Last updated: 24 July 2026
If you’re an interior designer specifying glazing for refurbished homes in the UK, Fabco is probably the name you’ll keep coming back to. They’re the go-to brand for slim-profile steel windows and doors when getting as much natural light in as possible is the main goal. Their W20 system hits 10mm sightlines (that’s the visible width of the frame between panes) on a 32mm face width, roughly a third of what you’d get with a typical aluminium system. That means significantly more glass in every opening.
And that difference really shows up in real life.
A slimmer frame means more daylight flooding in, cleaner views, and less visual clutter. When your clients are opening up a Victorian terrace or refurbishing a period flat, this is exactly what they’re after. Steel makes it possible because of its strength-to-profile ratio; basically, steel is strong enough to stay rigid at much thinner dimensions than aluminium can manage.
There’s also a regulatory side you can’t skip past. Replacement windows in occupied homes must meet Building Regulations Part L, which sets minimum standards for thermal performance (how well a window holds heat in). Here’s how Fabco’s systems stack up:
- Standard W20 achieves a B-rated Window Energy Rating (WER), which satisfies Part L via the rating route
- Enhanced W20+ reaches an A+ rating, with a whole-window U-value of 1.3–1.4 W/m²K, which actually exceeds current requirements
A U-value, by the way, measures how much heat passes through a material – lower numbers mean better insulation.
Both systems look completely identical from the outside. That’s a genuinely useful specification advantage it’s worth getting your head round, before you commit to any glazing system for a refurbishment project.
Steel vs. Aluminium: Why the Material Actually Matters in Slim-Profile Glazing
Not all slim-profile windows are made from the same material, and on a heritage or refurbishment project, that difference is a big deal.
Some aluminium systems are marketed using steel glazing language: slim sightlines, industrial looks, period credentials. But these two materials behave very differently when you’re talking about structural strength. Steel’s strength-to-profile ratio (how strong it is relative to how thin it can be made) is significantly higher than aluminium’s. That’s exactly why genuine steel systems like Fabco’s W20 can hit 10mm sightlines on a 32mm face width. Aluminium simply can’t reach those dimensions without losing structural integrity, so aluminium “slim” profiles typically lead to much thicker widths in practice.
That gap is visible to the naked eye. And it comes up pretty quickly in conversations with conservation officers.
Why heritage projects care so much
When you’re applying for listed building consent, or doing work inside a conservation area, replacement glazing often has to match the original steel proportions. There’s no shortcut here. Authentic hot-rolled steel sections (like Fabco’s hot-rolled W20 profile) carry a heritage credibility that aluminium alternatives just can’t replicate, no matter what finish or colour you choose.
Thermal performance is another distinguishing factor. Fabco’s W20+ hits a whole-window U-value (a measure of heat loss through the window) of 1.3–1.4 W/m²K and earns an A+ Window Energy Rating, all while exceeding current Building Regulations Part L requirements. And it does all of that while keeping those same 10mm sightlines. That combination (authentic proportions plus modern thermal performance) simply isn’t available in aluminium at equivalent sightline dimensions.
So if you’re a designer specifying materials for a refurbished residential project, the steel-versus-aluminium question isn’t just about looks. It’s structural, regulatory, and aesthetic all at once.
Thermal Performance Meets Heritage Aesthetics: How Modern Steel Windows Bridge Regulations & Conservation
Listed building consent and conservation area guidelines rarely leave much room for manoeuvre. Frames must look right. Sightlines must match the originals.
So how do modern steel windows also satisfy Building Regulations Part L?
The answer sits inside the frame – not on the surface of it.
Fabco’s W20+ system uses an 18mm slimline double-glazed unit built from three layers: a 4mm outer pane, a 10mm krypton-filled cavity, and a 4mm inner pane. Krypton gas is a better insulator than argon, which is what most standard units use. That extra performance helps the W20+ hit a whole-window U-value (a measure of how much heat escapes through a window) of 1.3–1.4 W/m²K, which beats current Part L requirements through both the U-value route and the A+ Window Energy Rating route.
Two other features help get there too:
- Low-emissivity (Low-E) coatings – a near-invisible layer on the glass that reflects heat back into the room
- Warm-edge spacer bars – the strips that hold the two panes apart, designed to reduce heat loss around the edges (optional upgrade)
None of this engineering shows on the outside. The W20+ keeps exactly the same 10mm sightlines as the standard W20, so from the street (or from a conservation officer’s point of view) the frame proportions look identical.
The gap in performance between the W20+ and traditional single-glazed steel is pretty striking. Single-glazed steel typically carries a U-value above 5.0 W/m²K. The W20+ closes that gap dramatically, and does it without touching the slim profile that planning conditions demand.
That’s what makes it a practical choice for designers working on listed properties, where both compliance and authenticity are non-negotiable.
Frequently Asked Questions: Specifier Guidance on Steel Window Selection, Performance & Compliance
These are the questions specifiers ask most often when working on listed and refurbished residential projects. The answers below cut through the confusion on compliance, fire rating, durability, and cost justification.
How do slim steel windows comply with Building Regulations Part L in a listed building?
Fabco’s W20 system achieves a B Window Energy Rating and a whole-window U-value of approximately 2.0 W/m²K, which meets Part L via the WER rating route for replacement windows. For projects where full compliance is required, the W20+ reaches a U-value of 1.3 to 1.4 W/m²K and carries an A+ rating, exceeding Part L via both the U-value and rating routes. Conservation officers cannot distinguish the W20+ from the W20 by sight, as both share identical 10mm sightlines.
Is there a fire-rated steel window option for residential projects with mixed-use or open-plan layouts?
Yes. Fabco’s W40 system is independently assessed by Warringtonfire for FD30 performance, meaning it resists fire for up to 30 minutes on one side, integrity only. The assessment covers glazed doorsets and screens, not standard windows, and applies within the assessed Ottostumm configuration. The W40 uses a larger profile than the W20 and requires mechanically fixed steel glazing beads to retain fire-rated glass. Each pane carries a visible etching confirming its fire-rated status.
How long will premium steel windows actually last, and how does that justify the cost?
Many original steel windows installed in the early twentieth century are still in service today. That is over 100 years of performance. Steel resists warping and twisting in ways that alternative frame materials cannot match, and Fabco applies hot-dip galvanising to British Standards before powder coating, giving strong corrosion resistance. Modern powder-coated steel requires relatively little maintenance compared with traditional systems. When the upfront cost is spread across a lifespan measured in generations rather than decades, the per-year cost of a steel window compares very favourably with lower-cost alternatives that need replacing every 20 to 30 years.
What thermal performance can specifiers realistically expect from slimline steel double glazing?
Fabco’s W20 system uses an 18mm slimline insulated double-glazed unit: a 4mm outer pane, a 10mm krypton-filled cavity, and a 4mm inner pane. Krypton outperforms the argon gas used in most standard units. The result is a whole-window U-value of approximately 2.0 W/m²K for the W20 and 1.3 to 1.4 W/m²K for the W20+, compared with a U-value above 5.0 W/m²K for traditional single-glazed steel. Low-E coatings come as standard in Fabco’s double glazing, and warm-edge spacer bars are available to push performance further.
How does Fabco ensure installation quality on complex heritage projects?
Fabco employs its own project engineers, surveyors, installation fitters, and glazing engineers directly. No stage is subcontracted. Every project begins with a state-of-the-art 3D laser site survey before production starts. A dedicated project engineer acts as a single point of accountability from survey through to final glazing sign-off. Wet glazing is completed in-house using RAL colour-matched silicone selected for UV resistance and colour-fastness. This five-step delivery process means the same team that measures the opening installs the frame and finishes the glazing, reducing the risk of coordination errors that are common on heritage sites.
If you’re an interior designer working on listed or refurbished homes, you already know the specification decision (choosing which products to use) is never just about looks. It’s about confidence – confidence that what you specify will perform, meet the rules, and stand the test of time.
Fabco’s W20 and W20+ systems make a strong case on all three counts. A 10mm sightline (the visible width of the frame between panes) keeps natural light flowing without weakening the structure. The W20+ hits a whole-window U-value of 1.3–1.4 W/m²K, which actually beats current Building Regulations Part L requirements. That’s simply ahead of the curve.
The materials back this up too. The steel is:
- Hot-dip galvanised to British Standards
- TIG welded at every critical joint
- Finished in UV-resistant, RAL colour-matched silicone
That’s a window built to serve for generations, not just decades.
Yes, the upfront cost is higher than alternatives. But measured across a lifespan that outperforms other options by decades, the value calculation shifts decisively in Fabco’s favour.
What Fabco also brings is something genuinely rare: complete accountability, start to finish. British manufacturing in West Sussex, directly employed installation teams, and 3D laser surveys carried out before a single frame is cut – all of it backs up that promise.
