Argon is the right gas for a standard double-pane window with a roughly half-inch air space. Krypton is the right gas when the space is narrow, about a quarter to three-eighths of an inch, as in thin triple-pane units. Both are denser and less conductive than air; krypton insulates better per inch but costs far more, so it only pays where the gap is too thin for argon.
This post goes deeper on the gas itself: why the gap width decides which one to use, how much really leaks out over the years, what the initial fill looks like coming off the line, and how to weigh it against the other parts of the glass package. For the overview of Low-E, argon and U-factor together, start at jmbbuildersmn.com/blog/low-e-glass-argon-windows-explained.
What does the gas between the panes actually do?
Heat crosses the space between two panes three ways. It radiates from the warm pane to the cold one, which a Low-E coating handles. It conducts through whatever gas fills the space. And it convects: gas warms on the room side, rises, cools on the outdoor side, sinks, and carries heat around the loop.
A denser, less conductive gas slows both of the last two. Argon and krypton are inert, colorless, odorless and non-toxic. Their atoms are heavier than the nitrogen and oxygen in air, which makes them sluggish: they conduct less heat and set up weaker convection currents. Glass makers report argon's conductivity at roughly a third lower than air and krypton's at close to two-thirds lower.
That is the whole trick. The gas is not doing anything exotic; it is a heavier, lazier version of the air that would otherwise be there. The gain is real but modest on its own, and it is the combination with a Low-E coating that makes a modern glass unit perform.
Argon vs. krypton vs. air: how do they compare?
Side by side, with cost described in words rather than numbers because it moves with the market and the manufacturer.
| Air | Argon | Krypton | |
|---|---|---|---|
| Thermal conductivity vs. air | Baseline | About one-third lower | About two-thirds lower |
| Best gap width | Any | Around 1/2 inch | About 1/4 to 3/8 inch |
| Typical use | Budget or older units | Standard double-pane; wide-gap triple-pane | Thin triple-pane; slim retrofit units |
| Cost | None | Small premium, often included | Many times argon; adds meaningfully to the glass price |
| Where it is common | Sheds, garages, cheapest replacements | Almost every Northern-zone window | High-end triple-pane and passive-house glass |
Why does the gap width decide which gas to use?
Every gas has a gap width where it insulates best. Too narrow and conduction straight across dominates, because there is not enough gas between the panes to slow it. Too wide and convection takes over, because the gas has room to circulate and carry heat around. In between is the sweet spot.
For argon that spot is around a half inch, which is why the standard double-pane unit is built with a gap close to that. For krypton, which is denser still, the sweet spot is narrower, about a quarter to three-eighths of an inch. In a half-inch gap krypton still beats argon, but not by enough to justify its price. In a quarter-inch gap argon has barely started working and krypton is at its best.
This is why krypton shows up almost entirely in triple-pane glass. A triple-pane unit built to the overall thickness of a double-pane, so it fits a standard sash, has two gaps of roughly a quarter inch each. Argon in those gaps underperforms; krypton in them is what lets the thin triple reach a low U-factor. A triple-pane unit with a thicker sash and two half-inch gaps can use argon and does not need krypton at all.
Does the gas leak out over time?
Yes, slowly. Gas molecules migrate through the edge seal, and the rate depends on the spacer system, the sealant, the quality of the factory seal and the temperature swings the unit lives through. The figure quoted most often in the industry, including by home-inspector training material, is on the order of one percent per year. At that rate a unit still holds most of its argon after twenty years, and the change in U-factor over that span is small.
The bigger truth is the initial fill. No production line gets a unit to one hundred percent argon; some air always remains. Manufacturers certify a fill level and the NFRC rating is calculated on the basis of that certified level, not a perfect one. Well-run lines fill high; poorly run ones fill lower and the unit never had the rating on the sticker to begin with. You cannot see this from outside, which is why buying from a manufacturer with a real certification program matters more than the word argon on the brochure.
Fast loss looks different. When the edge seal fails outright, the gas goes quickly, moist air comes in, and the unit fogs or streaks between the panes. That is a dead unit, not a slow leak, and it is covered in our foggy-window guide at jmbbuildersmn.com/blog/foggy-windows-failed-seals-what-to-do. The gas cannot be topped up; the insulated glass unit is replaced as a piece, usually without replacing the window frame.
How much does the gas change the U-factor?
Less than the Low-E coating, more than nothing. One European glass maker's published comparison puts argon's effect at about a tenth off the center-of-glass U-factor on plain double glazing and closer to a sixth when paired with a Low-E coating, with krypton improving on that by a further margin. The gas and the coating help each other: once radiant loss is handled by the coating, conduction and convection through the gap become a bigger share of what is left, so the gas has more to fix.
Whole-window numbers shift less than center-of-glass numbers because the frame and the spacer are unaffected by what gas is in the middle. On a typical Northern-zone double-pane, dropping argon from the order might raise the unit U-factor by a couple of hundredths, enough to fall off the ENERGY STAR line, which is why nearly every certified Northern window includes it.
The place gas matters most is triple-pane. Two gas spaces mean two chances to slow conduction, and in a thin triple with narrow gaps krypton is often the difference between a good U-factor and an excellent one. That is the case where paying for krypton is a performance decision rather than a marketing one.
Is krypton worth it in Minnesota?
For a standard double-pane replacement, no. Put the money into the parts that move the number more: a room-side Low-E coating on surface 4, a warm-edge spacer instead of aluminum, and a foam-filled or fiberglass frame. Any one of those does more for a half-inch double-pane than swapping argon for krypton.
For a triple-pane that has to fit an existing sash depth, such as an insert replacement into old wood frames or a historic house where a thicker sash would look wrong, krypton is often the only way to reach the rating the manufacturer advertises. In that case it is worth it, and you should confirm the U-factor of the exact unit with krypton rather than the series average.
For a full-frame replacement or new construction where a thicker triple-pane sash fits, a wide-gap argon triple-pane usually gets to the same place for less. The frame has to be built for the weight, and the glass sits further out toward the room, which slightly raises the inside surface temperature. That warmer glass is what you feel in February, and it comes from three panes, not from which gas is between them.
How do I tell whether my windows have argon?
You cannot see it; argon and krypton are colorless. Three ways to find out:
- The NFRC label, if it is still on the glass or you have the order paperwork. The U-factor listed is for the glass package as sold, and the manufacturer's spec sheet for that package will say whether it includes gas.
- Look at the spacer edge inside the unit for a small sealed hole or a short capped tube. Some fabricators fill through a hole in the spacer and plug it; others use breather tubes for units shipped over mountain passes and seal them on site. Either is a clue the unit was gas-filled, though absence proves nothing.
- Ask the manufacturer with the etched or stamped code on the spacer. Most units carry a maker's mark and date, and the maker can tell you what that line was built with.
What JMB Builders does
JMB Builders is a licensed Minnesota residential building contractor (BC809005) based in Zimmerman, working across the Twin Cities metro. We install every brand and are a dealer for none. One crew measures, installs, seals the frame gap and trims. Paid by check or direct deposit. Call 763-204-2099 or send photos through the site.
- Labor-only installation of windows you bought at a store or online — jmbbuildersmn.com/services/labor-only
- Install a window you bought online, checked against the opening before you order — jmbbuildersmn.com/services/doors/install-window-bought-online
- Insert and full-frame replacement windows and doors, measured and ordered by us — jmbbuildersmn.com/services/full-service
- Marvin Essential and Elevate installation, labor only — jmbbuildersmn.com/services/labor-only/marvin-essential-window-install-labor-only
- Send three photos, written quote within one business day.
Frequently asked questions
Is krypton better than argon in windows?
Krypton insulates better per inch of gap because it is denser and less conductive, roughly two-thirds less conductive than air compared with about one-third for argon. But it costs many times more, and in the half-inch gap of a normal double-pane unit its advantage over argon is small. Krypton is the better choice only in narrow gaps of about a quarter to three-eighths of an inch, which are found in thin triple-pane units.
How long does argon gas last in windows?
Industry estimates put the loss through a sound edge seal at around one percent a year, so a well-made unit keeps most of its argon for twenty years or more. The gas is lost quickly only when the edge seal fails, and that shows up as fog or streaks between the panes. At that point the insulated glass unit is replaced; it cannot be refilled.
Can you refill argon in a window?
Not in any practical way. The gas leaves because the edge seal has failed, and a failed seal lets moist air back in as fast as new gas could be added. The fix is to replace the sealed glass unit, which can usually be done in the existing sash without replacing the whole window.
Do argon-filled windows make a difference in Minnesota?
Yes, a moderate one. Argon lowers the center-of-glass U-factor by a noticeable margin when combined with a Low-E coating, and on a typical double-pane it is often the difference between meeting and missing the ENERGY STAR Northern criteria. It is standard on nearly every window sold for a cold climate and is rarely worth leaving off.
What is the ideal gap between window panes for argon?
Around a half inch. Narrower than that and conduction straight across the gap grows because there is less gas in the way; wider and convection currents start moving heat around the loop. Krypton's ideal gap is narrower, about a quarter to three-eighths of an inch, which is why it is used in thin triple-pane units.
Are windows really 100 percent argon filled?
No. Factory filling always leaves some air in the unit. Manufacturers certify a fill level and the published U-factor is based on that certified level. Good production lines fill high and test it; the word argon on a brochure says nothing about how full the unit actually is, which is one reason to buy from a manufacturer with a real certification program.
