouldn't worry too much about krypton. However, ionizing radiation is cumulative, and when we can avoid exposure we should try to do so.

Does the gas stick around?

The question of whether the low-conductivity gas lasts in an IGU is huge. If it leaks out in a few years, it wouldn't be worth spending more for it. The rule-of-thumb, based on laboratory testing, is that 1% of the gas will be lost per year. Oddly, there has been very little research done on gas retention rates in the field.

Fortunately, the research that has been done offers generally good news. Randi Ernst has done about the only field testing of gas retention rates that I know of. From repeatedly testing several dozen windows over a period of years, he has found that about 0.6% of the gas leaks out per year.

That's a pretty low leakage rate: a window starting with 95% argon would be down to 79% argon after 30 years and 70% argon after 50 years. Even assuming 1% annual loss, after 30 years, there will still be 70% of the original argon, and after 50 years 58%. Most windows don't last 50 years for other reasons, so I'm comfortable with the gas retention.

Bottom line

It's always worth adding low-conductivity gas fill to an IGU. While I'm not terribly worried about the radioactivity of krypton, it does give me pause, and we get far more bang for the buck with argon. If I want better energy performance than can be achieved with low-e and argon in an IGU, rather than replace the argon with krypton, I'll specify a third layer of glass with another low-e coating or a second low-e coating on the inner (#4 surface) of a double-glazed IGU (see last week's blog).

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. Archives of the Energy Solutions blog can be found here. To keep up with his latest articles and musings, you can sign up for his Twitter feed