• 1 September 2026|
• News|
• Daniel Anderson

If you’ve fallen for the slim steel profiles and unmistakable aesthetic appeal of Crittall windows, you may have wondered whether that classic appearance comes at the expense of energy efficiency.

It’s a question we hear regularly from homeowners: are Crittall windows double glazed, or are they still essentially single-glazed steel windows?

The answer is that modern Crittall systems can absolutely be specified with double glazing. Today’s products combine the slim sightlines associated with traditional steel windows with significantly improved thermal performance, making them suitable for modern homes, extensions and renovation projects.

At Metwin, we’ve been working with steel windows since 1956, helping homeowners find the right balance between the appearance they want and the performance their property needs.

Why do people associate Crittall windows with single glazing?

The confusion largely comes from the age of many of the Crittall windows still found in homes across England.

Original steel windows were commonly manufactured with single glazing, and many surviving examples can still be found in period properties, particularly early and mid-20th-century homes and buildings associated with the Art Deco and Modernist eras. These windows are often instantly recognisable because of their narrow steel frames and individual panes of glass.

That traditional appearance is a major part of their appeal. However, the fact that many older Crittall windows were single glazed doesn’t mean modern Crittall products have to be.

Crittall has continued to develop its steel window systems, with modern products designed to accommodate higher-performance glazing and improve thermal efficiency while retaining the slim appearance that makes such windows so popular.

Can Crittall windows be double glazed?

Yes. Modern Crittall windows can be double glazed.

The exact specification depends on the Crittall system, the size and configuration of the window and the performance requirements of the project.

For example, Crittall’s Corporate W20 system can accommodate double glazing, while the thermally enhanced W20 TE system is designed to deliver improved thermal performance. Crittall’s T60 system goes further, using a thermally broken steel frame to achieve whole-window U-values as low as 0.8 W/m²K when appropriately specified.

At Metwin, we can help you understand which system and glazing specification is appropriate for your property rather than assuming that one solution will suit every project.

You can explore our Crittall windows to see the range available.

What is a U-value?

If you’re comparing the thermal performance of different windows, you’ll probably come across the term U-value.

Put simply, a U-value measures how readily heat passes through a building element. For windows, the lower the U-value, the better the thermal insulation.

This is important because a window isn’t just about the glass. The overall performance is influenced by the frame, glazing, spacers, seals and the way the complete window has been designed.

Crittall’s modern thermally enhanced systems demonstrate how far steel windows have come. The W20 TE system can achieve U-values of around 1.4 W/m²K, while T60 can achieve values as low as 0.8 W/m²K, depending on the specification.

That means homeowners no longer have to assume that choosing such windows automatically means accepting poor thermal performance.

How does double glazing improve thermal performance?

Double glazing works by placing two panes of glass within a sealed insulating unit, creating a gap between the panes that helps slow the transfer of heat.

Modern double-glazed units can also incorporate low-emissivity coatings and gas fills to further improve their insulating performance.

The result is a window that can help reduce heat loss compared with traditional single glazing.

For a homeowner, that can mean a more comfortable room, less reliance on heating and reduced cold spots around the glazing.

Double glazing can also help reduce external noise, although the level of acoustic improvement depends on the exact glass specification.

It’s worth remembering that upgrading the glazing alone isn’t the whole answer. The frame, seals, installation and surrounding construction all contribute to the final performance of the window.

What is a thermal break?

You may also have come across the term thermal break when researching modern steel windows.

Steel is an excellent structural material, but it is also a good conductor of heat. A traditional steel frame can therefore create a thermal bridge between the inside and outside of a building.

Thermally broken steel systems introduce an insulating barrier within the frame to reduce this heat transfer.

Crittall’s T60 is a thermally broken steel window system and can achieve whole-window U-values as low as 0.8 W/m²K with suitable glazing.

The W20 TE system takes a different approach. It is a thermally enhanced, rather than thermally broken, system and can achieve U-values around 1.4 W/m²K.

This distinction is worth understanding when comparing different products, because “thermally enhanced” and “thermally broken” don’t mean exactly the same thing.

How do Crittall windows maintain their slim appearance?

This is where modern steel windows really stand out.

One of the reasons such windows have remained popular for so long is their ability to combine structural strength with remarkably slim frames.

That gives Crittall windows their distinctive elegance and allows large areas of glass without bulky sections dominating the view.

The challenge is fitting modern insulating glass into a frame while maintaining those proportions.

Modern Crittall systems have been engineered specifically to achieve this balance. For example, Corporate W20 retains traditional slender steel framing while accepting double glazing, while W20 TE provides enhanced thermal performance without abandoning the visual character associated with the system.

For homeowners who love the Art Deco aesthetic, industrial character or traditional steel-window appearance, this can be particularly important. You don’t necessarily have to choose between the appearance you want and the performance you need.

What about original single-glazed Crittall windows?

If you already have Crittall windows installed in your property, the answer isn’t necessarily to replace them.

At Metwin, we also work on the restoration and refurbishment of existing steel windows. In some cases, the original frames can be retained and repaired, allowing homeowners to preserve the character and architectural integrity of the building.

The most appropriate approach depends on the condition of the existing windows, the glazing specification, the property’s requirements and, where relevant, conservation or planning restrictions.

For listed and historic buildings, preserving original fabric can be particularly important. That’s why we would always recommend having the existing windows assessed before deciding that replacement is the only option.

Our Crittall and steel restoration service can help determine what is possible with an existing installation.

Which Crittall system is right for your home?

There isn’t one Crittall window that is automatically right for every project.

If preserving a traditional appearance is the priority, a system such as Corporate W20 may be worth considering.

If you’re looking for a combination of slim sightlines and improved thermal performance, W20 TE provides a thermally enhanced option with U-values around 1.4 W/m²K.

For projects where particularly strong thermal performance is required, T60 offers a thermally broken steel solution capable of achieving U-values as low as 0.8 W/m²K with the appropriate specification.

The right choice will ultimately depend on the property, opening sizes, glazing, planning requirements and the performance you’re trying to achieve.

Why choose Metwin?

At Metwin, we specialise in Crittall steel windows and doors, working across both residential and commercial projects.

We’ve been established since 1956, originally beginning as a family-run business repairing and replacing steel windows around London. Today, we’re an approved Crittall Specialist Partner with experience in installation, restoration and the supply of genuine Crittall parts.

That experience means we can look at the bigger picture rather than simply recommending a new window.

Whether you’re replacing existing windows, planning an extension or trying to improve the performance of original steel frames, we can help you understand the available options.

You can also visit our showroom to see Crittall products in person.

So, are Crittall windows double glazed?

Yes. Crittall windows can be double glazed, and modern systems can provide significantly better thermal performance than the single-glazed steel windows commonly found in older properties.

The important thing is to look beyond the name “Crittall” and consider the exact system and specification.

Modern options such as W20 TE and T60 demonstrate how steel windows can combine energy efficiency with the slim profiles, elegance and aesthetic appeal that have made Crittall so distinctive for generations.

If you’re unsure whether you should restore your existing windows, upgrade the glazing or install a modern double-glazed system, we’d recommend starting with an assessment of your property and its requirements.

Get in touch with Metwin

If you’re considering Crittall double glazing for your home, we can help you explore the available options and find a specification that works for both your property and your priorities.

Contact Metwin to discuss your project, or explore our full range of Crittall products.

Call: 020 8523 2081
Email: enquiries@metwin.co.uk