Last updated: 13 July 2026
Introduction
The slimmest steel windows on the UK market and modern thermal performance used to be a trade-off. Fabco’s hot-rolled W20 range now offers both from one profile: the standard W20 achieves a whole-window U-value of approximately 2.0 W/m²K with a B-rated Window Energy Rating, and the enhanced W20+ achieves an A+ rating with a U-value of 1.3–1.4 W/m²K. Both with 10mm sightlines, and both visually identical from the outside. This comparison sets out the verified figures, how each system meets Building Regulations Part L, and what to ask any manufacturer before you specify.
Thermal Performance Benchmarks: U-Values Across Sightline Specifications
A U-value measures the rate at which heat passes through a window or door: the lower the value, the better the insulation and the less heat escapes. Sightline (the visible frame edge around each pane) is the figure that defines how much glass and daylight you actually get. Traditionally, improving one meant sacrificing the other: original single-glazed steel windows have exceptionally slim frames but typically lose heat at 5.0 W/m²K or more.
The Window Energy Rating (WER), and Doorset Energy Rating (DSER) for doors, assesses the whole unit, taking in heat loss through frame and glass, solar heat gain, and air-leakage performance. It is one of the two routes to Building Regulations Part L compliance, and for steel windows it is usually the route that matters.
| System | Typical whole-window U-value | Energy rating | Sightline |
|---|---|---|---|
| Traditional single-glazed steel | 5.0+ W/m²K (single glazing typically 4.8–5.8) | Unrated | Very slim |
| Traditional leaded single glazing | 5.0–6.0 W/m²K | Unrated | Very slim |
| Heritage SMW steel (single or 16mm slim double glazing) | Not thermally rated – specified only where a heritage officer sets aside Building Regulations to preserve original character | Unrated | Heritage detailing |
| Fabco W20 | Approx. 2.0 W/m²K (2.0–2.2) | WER/DSER B | 10mm |
| Fabco W20+ | 1.3–1.4 W/m²K (fixed lights 1.3; opening windows and doors 1.4) | WER/DSER A+ | 10mm |
Two things stand out. First, moving from original single glazing to a modern slimline steel system cuts heat loss by more than half. Second, the step from W20 to W20+ happens without any change to the window’s appearance: from the outside the two systems are visually identical, with the engineering advancements concealed within the system.
Fabco W20 and W20+: Specifications and Thermal Performance
| Specification | Detail |
|---|---|
| Profile | Hot-rolled W20 steel – the original slimline steel glazing system, with a 32mm face width |
| Sightline | 10mm across both W20 and W20+ |
| W20 thermal performance | Whole-window U-value approx. 2.0 W/m²K; WER/DSER B; meets current Building Regulations Part L requirements via the WER/DSER route |
| W20+ thermal performance | Whole-window U-value 1.3–1.4 W/m²K; WER/DSER A+; exceeds current Part L requirements |
| Glazing | Single glazing, or 18mm slimline insulated double glazing: 4mm outer pane, 10mm krypton-filled cavity, 4mm inner pane |
| Gas fill | Krypton as specified – the industry standard is argon, which performs worse in the narrow cavities slimline units require |
| Coatings | Low-E glass as standard in Fabco double glazing; warm-edge spacer bars available |
| Glazing bars | Genuine steel sections, fully TIG-welded into the frame (not glued, clipped, or surface-applied) so sightlines stay consistent across the whole installation |
| Construction | TIG-welded joints throughout; durable powder-coated finish; corrosion-resistant stainless steel fixings |
| Related systems | The larger W40 profile is used for Fabco’s FD30 fire-rated doors and screens, not for standard windows |
Fabco’s systems feature some of the slimmest steel sightlines available, and because W20 and W20+ share the same external appearance, specifiers can choose the thermal tier without changing the visual design.
Why Krypton Matters in a Slim Frame
Slim sightlines demand slim glazing units, and slim glazing units mean narrow cavities. Standard double glazing relies on argon gas, which underperforms in a narrow gap. Krypton performs significantly better within narrow cavities, which is what allows an 18mm unit (4mm glass, 10mm krypton cavity, 4mm glass) to deliver modern thermal performance while closely resembling original single-glazed steel fenestration. Combined with Low-E coatings, warmer internal glass and frame surfaces also reduce the likelihood of condensation forming.
What Slim Sightlines Mean for Daylight
The arithmetic is simple and worth doing on any window you’re comparing. Take a 1,200mm × 1,500mm fixed light. At a 10mm sightline, the visible glass measures roughly 1,180mm × 1,480mm – about 97% of the opening. At a 45mm sightline, visible glass drops to roughly 1,110mm × 1,410mm – about 87%. Divided designs widen that gap further, because every glazing bar repeats the saving or the loss.
Glazed area also interacts with Building Regulations in both directions: higher glazing ratios feed useful solar gain into SAP energy calculations under Part L, while Part O caps glazing area by orientation on new dwellings to limit summer overheating. More glass helps one calculation and has to be managed against the other, which is why it pays to ask any manufacturer for documented sightline figures and dimensioned drawings rather than headline numbers alone.
Part L Compliance
Under Approved Document L (England, 2021 edition, in force since June 2022), new and replacement windows in existing dwellings must achieve either a U-value of 1.4 W/m²K or better, or a Window Energy Rating of Band B or better. For glazed doorsets (more than 60% glazed), the rating route is DSER Band C or better.
- Fabco W20 meets Part L for replacement windows through its B-rated WER, the rating route, which assesses the whole window including solar gain rather than the U-value alone.
- Fabco W20+ exceeds the requirements on both routes, with an A+ rating and a 1.3–1.4 W/m²K U-value, and feeds stronger figures into SAP calculations on new-build and deep-retrofit schemes, where the notional target for windows is 1.2 W/m²K.
Wales and Scotland set their own requirements, so check the national documents for projects outside England.
Frequently Asked Questions: Sightline, Thermal Performance, and Real-World Application
What is the difference between W20 and W20+?
W20 is the original slimline hot-rolled steel system, achieving a B Window Energy Rating and a U-value of approximately 2.0 W/m²K. W20+ incorporates enhanced thermal technology to achieve an A+ rating and a U-value of 1.3–1.4 W/m²K, without changing the appearance of the frame.
Do slimmer sightlines make U-values worse?
Not with modern steel systems. Fabco’s W20 and W20+ both hold a 10mm sightline: the standard system rates B and the enhanced system rates A+, so the slim frame and the energy rating come together rather than trading off.
Can slim steel windows really meet current Building Regulations?
Yes. The B-rated W20 meets Part L for replacement windows via the WER route, and the A+-rated W20+ exceeds current requirements.
Can W20 steel windows be double glazed?
Yes. W20 accommodates single glazing or 18mm slimline insulated units (4mm/10mm krypton/4mm) while retaining traditional sightlines, which is why it is widely accepted on heritage and conservation projects.
Why krypton rather than argon?
Krypton performs significantly better in the narrow cavities that slimline heritage glazing units require. Argon is the industry standard in conventional double glazing, but it is the poorer-performing choice in an 18mm unit.
How much maintenance do steel windows need?
Modern powder-coated steel windows require relatively little maintenance: durable powder-coated finishes, corrosion-resistant components, and stainless steel fixings keep upkeep to routine inspection and cleaning. See Fabco’s Care & Maintenance guide for the full schedule.
Conclusion
For replacement and renovation projects where a B rating satisfies Part L, the standard W20 delivers authentic slimline steel at approximately 2.0 W/m²K. For energy-conscious projects, new builds, and anywhere condensation control or running costs matter, the A+-rated W20+ at 1.3–1.4 W/m²K is the stronger specification, at the same 10mm sightline and with an identical external appearance. In highly sensitive heritage settings, single-glazed or SMW configurations remain available where conservation requirements take precedence over thermal ratings.
Whichever system you shortlist, from Fabco or anyone else, ask for the Window Energy Rating certificate, the whole-window U-value for your configuration, and dimensioned drawings showing the sightline. Those three documents settle the comparison better than any headline claim.
Fabco manufactures, surveys and installs its steel windows and doors from its West Sussex facility through directly employed teams. To discuss a project or request system documentation, contact +44 (0)1903 718808 or enquiries@fabco.co.uk.
