Argon vs Krypton: What the Gas in Your Windows Does

Argon and krypton are inert, nontoxic gases sealed between the panes of an insulated window in place of ordinary air, to slow the heat that moves through the glass ³. The U.S. Department of Energy lists double or triple panes with inert gases such as argon between them among the features of ENERGY STAR windows, and says the gas improves the window's ability to insulate against unwanted heat flow in either direction ¹. On the label, the effect shows up as a lower U-factor ³.

Argon is the common, lower-cost fill. Krypton insulates better but costs more, and it is used mainly where the gaps between panes are narrow, as in thin triple-pane glass ^{4 5}. Both gases slowly escape over the years; NFRC's consumer site puts the leakage for an efficient window at about 0.5% of the fill per year ⁴.

What the gas actually does

Manufacturers use argon and krypton because they are denser than air and so block more heat transfer ⁴. NFRC defines a gas fill simply as a gas other than air, usually argon or krypton, placed between the panes to reduce the U-factor ³.

DOE's Energy Saver Guide pairs the two features: in colder climates it suggests gas-filled windows with low-E coatings to reduce heat loss ². That pairing matters. Cardinal, a glass maker, does not recommend argon in units without a low-E coating, because on its own the gas gives only a small improvement ⁷. Its modeled clear double-pane unit goes from a U-factor of 0.48 with air to 0.46 with 90% argon, with no change in visible light (82%) or solar heat gain coefficient (0.78) ⁷. Vitro, another glass maker, says 90% argon in a low-E unit can improve the insulating value by up to 16% compared with air, and that krypton can improve insulation by up to 27% ⁸.

The gas does not change how much sunlight or daylight comes through; those are set by the glass and its coating ⁷. For those numbers, see solar heat gain coefficient explained and low-E glass explained.

Argon vs krypton

NFRC adds that some manufacturers sell a mix of the two gases to balance performance and cost ⁴. If a quote says "gas filled" without naming the gas, ask which one and in what share.

When krypton is used: thin triple-pane glass

A standard triple-pane unit is thicker and heavier than a double-pane one; PNNL, a DOE national laboratory, says triple-pane windows can be 25% to 50% heavier than double-pane windows of similar size ⁶. The newer "thin triple" design keeps the unit slim by using a narrow center pane and narrower gaps between the panes, often filled with krypton rather than argon, so that an insulated glass unit with triple-pane performance fits in a frame built for double-pane glass ⁵. PNNL says that makes thin triples easier to retrofit into existing double-pane frames ⁶.

Outside thin triples, NFRC describes argon as the popular and affordable choice, with krypton the costlier option that gives the biggest efficiency gain ⁴. More on three-pane units, and when they make sense, is in triple-pane windows.

Does the gas leak out over time?

Some, slowly. NFRC's consumer site says some gas can leak from a window over time, that this is not a safety or health concern because the gases are non-toxic, and that it causes a small drop in efficiency ⁴. It puts the leakage rate for an efficient window at about 0.5% of the fill per year, so that after 20 years the window would still perform at 90% of its original capacity ⁴.

Manufacturers describe it in similar terms:

• The starting fill is not 100%. Cardinal says sealed units cannot be completely filled with any gas, designs its units for an average initial argon fill of 90% or more, and notes that grilles inside the airspace hold ordinary air and can lower the fill by several percentage points ⁷.
• The industry test. Under ASTM E2190, Cardinal says test units must start at an average of at least 90% argon and still hold at least 80% after the weathering cycle ⁷. Vitro describes the related test as measuring argon before and after the equivalent of 10 years of weathering, allowing a loss of up to 10%, or an average of 1% per year ⁸.
• Bigger units lose less. The test specimen is small, 14 by 20 inches. Vitro calculates 0.52% per year for that size and 0.11% per year for a 60 by 100 inch unit, and says the loss slows as the gas escapes ⁸.
• No unit-by-unit promise. Cardinal reports independent lab results below 1% per year under the European EN 1279-3 test, but says it cannot assure that any particular unit will match that rate ⁷.

Slow leakage through an intact seal is different from a failed seal. NFRC lists failed seals among the causes of fogging inside a sealed unit ³. That case is covered in foggy window between panes.

What it adds to the price

The gas is usually part of a glass package rather than a line item. In its window glass replacement guide (updated August 29, 2025), Fixr lists material-only ranges of $375 to $850 for argon-filled glass and $525 to $1,190 for krypton-filled glass, and says labor adds about $50 to $125 ⁹. Because the fill works together with the coating and the number of panes, compare the NFRC U-factor on each quote rather than the gas alone; window U-factor explained shows how to read it.

Questions to ask an installer

1. Which gas is in the quoted units, argon, krypton or a mix?
2. Is the gas paired with a low-E coating, and which one?
3. What are the NFRC U-factor and solar heat gain numbers for the exact unit quoted?
4. Do internal grilles sit between the panes, and does that change the rating?
5. What does the glass warranty say about seal failure? See window warranties explained.

The bigger picture on efficient windows is in energy-efficient windows explained. When you are ready, you can compare quotes from window installers and ask each one to name the fill.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Official record: DOE consumer guide (2021 PDF) listing double or triple panes with inert gases such as argon among ENERGY STAR window features, checked October 26, 2026.
• ² Official record: DOE Energy Saver Guide (2022 PDF) suggesting gas-filled windows with low-E coatings in colder climates, checked October 26, 2026.
• ³ Official record: NFRC glossary definitions of argon, krypton, gas fill and fogging, checked October 26, 2026.
• ⁴ Official record: NFRC consumer page on gas fills (denser than air, krypton costlier, mixes, non-toxic, about 0.5% leakage per year and 90% performance after 20 years), checked October 26, 2026.
• ⁵ Official record: DOE Building America Solution Center description of thin-triple windows with narrower, often krypton-filled gaps that fit double-pane frames, checked October 26, 2026.
• ⁶ Official record: PNNL article (March 3, 2022) on triple-pane windows being 25% to 50% heavier and thin triples fitting existing double-pane frames, checked October 26, 2026.
• ⁷ Reported: Cardinal IG technical bulletin IG02 (August 2022) on 90% initial argon fill, grilles, the ASTM E2190 90% and 80% thresholds, EN 1279-3 results below 1% per year, and the 0.48 vs 0.46 U-factor example without low-E, checked October 26, 2026.
• ⁸ Reported: Vitro article (updated August 21, 2026) on up to 16% and 27% insulating gains, krypton costing up to 40 times as much as argon, the 10% over 10 years test allowance, and 0.52% vs 0.11% yearly loss by unit size, checked October 26, 2026.
• ⁹ Reported: Fixr material-only glass ranges of $375 to $850 (argon) and $525 to $1,190 (krypton), plus $50 to $125 labor, from pricing databases, contractors and industry experts, updated August 29, 2025, checked October 26, 2026.