Argon vs Krypton Gas Fill: Which Window Insulation is Best for UK Homes?

In the UK, approximately 90% of new double-glazed units are filled with Argon gas, as it provides the most cost-effective balance between thermal insulation and manufacturing expense. While Argon remains the industry standard, the rise of high-performance retrofits and Passivhaus standards has brought Krypton gas into the spotlight for those seeking the absolute lowest U-values possible. For the average homeowner, the choice between these two inert gases is rarely about “which is better” in isolation, but rather “how much additional thermal gain justifies the price premium.”

Understanding what happens inside the vacuum-sealed gap of your window is the key to reducing heat loss. When you remove standard air and replace it with an inert gas, you are essentially slowing down the process of conduction and convection, keeping your heat inside where it belongs during a bleak January freeze.

The Role of Inert Gases in Glazing

In a standard Insulating Glass Unit (IGU), two or three panes of glass are separated by a spacer bar. If this gap were filled with normal atmospheric air, heat would move relatively quickly from the warm interior glass to the cold exterior glass.

Inert gases—so called because they do not react with other chemicals—are denser than air. Because they are heavier and move more slowly, they create a more effective thermal barrier. This reduces the “U-value” of the window; the lower the U-value, the better the window is at insulating. According to the U-value pricing matrix, moving from basic air to a high-quality Argon fill can significantly shift a window from a B-rated to an A++ rated energy performance.

Argon: The UK Industry Standard

Argon is the most common gas used in UK windows, and for a good reason. It is abundant, non-toxic, and provides a substantial leap in efficiency over air-filled units.

The Benefits of Argon

Argon is highly effective at reducing the rate of heat transfer. In a typical 16mm to 20mm gap, Argon can reduce the U-value of a double-glazed unit by roughly 15-20% compared to air. This is why most reputable UK installers, including Anglian Windows, specify Argon as their baseline for energy-efficient installations.

Cost and Availability

Because Argon is extracted from the air we breathe (where it makes up about 0.93% of the atmosphere), it is cheap to produce. This means most homeowners get Argon fill included in the base price of their uPVC or aluminium windows without needing a bespoke upgrade.

The Downside: The “Sweet Spot” Problem

Argon has a specific “optimal gap” width. If the gap between the glass panes is too narrow (under 12mm) or too wide (over 24mm), the insulating properties of Argon actually begin to diminish. This is why standard UK window heights and widths are often designed around the 16mm-20mm spacer bar.

Krypton: The High-Performance Alternative

Krypton is a rarer noble gas that provides even greater thermal resistance than Argon. It is primarily used in triple glazing or in windows where the frame profile is too slim to allow for a wide Argon gap.

Thermal Superiority

Krypton is significantly denser than Argon. This means it can achieve the same (or better) insulating properties in a much narrower gap. For example, a Krypton-filled unit with a 10mm gap can often outperform an Argon-filled unit with a 16mm gap. For those pursuing the Passivhaus standard, Krypton is often the only way to hit the strict U-value targets without resorting to prohibitively thick glass units.

The Cost Penalty

The primary barrier to Krypton adoption is price. Krypton is much rarer than Argon, making it significantly more expensive to source and inject. In the UK market, choosing Krypton over Argon can increase the cost of a glazing unit by 20% to 50% or more. For the vast majority of residential homes, the marginal gain in energy savings does not offset this initial capital outlay over a 20-year period.

Use Cases for Krypton

• Slim-profile Heritage Windows: Where you need high efficiency but cannot fit a thick Argon spacer.
• Extreme Energy Retrofits: Homes aiming for a near-zero energy balance.
• High-End Aluminium Systems: Where sightlines are kept to a minimum.

Comparing Argon vs Krypton: Side-by-Side

Feature	Argon	Krypton
UK Prevalence	Extremely High (Standard)	Low (Specialist)
Insulating Power	Very Good	Excellent
Optimal Gap Width	16mm - 20mm	8mm - 12mm
Cost	Low / Included	High Premium
U-Value Impact	Significant drop vs Air	Maximum possible drop
Common Application	Standard Double/Triple Glazing	Triple Glazing / Passivhaus

How to Identify Which Gas Your Windows Use

Unfortunately, you cannot see Argon or Krypton with the naked eye—both are colorless and odorless. However, there are a few clues you can look for.

1. The Spacer Bar

Look at the metal or plastic strip that separates the two panes of glass. Many modern “warm-edge” spacers have the gas type printed on them in tiny letters (e.g., “Argon Filled”). If you see a very thin gap (under 12mm) in a high-end window, it is more likely to be Krypton.

2. The Installation Certificate

If you have a FENSA or CERTASS certificate from your installer, the technical specifications of the glass units are often listed. Look for the U-value; if it’s exceptionally low (e.g., below 0.8 for double glazing), it may be using a specialty gas.

3. The “Misting” Symptom

When a seal fails, the inert gas escapes and is replaced by moist air, leading to condensation inside the panes. Because Krypton is more expensive, it is often paired with higher-quality seals. If your windows are misting after only 5 years, you likely have a budget Argon installation with poor sealing. For a deeper look at why this happens, see our guide on uPVC window lifespan.

Does the Gas Actually Pay for Itself?

The “payback period” is the most critical calculation for any UK homeowner.

Moving from single glazing to Argon-filled double glazing provides an immediate and massive return on investment, often saving hundreds of pounds per year on heating bills. However, moving from Argon to Krypton is a case of diminishing returns.

For a standard 3-bedroom semi-detached house, the difference in annual heat loss between a high-quality Argon triple-glazed window and a Krypton triple-glazed window might be as little as £10-£30 per year. Given that the upfront cost of Krypton could be thousands of pounds higher across a whole house, the “payback period” would be decades—likely longer than the realistic lifespan of the window seals.

Final Verdict: Which Should You Choose?

For 95% of UK homes, Argon is the correct choice. It provides the efficiency needed to meet current Building Regulations (Part L) while remaining affordable. When paired with low-E coatings and warm-edge spacers, Argon delivers an A++ rating that is sufficient for almost any climate in the British Isles.

Choose Krypton only if:

1. You are building a certified Passivhaus home.
2. You are installing ultra-slim heritage frames where a 16mm gap is physically impossible.
3. Budget is not a constraint and you want the absolute theoretical maximum in thermal performance.

Ultimately, you will gain more energy efficiency by ensuring your windows are installed with a perfect airtight seal than by swapping Argon for Krypton. A perfectly installed Argon window will always outperform a poorly fitted Krypton window.

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