Jansen Steel Windows: The Heritage-Look Option Worth Considering

Ninety percent of the aluminium door and window conversations happening in Potters Bar and Barnet are about modern extensions with grey powder-coated frames. The other ten percent are trying to solve a genuinely different problem: how do you replicate the look of original steel windows on a heritage property without butchering the character?

The two credible answers are Jansen steel systems and heritage aluminium ranges. Both have their place.

What Jansen Actually Are

Jansen is a Swiss steel window and door system, distributed in the UK by a handful of specialist fabricators. Unlike Crittall (the original British name, still around and still making steel windows), Jansen's modern systems are thermally broken — you can hit realistic modern U-values with genuinely slim steel frames.

Key features:

• Frame sightlines as slim as 30–40 mm.
• Thermally broken construction for Uw values in the 1.4–1.7 range.
• Genuine steel — not an aluminium lookalike.
• Available as windows, doors, and screens.

Why Not Just Use Aluminium With Fake Bars?

Fair question, and there's an honest answer: because from about three metres away it starts to look wrong. Slim aluminium is genuinely narrower than it used to be, but next to a real steel frame the difference reads immediately. On a Grade II listed cottage in Brookmans Park, or a period property in the older parts of Cuffley, planners will often push back on aluminium lookalikes.

For heritage-look aluminium there are decent options — 5EC's heritage aluminium range and comparable systems from other UK fabricators cover this well — but they're a compromise. Jansen isn't.

Where Jansen Makes Sense

Realistic use cases:

• Restoration of period Crittall-style windows where original steel is beyond saving.
• New-build in a conservation area where planners want a period-consistent look.
• Contemporary industrial-style interiors — internal steel screens and doors in loft-style conversions.
• Very slim external doors where aluminium can't get slim enough.

Internal steel screens: the growing use case

The fastest-growing use of Jansen in domestic UK work isn't external windows — it's internal steel-and-glass screens dividing a kitchen from a hall or a bedroom from an ensuite. Genuine steel gives you strength at very slim sightlines that aluminium simply can't match.

What About Cost?

Jansen is expensive. There's no gentle way to phrase it. Expect to pay somewhere between 1.5x and 2.5x the equivalent aluminium option. The steel raw material is more, the fabrication is more specialised, and the pool of installers who can do it well is smaller.

That said — for the specific use cases above, there isn't really a substitute. And on a heritage property, using the wrong window can knock the resale value more than the extra spend on the right window costs you.

Alternatives Worth Knowing

Finding a Jansen Installer Locally

The pool of installers doing Jansen work in Hertfordshire is small. 5EC's Jansen page is worth a read if you're at the "specifying" stage — they cover the system alongside their aluminium ranges, which is genuinely helpful when you're weighing up whether steel is the right answer or whether heritage aluminium will do.

Our Verdict

Jansen is a specialist product for specialist situations. Don't use it on a modern rear extension where aluminium would look right — you'll pay for something the design doesn't need. But on a genuine heritage restoration, or a Crittall-look replacement, or a slim internal screen — it's the right answer and the extra cost is justified.

For the mainstream aluminium comparison see our Schuco AWS windows guide. For a modern take on the timber-alu composite option, our Velfac vs Rationel review is worth a look.