Last updated: 21 July 2026
If you own a listed building or a property in a conservation area, you’re probably stuck between two problems: keeping your home looking authentic and making it warm enough to meet modern standards. Fabco has become the go-to answer for that exact challenge. In 2026, its W20+ system holds an A+ Window Energy Rating with a whole-window U-value (a measure of how much heat escapes through a window, lower is better) of 1.3–1.4 W/m²K. That clears the current Building Regulations Part L threshold while keeping sightlines of just 10mm – basically identical to original steel glazing.
So what’s the actual problem? It comes down to numbers.
Traditional single-glazed steel windows (the kind found in most listed building) have U-values of 5.0 W/m²K or worse. Current Part L rules set a limit of 1.4 W/m²K for new installations. That gap means you need roughly a 70% improvement in how well a window holds heat just to meet today’s legal standard. That’s a big jump.
Steel can get you there in a way other materials simply can’t. Here’s why:
- Strength-to-profile ratio – steel is strong enough to hold slim double glazing in frames that would be impossible in timber or uPVC.
- Fabco’s 18mm krypton-filled unit uses a 4mm / 10mm cavity / 4mm setup that fits neatly inside a 32mm face-width frame.
- Conservation officers are on board – local authorities increasingly accept this approach because the sightlines still look period-accurate.
What changes is invisible. The glass and frames look the same from the outside. It’s only what sits between the panes and the engineering in the frame that’s different.
Traditional single-glazed steel windows have U-values of 5.0 W/m²K or higher.Building Regulations Part L sets a maximum of 1.4 W/m²K for replacement windows assessed via the U-value route. That gap is roughly 70%, which is why so many heritage homeowners assume steel windows and energy efficiency can’t go together.
They can.
The real issue comes down to glazing specification – basically, what goes inside the window unit and how it’s put together. When correctly engineered, steel windows can hit modern thermal standards without losing their slim, elegant profiles.
Fabco’s W20 system is a good example of this. It uses an 18mm slimline double-glazed unit built like this:
- 4mm outer pane
- 10mm krypton-filled cavity
- 4mm inner pane
That setup achieves a whole-window U-value of around 2.0 W/m²K and a B-rated Window Energy Rating (WER). This satisfies Part L building regulations – via the WER/DSER rating route, not the U-value route, where 2.0 W/m²K would fall short of the 1.4 W/m²K threshold.
Need to meet both assessment routes at once? That’s where the W20+ system comes in. It delivers 1.3 W/m²K on fixed lights and 1.4 W/m²K on opening windows and doors, earning an A+ WER rating.
While most windows use argon as standard, krypton is what powers the W20+’s performance boost together with an engineered frame. Krypton has lower thermal conductivity (meaning it transfers less heat) which gives it measurably better insulation within the same 10mm cavity depth. And keeping that shallow cavity is what preserves the authentic 10mm sightline width that gives heritage windows their distinctive look.
Here’s the clever part: the W20 and W20+ look completely identical from the outside. All that thermal improvement is hidden within the frame, where no one can see it.
Typical Heritage Specifications: Three Common Project Scenarios
Every heritage project is different, but most fall into recognisable patterns. The three scenarios below show how a typical project of each kind is specified, and what approval usually hinges on.
Scenario 1: A Grade II Listed Farmhouse (W20+ Specification)
The starting point: original mid-twentieth-century single-glazed steel frames. Published figures put single-glazed steel at typically 5.0+ W/m²K – original units are rarely tested, but the performance gap is not in doubt.
The typical specification: the W20+ system glazed with 18mm slimline krypton-filled units (4mm outer pane, 10mm cavity, 4mm inner pane), achieving a whole-window U-value of 1.3 W/m²K on fixed lights and 1.4 W/m²K on opening windows and doors, with an A+ Window Energy Rating.
What approval typically hinges on: listed building consent is required for alterations affecting the building’s character, and the case usually turns on whether the replacement matches the original profiles. The W20+ holds the same 10mm sightlines and 32mm face width as the original W20 profile, which is what conservation officers assess against the existing fenestration. Where an officer judges that even slimline double glazing would harm the building’s significance, single glazing remains available in the same profiles. The decision always rests with the local planning authority.
Scenario 2: A Conservation-Area Terraced House (W20 Specification)
The typical specification: the standard W20 system with slimline double glazing, achieving a whole-window U-value of approximately 2.0 W/m²K and a B-rated Window Energy Rating.
What approval typically hinges on: in a conservation area, like-for-like replacement is often permitted development, but some conservation areas are covered by Article 4 Directions, which remove those rights and make a planning application necessary. Where a direction applies, the application usually succeeds or fails on material, profile and sightline authenticity. Hot-rolled steel sections at 10mm sightlines replicate what planning officers are comparing against.
Scenario 3: An Edwardian or Interwar Semi-Detached (W20+ Specification)
The typical specification: a mixed configuration of fixed lights and side-hung casements (common in this housing stock) specified in W20+ with 18mm slimline krypton-filled units.
What approval typically hinges on: most houses of this type sit outside listed or conservation-area controls, so Building Regulations Part L is usually the only approval that applies, and the W20+ satisfies it via both routes.
The common thread across all three: the same 32mm face width and 10mm sightlines that satisfy conservation officers are what deliver the glass area, daylight and proportions these buildings were designed around. Heritage authenticity and modern thermal performance are specified together, not traded against each other.
How We Build Our Steel Systems For Heritage Projects
Thermal performance figures sound great on a spec sheet – but they only mean something if the manufacturing and installation actually back them up. How a product is made and who makes it matters more than most buyers ever realise.
Fabco builds its windows at a facility in West Sussex using hot-rolled W20 steel sections. All the critical frame joints are TIG welded. TIG stands for Tungsten Inert Gas, a welding method that causes very little distortion and creates stronger, cleaner joints than the more common MIG alternative. The glazing bars (the thin strips that divide the glass panels) are fully welded directly into the frame rather than clipped or bonded on. That might sound like a small detail, but it stops the movement that can slowly break down the window’s seals over time.
Before powder coating, every frame goes through hot-dip galvanising to British Standards. That gives you two separate, independently verifiable layers of protection against corrosion.
Rather than farming out different parts of the job to different companies, Fabco keeps everything in-house. Their own surveyors, project engineers, installation fitters, and glazing engineers handle each stage. Every project gets a dedicated project engineer who stays accountable from the initial 3D laser survey all the way through to the final wet glazing, which uses RAL colour-matched, UV-resistant silicone.
Quality control runs across eight documented checkpoints:
- Material quality
- Fabrication accuracy
- Weld quality
- Component fitment
- Surface preparation
- Finish quality
- Hardware installation
- Final product inspection
This level of control is especially important for heritage buildings. A U-value of 1.3–1.4 W/m²K for W20+ or 2.0 W/m²K for W20 is only achievable in the real world if the 18mm krypton-filled 4/10/4 glass unit is fitted correctly and sealed without any thermal bridging at the frame junction. Get that wrong, and the performance figures you specified simply won’t show up in practice.
Summary
Bespoke steel windows earn their premium price through decades of real, measurable performance, not just good looks. A W20+ system cuts heat loss dramatically compared to the 5.0+ W/m²K you’d get from original single glazing. And that improvement doesn’t stop at year one. Lower heating demand and reduced condensation risk keep paying off year after year.
So what makes Fabco stand out? It comes down to a specific mix of things working together:
- Verified thermal performance backed by independent testing
- British manufacturing based in West Sussex
- TIG-welded hot-rolled W20 steel construction
- A team of directly employed surveyors, installers, and glazing engineers – no subcontractors
That last point matters more than you might think. When something’s built to last fifty years or more, you want to know exactly who’s responsible for it.
