IGUs: What You Need to Know
Forget the notion of glaziers setting individual panes of glass in a window frame. That model no longer applies. You might be surprised to learn that what we call “window glass” is an assembly of several parts called the IGU. Parts are assembled in a factory—the idea was patented in 1865—as a single unit. Large manufacturers such as Cardinal, Saint-Gobain, or Pittsburgh-based PPG Industries supply IGUs to window manufacturers such as Pella, Andersen, and Marvin. Because of this IGU assembly, you cannot easily repair modern windows. When your child breaks your neighbor’s window with a baseball, you are on the hook for a couple of hundred dollars to replace the IGU, sash, or entire window.
Gas-Filled IGUs: Argon and Krypton
Inside this sealed assembly are inert gasses. These gasses slow heat conductivity better than air because they are denser. Krypton and argon are the most commonly used gases sealed inside of the IGU. These gases are dense, so they impede the transfer of outside cold to a warm house better than air. Xenon is the densest gas, but it is not yet widely used in windows.
Most Windows Are Gas-Filled
Unless you can manage to find an off-brand air-filled window, most new construction and replacement windows are filled with argon or krypton. Gas-filled windows are standard.
Which Gas Should You Choose?
It is always better to have a window that is filled with gas, rather than one filled with air. Look at the thermal conductivity numbers for three gasses: To put the gasses’ numbers in perspective, consider copper’s sky-high thermal conductivity rating of 401.
How Gases Improve Performance
A double-pane argon filled window will have a U-value of .25, while a double-pane air-filled window has a U-value of .30. It’s an improvement but not by leaps and bounds. Even PPG says that “[w]hen 90 percent argon gas fill is used in a low-e IGU instead of air, the window’s u-value can be improved by up to 16 percent. Similarly, krypton improves the u-value in a low-e IGU by up to 27 percent.” Once again, an improvement. But you are not doubling or tripling your window’s performance with these gases.
Not an Air Vacuum, Not Pure Gas
One myth about window IGUs is that that they are an “air vacuum.” IGUs are not voided of all air. In its “Pella 2016 Architectural Design Manual,” Pella notes that: “Using available argon filling equipment, it is not possible to achieve 100 percent argon filling.” Window IGUs always contain some air, to begin with, and they will gain more air as time goes by. Pella states that the reason for keeping air in the IGU is because higher concentrations of argon is a “driving force that causes the argon to permeate through insulating glass edge seals slowly.”
You Will Lose Gas Anyway
Loss of gas is a fact of life with IGUs. The question is, How much and how fast? PPG notes that even perfectly constructed IGUs can be expected to lose about 1 percent of its gas per year. Many windows’ seals will fail more dramatically and earlier, so 1 percent is a best-case scenario.
Reframing Gas Loss as “Argon-Air Exchange”
Pella reframes seal failure and gas loss as “an argon-air exchange.” While this may sound like marketing double-speak, it is a valid argument. They note that one unit they researched had an initial argon level of 90 percent. This resulted in a total unit U-Factor of 0.37. With the “1 percent per year” rule, within 20 years the argon level would be 70 percent. The U-Factor is now at 0.38.
