When shopping for replacement windows, you will find that salespeople love to latch onto your dreams of beautiful frame colors and shiny hardware. Complex technical matters tend to get brushed over.
However, one technical element of the window they love to talk about is the insulating gas. This is because of these beautiful words—krypton, argon, xenon—which help them sell you windows.
- Windows are sealed units called IGUs.
- IGUs are filled with dense gasses.
- Krypton is the best gas, but argon is more widely used.
- All window IGUs will lose gas.
- Even windows that "exchange gas for air" can perform well.
The IGU and You
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 Insulating Glass Unit or 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, PA-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.
And now we have come to a major selling point in the replacement salesperson's presentation. Let those words roll off your tongue—krypton, argon, xenon.
Mainly krypton and argon are sealed inside of the IGU. These gasses are dense, and 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.
Gas Is Nothing Special: 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. Saying that gas in a window is a bonus is like saying that it is a bonus for your new car to have four tires.
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:
Lower numbers represent gasses that are better for slowing down thermal transmission. Argon is better than air; krypton is better than argon, xenon is better than krypton.
To put the gasses' numbers in perspective, consider copper's sky-high thermal conductivity rating of 401.
Improvement, But Not Dramatically
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.
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 gasses.
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% argon filling."
Window IGUs always contain some air, to begin with, and they will gain more air as time goes by (see next section for this).
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% of its gas per year. Many windows' seals will fail more dramatically and earlier, so 1% 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%. This resulted in a total unit U-Factor of 0.37.
With the "1% per year" rule, within 20 years the argon level would be 70%. The U-Factor is now at 0.38,