Last updated: 13 October 2026
Why Black Steel Frames Dominate Contemporary Design
Fabco’s W20+ system achieves an outstanding balance of style and energy efficiency., achieving an A+ Window Energy Rating with a whole-window U-value of 1.3-1.4 W/m²K while maintaining 10mm sightlines. For most projects, that combination is the benchmark everything else gets measured against.
So why has black steel become the go-to choice for architects and self-builders? The honest answer is geometry.
Steel’s strength-to-profile ratio is simply unmatched. Where a uPVC frame might require significantly wider face widths to hold its shape, Fabco’s W20 system achieves a 32mm face width. That’s a dramatic difference in how much glass you actually see versus how much frame you’re looking at.
That slimness isn’t just aesthetically pleasing. It’s structurally honest. The glazing bars are TIG (Tungsten Inert Gas) welded directly into the frame, not clipped on for appearance. What you see is what’s holding the building together.
Black powder coating amplifies all of this. The dark finish recedes visually, making sightlines feel even slimmer and drawing the eye to the glass.
It’s why projects ranging from Victorian terrace extensions to new-build contemporary homes consistently land on the same answer: black steel.
The Energy Efficiency Challenge with Steel
Steel is a far more conductive material than timber. That’s not a typo – heat escapes through a steel frame much faster than through wood or uPVC. This is why older steel windows feel cold to the touch and collect condensation in winter.
Traditional single-glazed steel has significantly higher heat loss than modern double-glazed systems. By modern standards, that’s genuinely poor. Current Building Regulations Part L require replacement windows to meet at least a B Window Energy Rating, which most modern steel systems struggle to hit without serious engineering effort.
So how do you get black steel frames that actually perform?
The answer lies in the glazing unit itself, not the steel profile. Fabco’s W20 system uses an 18mm slimline double-glazed unit – 4mm glass, a 10mm krypton-filled cavity (krypton insulates better than the more common argon), then 4mm glass again. This achieves a whole-window U-value of approximately 2.0 W/m²K and a B energy rating. Their W20+ system pushes that to 1.3-1.4 W/m²K, earning an A+ rating. Same 10mm sightlines. Genuinely different thermal performance.
Thermal Engineering Technology
Steel conducts heat roughly 1,000 times faster than uPVC. That’s why people often assume steel windows must be cold, draughty, and expensive to heat. But modern manufacturing has changed that assumption.
The real question isn’t whether steel can perform thermally. It’s how to achieve that performance without making the frame thicker or losing the slim sightlines that make steel windows special.
Fabco’s W20 system does this with an 18mm slimline double-glazed unit – 4mm glass, a 10mm krypton-filled cavity (krypton works better than the standard argon), then 4mm glass. The result: a whole-window U-value of approximately 2.0 W/m²K and a B Window Energy Rating. That meets Building Regulations Part L via the WER route.
The W20+ goes further still. It reaches 1.3-1.4 W/m²K and earns an A+ rating, beating Part L on both U-value and rating measures. Both systems have identical 10mm sightlines. Stand outside and you won’t see any difference between them.
That’s the real win. All the thermal engineering happens inside the frame, invisible to anyone looking at the window. You get the look of traditional steel paired with performance that crushes older single-glazed systems – which typically sit above 5.0 W/m²K.
Finding Your Balance Point
By this point, you’ve seen how glazing choice, thermal engineering, and frame design all pull in different directions. So how do you actually pick? The answer comes down to three questions: What’s your Building Regulations requirement? How much glass area are you planning? And what’s your budget for running costs versus upfront cost? The W20 hits a U-value of 2.0 W/m²K and earns a B Window Energy Rating, which meets Part L for replacement windows. The W20+ reaches 1.3-1.4 W/m²K and an A+ rating, which exceeds Part L on both the U-value and rating routes. Both systems share identical 10mm sightlines, so the visual result is the same. The difference lives entirely inside the frame.
Fabco W20 (Standard)
Pros:
- Meets Building Regulations Part L for replacement windows via the B Window Energy Rating route – a meaningful compliance benchmark for renovation projects
- U-value of approximately 2.0 W/m²K is a significant improvement over traditional single-glazed steel, which typically measures 5.0+ W/m²K
- 10mm sightlines preserved with 18mm slimline double glazing using a krypton-filled 4/10/4 unit – no aesthetic compromise
- Lower entry cost than W20+, making it more accessible for larger projects with tighter budgets
Cons:
- Does not meet Part L via the U-value route (1.4 W/m²K limit) – compliance depends entirely on the WER rating pathway, which may not apply to all project types
- Less suitable for new-build or extension projects where stricter energy targets apply
Fabco W20+ (Enhanced Thermal)
Pros:
- U-value of 1.3 W/m²K for fixed lights and 1.4 W/m²K for opening windows and doors – exceeds current Part L requirements on both compliance routes
- A+ Window Energy Rating, the highest available classification, supporting better SAP scores and EPC outcomes
- Visually identical to W20 from the outside – same 10mm sightlines – so there is no trade-off between performance and aesthetics
- Best choice for extensions, new-builds, or anyone prioritising lower long-term heating bills over upfront savings
Cons:
- Higher initial investment than the W20 system – exact pricing depends on configuration, so request a project-specific quote
- Engineering advancements are concealed within the system, meaning the performance gain is invisible to the eye – which can make it harder to justify to clients who focus on visible specification
Verdict: If you’re replacing windows in an existing home, the W20 delivers genuine compliance and a dramatic improvement over old steel at a lower cost. If you’re building new, extending, or want the strongest long-term energy performance without changing the look of your glazing by a single millimetre, the W20+ is the clear answer.
Summary
Which black steel system actually delivers the best balance? The honest answer depends on your performance target.
Period properties need the W20. If you’re replacing windows in an older home and must meet Building Regulations Part L, the W20 scores a B-rated WER with a U-value of approximately 2.0 W/m²K. It’s compliant via the energy rating route and genuinely slim at 10mm sightlines. Compare that to traditional single-glazed steel, which typically sits at 5.0+ W/m²K – the W20 is a meaningful upgrade.
New builds and extensions tell a different story. When Part L compliance via the U-value route matters, the W20+ reaches 1.3-1.4 W/m²K and an A+ rating, exceeding current requirements without sacrificing the slim steel aesthetic.
Both systems use the same 4/10/4 krypton-filled slimline double glazing, so the proportions stay identical from the outside looking in.
Fabco manufactures both systems in West Sussex with directly employed installation teams. That matters because it removes the quality variation that often undermines performance data in real-world conditions.
Style versus efficiency isn’t actually a dilemma. With the right system choice, you get both.
