When a design brief calls for the slimmest possible sightlines — Georgian glazing bars, industrial Crittall-style aesthetics, modernist framed glass — the honest answer is often not aluminium at all, but steel. This is an editorial guide to steel door systems from Jansen, the Swiss manufacturer whose systems have quietly become the reference for premium steel glazing in UK residential work.
Why steel
Aluminium is a beautiful window material — it takes powder-coating well, it's dimensionally stable, it's fully recyclable, and it can be engineered to Passivhaus thermal performance. What it can't do is match the tensile strength of steel. And that matters because sightline is a function of how much material you need to hold a piece of glass safely against wind loads.
A Jansen steel window at ~20mm face width holds the same glass as an aluminium window at ~60mm face width. That factor-of-three difference in visible metal is why steel is specified where the aesthetic requires disappearing frames.
Jansen's residential systems
Jansen produces several system families relevant to UK residential work. The two most commonly specified:
- Janisol Arte 2.0 — the slim-line Crittall-style system. Frame face widths from around 25mm, thermally broken, suitable for casement, fixed and door configurations.
- Janisol HI — enhanced-insulation heavier-duty steel systems for larger door configurations and Passivhaus specifications.
Thermal performance
Modern thermally-broken steel windows perform surprisingly well. Janisol Arte 2.0 achieves Uw from around 1.4 W/m²K in typical configurations — comparable to a mid-market aluminium window. Janisol HI reaches Uw below 1.0 W/m²K. Neither is Passivhaus in the base configuration, but both clear Building Regulations comfortably.
Steel vs steel-look aluminium
The most important distinction in this segment. Two products both look like "Crittall-style" doors but are fundamentally different:
- Real steel (Jansen, or genuine Crittall) — actual steel profiles, achievable face widths ~20–25mm. Higher cost, higher thermal, higher weight.
- Steel-look aluminium — aluminium profiles designed to resemble steel, face widths typically ~40–55mm. Lower cost, lower thermal, lower weight.
Steel-look aluminium is not "worse" — it's a legitimate specification for many projects. But it isn't the same product, and a supplier selling one as the other is being at best imprecise. If the design brief specifies steel-look aluminium and the client's expectation is a real Crittall aesthetic, there will be a moment of surprise when the frames arrive.
Where Jansen steel fits in the UK market
- Georgian and Victorian renovations where original steel Crittall windows are being replaced with modern thermally-broken equivalents.
- Industrial-style loft conversions and warehouse projects where the aesthetic is intrinsic to the design.
- Contemporary architecture using slim internal steel doors as room dividers.
- Listed buildings and conservation areas where planning requires like-for-like steel replacement.
Cost expectations
Steel systems are noticeably more expensive than aluminium at equivalent sizes. Rough guidance for installed cost, ex-VAT:
- Jansen internal single door (2.0m × 0.9m): £4,500–£7,000
- Jansen double door with side lights (~3m wide): £11,000–£18,000
- Jansen external door system (~4m wide, insulated): £16,000–£28,000+
Sourcing Jansen locally
Jansen is distributed through a specialist fabricator network. 5EC Aluminium's steel Jansen range covers the residential systems above, alongside their heritage aluminium alternative where the design intent allows aluminium in place of steel.
For manufacturer product information, Kobbe also produces steel-look aluminium systems worth reviewing where the aesthetic is close but the budget or specification favours aluminium.
Editorial verdict
Real steel is the correct specification for a specific and narrow set of design problems. Where aluminium can look convincing enough — often it can — the practical and financial case for aluminium is strong. Where it can't, Jansen is the reference system, and cheap alternatives generally show themselves quickly. The right question is what the architectural brief actually requires, not which is "better."