Gas-Filled Double Glazing Units
How argon, krypton, and xenon gas fills affect thermal performance, energy ratings, and heating costs.
Why Gas Fill Matters in Double Glazing
Every double glazing sealed unit consists of two (or three) panes of glass separated by a cavity. What fills that cavity has a direct and measurable impact on how well the unit insulates your home. The principle is straightforward: gases that are denser and less conductive than ordinary air slow the transfer of heat from the warm interior pane to the cold exterior pane more effectively.
In a standard air-filled sealed unit, convection currents form inside the cavity as warm air rises on the interior side and falls on the exterior side, creating a loop that transfers heat outwards. By replacing air with a heavier, less conductive noble gas – such as argon, krypton, or xenon – these convection currents are suppressed, heat transfer is reduced, and the overall U-value of the unit drops significantly.
A lower U-value means better insulation. For context, Building Regulations Part L in England currently requires replacement windows to achieve a whole-window U-value of 1.4 W/m²K or lower. The gas fill inside your sealed unit is one of the easiest and most cost-effective ways to meet and exceed this standard.
Gas-filled units also contribute to a higher BFRC (British Fenestration Rating Council) energy rating – the A++ to E scale you'll see on window energy labels throughout the UK. The better the gas fill, the higher the rating, and the greater the energy savings over the lifetime of the unit.
In this guide, we'll walk through each gas option in detail – including argon, krypton, xenon, and standard air – so you can understand the differences in thermal performance, cost, cavity requirements, and best-use scenarios. Whether you're replacing misted units in a semi-detached house or specifying glazing for a listed building, the right gas fill choice can make a real difference to comfort and running costs.
Argon Gas – The UK's Most Popular Choice
Argon is by far the most widely used gas fill in UK double glazing. It accounts for the vast majority of gas-filled sealed units manufactured in Britain, and for good reason: it offers an excellent balance of thermal performance, availability, and cost.
With a thermal conductivity of just 0.016 W/mK – roughly 34% lower than air (0.025 W/mK) – argon significantly reduces heat transfer across the cavity. In practical terms, this means a standard 4-16-4 sealed unit (4mm glass, 16mm cavity, 4mm glass) filled with argon will typically achieve a centre-pane U-value of around 1.1 W/m²K when combined with a low-emissivity (Low-E) coating. The same unit filled with air would sit at approximately 1.4–1.5 W/m²K.
Argon performs optimally in a 16mm cavity. At this width, the gas provides maximum resistance to convective heat transfer. Narrower cavities (below 12mm) do not allow argon to perform at its best, as the gas movement is constrained before the benefit of reduced convection is fully realised. Wider cavities (above 20mm) can actually worsen performance, as the larger space allows convection currents to re-establish.
BS EN 1279-3 Certification
All reputable UK manufacturers, including ourselves, produce argon-filled units to BS EN 1279-3 – the European standard governing gas fill in insulating glass units. This standard requires a minimum 90% gas fill concentration at the point of manufacture, verified through regular third-party testing. The standard also prescribes gas retention rates over the expected service life of the unit, ensuring the gas concentration remains effective for 20 years or more.
Cost Considerations
Argon is abundantly available – it makes up roughly 0.93% of the Earth's atmosphere and is a by-product of oxygen and nitrogen production. This abundance keeps the cost low. Upgrading from an air-filled to an argon-filled sealed unit typically adds just £10–£30 per unit, making it one of the most cost-effective thermal upgrades available in glazing. For a full breakdown of pricing, visit our cost guide.
Who Should Choose Argon?
Argon is the right choice for the overwhelming majority of UK homeowners, landlords, and trade professionals. It is ideal for standard replacement sealed units, new-build glazing, and any application where a standard 16mm cavity width is feasible. If you're replacing misted units or upgrading from older air-filled glazing, argon should be your default choice – the performance improvement is substantial, and the cost premium is negligible.
Argon gas filled sealed unit with a 16mm cavity – the UK's standard for energy-efficient glazing.
Krypton Gas – Superior Performance in Slimline Units
Krypton is a denser, rarer noble gas that offers significantly better thermal insulation than argon. With a thermal conductivity of 0.0095 W/mK – approximately 40% lower than argon and 62% lower than air – krypton is the preferred gas fill when you need maximum thermal performance in a restricted space.
Unlike argon, which requires a 16mm cavity to perform well, krypton achieves its optimal performance in a 10–12mm cavity. This makes it particularly valuable in situations where a standard-width sealed unit cannot be accommodated – most notably in heritage and listed buildings where the original window profiles are narrow, and planning regulations restrict how much the glazing depth can be increased.
When to Choose Krypton Over Argon
Krypton should be considered when:
- Heritage or listed buildings – where Conservation Officers require slimline double glazing units that replicate the appearance of original single-glazed windows.
- Slim-profile window systems – such as steel-framed windows or heritage aluminium systems with narrow rebates that cannot accommodate a 16mm+ cavity.
- Triple glazing with limited overall depth – where two gas-filled cavities must fit within a tight overall unit thickness.
- Maximum U-value performance – where Building Regulations or project specifications demand the lowest possible U-values and the additional cost is justified.
Cost and Availability
Krypton is significantly rarer than argon in the atmosphere (roughly 1 part per million compared to argon's 9,340 parts per million), which makes it 3–5 times more expensive as a gas fill. A krypton-filled sealed unit might add £50–£120 per unit over an air-filled equivalent, depending on size. Despite the higher cost, krypton remains the only practical solution for slimline glazing that needs to meet modern thermal standards.
Performance Data
A typical 4-12-4 sealed unit with krypton fill and a Low-E coating can achieve a centre-pane U-value of around 1.0 W/m²K – comparable to, or better than, an argon-filled 4-16-4 unit. This is remarkable considering the cavity is 25% narrower, and it underlines why krypton is the go-to gas for applications where space is at a premium.
Krypton gas units are ideal for heritage properties where slimline glazing profiles are required.
Xenon Gas – The Ultimate in Thermal Insulation
Xenon is the densest and most thermally efficient noble gas used in insulating glass units. With a thermal conductivity of just 0.0054 W/mK – almost five times lower than air and roughly a third of argon's figure – xenon offers the absolute best performance available in gas-filled glazing.
Xenon performs optimally in an 8–10mm cavity, making it theoretically ideal for ultra-slim glazing profiles. A 4-8-4 unit filled with xenon and fitted with a Low-E coating can achieve a centre-pane U-value below 0.9 W/m²K – a level of insulation that air and argon simply cannot match in such a compact profile.
Why Xenon Is Rarely Used Domestically
Despite its outstanding thermal properties, xenon is rarely specified for UK domestic glazing for one primary reason: cost. Xenon is extremely rare in the atmosphere (approximately 0.087 parts per million), making it one of the most expensive noble gases available. The cost of xenon fill can be 10–20 times higher than argon, adding £160–£400+ per sealed unit depending on size.
For most homeowners, the additional thermal benefit over krypton does not justify this premium. The law of diminishing returns applies firmly: moving from air to argon delivers a transformative improvement at minimal cost, moving from argon to krypton provides a meaningful further step for specific applications, but moving from krypton to xenon yields a relatively modest additional gain at a disproportionately high price.
Commercial and Specialist Applications
Xenon finds its niche in specialist commercial applications where every fraction of a U-value point matters – for example, high-performance curtain walling on BREEAM Excellent-rated commercial buildings, passive house (Passivhaus) certified projects, or research facilities with extremely stringent environmental control requirements.
If you are working on a commercial project with demanding thermal specifications, our team can advise on whether xenon fill is the right option. Get in touch to discuss your requirements.
Xenon gas is primarily used in high-specification commercial projects where the highest thermal performance is required.
Air-Filled Units – The Standard Baseline
Before gas-filled units became the industry norm, all double glazing units were filled with standard dehydrated air. Air has a thermal conductivity of 0.025 W/mK – higher than any of the noble gas alternatives – which means it offers the lowest level of insulation in a sealed unit.
Air-filled sealed units are still manufactured and have their place. They are the cheapest option available, and for non-heated or low-priority spaces, the reduced thermal performance is perfectly acceptable. Common applications include:
- Garages and detached outbuildings – where the space is unheated or only occasionally used.
- Sheds, workshops, and garden rooms – especially where the structure itself has poor insulation, making gas-filled glazing a marginal improvement.
- Temporary or short-term installations – where the budget does not justify the premium of gas fill.
- Internal partitions – where thermal insulation between the two sides is not required.
Performance Comparison
A standard 4-16-4 sealed unit with air fill and no Low-E coating will typically achieve a centre-pane U-value of around 2.7–2.8 W/m²K. Add a Low-E coating and the figure improves to approximately 1.4–1.5 W/m²K, but this still falls short of Building Regulations Part L requirements for replacement windows in habitable rooms without further upgrades.
For any heated living space in the UK, we strongly recommend upgrading to at least argon gas fill. The cost difference is minimal, and the energy savings over the life of the unit far exceed the initial premium. View our full range of unit types to find the right specification for your project.
Air-filled sealed units remain a budget-friendly option for outbuildings and unheated spaces.
Gas Fill Comparison Table
A side-by-side comparison of the four gas fill options available for double glazing sealed units, including thermal conductivity, optimal cavity width, relative cost, and best-use scenarios.
| Gas Type | Thermal Conductivity (W/mK) | Optimal Cavity (mm) | Relative Cost | Best Use | U-Value Improvement vs Air |
|---|---|---|---|---|---|
| Air | 0.025 | 16 | £ (lowest) | Garages, outbuildings, internal partitions | Baseline |
| Argon | 0.016 | 16 | ££ (low premium) | Standard domestic & commercial glazing | ~34% improvement |
| Krypton | 0.0095 | 10–12 | £££ (3–5× argon) | Heritage buildings, slim-profile units | ~62% improvement |
| Xenon | 0.0054 | 8–10 | ££££ (10–20× argon) | Specialist commercial, Passivhaus | ~78% improvement |
Note: U-value improvements are approximate centre-pane figures and will vary depending on glass type, Low-E coating, spacer bar, and overall unit configuration. All gas fills should be specified to BS EN 1279-3 with a minimum 90% fill concentration.
BFRC Energy Ratings & Building Regulations Part L
The British Fenestration Rating Council (BFRC) operates the UK's national window energy rating scheme. Every window sold in the UK can be rated on a scale from A++ (best) to E (worst), based on three factors: the U-value (heat loss), the solar heat gain (free energy from sunlight), and air leakage through the frame.
The gas fill inside a sealed unit directly influences the U-value component of this rating. Switching from air to argon alone can shift a window's energy rating by one or two bands – for example, from a C rating to an A rating. This makes gas fill one of the most impactful and cost-effective upgrades when targeting a higher BFRC rating.
Part L Compliance
Building Regulations Part L (Conservation of fuel and power) sets the minimum thermal performance standards for windows in England. As of the current regulations, replacement windows in existing dwellings must achieve either:
- A whole-window U-value of 1.4 W/m²K or lower, or
- A BFRC energy rating of Band C or above.
In practice, achieving 1.4 W/m²K consistently requires a Low-E coated, gas-filled sealed unit. An air-filled unit with Low-E coating may just scrape through on a small window, but argon fill provides a comfortable margin of compliance and is essentially the minimum standard for responsible specification.
Reaching A++ Ratings
To achieve the highest A++ energy ratings in the BFRC scheme, you'll typically need a combination of:
- High-performance Low-E glass (such as Planitherm Total+ or Pilkington Energikare)
- Argon or krypton gas fill at 90%+ concentration
- Warm-edge spacer bars (e.g., Thermobar, Swisspacer, or Super Spacer)
- A thermally efficient frame system (multi-chamber uPVC, timber, or thermally broken aluminium)
Gas fill is therefore a critical component but works best as part of a holistic approach to window energy performance. Our team can advise on the right combination of glass, gas, and spacer for your required rating – get a free quote to find out more.
A BFRC energy label – gas fill directly influences the U-value component of the window's rating.
How Long Does Gas Fill Last?
One of the most common questions we receive is about the longevity of gas-filled sealed units. The good news is that a properly manufactured, BS EN 1279-compliant unit will retain its gas fill for a very long time.
Expected Lifespan
Industry data and independent testing consistently show that well-manufactured sealed units retain the vast majority of their gas fill for 20–25 years or more. The rate of gas loss is extremely slow – typically around 1% per year under normal conditions. This means that after 20 years, an argon-filled unit manufactured at 95% concentration would still contain approximately 75% argon, which is more than sufficient to deliver meaningful thermal benefits.
Seal Degradation
The primary cause of gas loss is seal degradation. Modern sealed units use a dual-seal system – typically a primary seal of polyisobutylene (PIB) for gas retention and a secondary seal of polysulphide, silicone, or hot-melt butyl for structural integrity. Over time, UV exposure, thermal cycling, and mechanical stress can gradually degrade these seals, allowing gas to escape and moisture to enter.
Once moisture enters the cavity, internal condensation (misting) occurs – the visible sign that a unit has failed. At this point, the gas fill has usually been significantly or fully lost, and the unit needs replacing.
BS EN 1279 Gas Retention Testing
The BS EN 1279 standard includes rigorous gas retention testing. Under BS EN 1279-3, manufacturers must demonstrate that their units maintain a minimum gas fill concentration over an accelerated ageing period. This involves testing at elevated temperatures and humidity levels to simulate years of real-world exposure in a compressed timeframe.
Units that pass this testing are certified as compliant and can be expected to deliver their rated thermal performance for the duration of their intended service life. When ordering gas-filled units, always ensure your supplier is BS EN 1279-3 certified – as we are – to guarantee you're receiving a product that meets these stringent durability standards.
Maximising Longevity
While gas retention is primarily a function of manufacturing quality, a few factors can extend or reduce the lifespan of your sealed units:
- Quality of installation – Properly drained and ventilated frames allow moisture to escape and reduce the risk of premature seal failure. Poor installation is a leading cause of early unit failure.
- Exposure conditions – Units in south-facing positions experience more UV and thermal cycling, which can accelerate seal degradation over decades.
- Frame maintenance – Keeping drainage channels clear and timber frames painted or stained protects the unit perimeter from prolonged moisture contact.
If your existing sealed units are showing signs of misting, condensation, or visible seal failure, it's time for a replacement. Learn about our replacement services or request a free quote.
Frequently Asked Questions About Gas Fills
Yes. Argon gas is absolutely worth it for the vast majority of UK homeowners. It improves the thermal insulation of a sealed unit by approximately 34% compared to standard air-filled units, yet adds only a modest amount to the overall cost – typically £10–£30 per unit. Over the lifespan of the sealed unit (usually 20–25 years), the energy savings on heating bills far outweigh the upfront premium. Argon-filled units also help your windows achieve a better BFRC energy rating and make it easier to comply with Building Regulations Part L.
Unfortunately, you cannot see, smell, or detect noble gas leakage with the naked eye – argon, krypton, and xenon are colourless and odourless. The most common sign of seal failure is internal condensation (misting) between the panes, which indicates moisture ingress after the seal has degraded. If your sealed units are misting up, it is likely that the gas fill has already been partially or fully lost. A specialist glazing company can test gas concentration levels using a spark emission spectrometer, though in practice most homeowners simply replace misted units.
No. Once the seal on a double glazing unit has failed, it is not possible to refill the gas. Sealed units are manufactured under controlled factory conditions with the gas injected during assembly and the unit hermetically sealed. If the seal degrades and gas escapes, moisture will also have entered the cavity, meaning the unit must be replaced entirely. Attempting to drill and refill a sealed unit would compromise its structural integrity and void any warranty.
Argon and krypton are both noble gases used to improve the thermal performance of double glazing units, but they differ in several key areas. Argon has a thermal conductivity of 0.016 W/mK and performs best in a standard 16mm cavity. Krypton is denser with a lower thermal conductivity of 0.0095 W/mK, making it roughly 40% more efficient than argon – but it performs optimally in a narrower 10–12mm cavity. Krypton is significantly more expensive (typically 3–5 times the cost of argon fill), so it is mainly used in heritage or listed buildings where slimline glazing profiles are required to preserve the building's character. For standard domestic windows, argon offers the best value.