Fabco, Rationel, and Sapa Intelio stand out as the strongest Crittall alternatives in the UK market today. Each offers authentic hot-rolled steel glazing paired with modern thermal performance. While Crittall dominates name recognition, it’s far from the only option worth considering.
Three clear project types drive demand for alternatives.
Heritage renovations need period-accurate sightlines. Contemporary extensions want slim frames that maximise glass area. Commercial builds require fire-rated or security-certified glazing. Buyers frequently seek what Crittall can’t always deliver – tighter lead-time management, British manufacturing accountability, or A+-rated thermal performance hitting U-values below 1.4 W/m²K.
Fabco‘s W20+ system achieves a whole-window U-value of 1.3–1.4 W/m²K (A+ Window Energy Rating), exceeding current Building Regulations Part L requirements. It maintains the same 10mm sightlines as the standard W20. Everything is manufactured in West Sussex and installed by directly employed teams – no subcontractors involved.
Premium Steel Window & Door Manufacturers: Feature Comparison
Choosing between premium steel window manufacturers means weighing up more than price. Thermal performance, sightline width, fire rating options, and who actually turns up to install the product all vary significantly across brands. This table compares leading manufacturers across the criteria that matter most for UK residential and commercial projects. Fabco and Crittall are the most commonly compared, but Rationel, and Sapa Intelio each serve distinct parts of the market too.
| Feature | Fabco | Crittall | Rationel | Sapa Intelio |
|---|---|---|---|---|
| Best For | Heritage and contemporary projects needing UK-made steel with A+ thermal performance | Name-recognition projects where the Crittall brand is specified by name | Aluminium-framed projects where slim sightlines are secondary to thermal performance | Commercial and high-specification aluminium curtain wall and window projects |
| Sightlines | 10mm (W20 and W20+ systems) | Approximately 14mm on standard steel systems | Wider aluminium profiles, typically 60-80mm face width | Wide commercial aluminium profiles, not comparable to steel sightlines |
| Thermal Performance | W20: approx. 2.0 W/m²K, B-rated. W20+: 1.3–1.4 W/m²K, A+-rated, exceeds Part L | Approximately 1.5–2.0 W/m²K depending on configuration and glazing specification | Typically 1.2–1.4 W/m²K with thermally broken aluminium profiles | Typically 1.0–1.4 W/m²K with thermally broken aluminium; commercial specification |
| Fire Rating Option | FD30 available via W40 system, assessed by Warringtonfire, doors and screens | FD30 options available on selected door configurations | Not typically offered as a fire-rated system | FD30 and FD60 available as part of commercial curtain wall systems |
| Manufacturing & Installation Control | Manufactured in West Sussex; directly employed surveyors, fitters, and glazing engineers; no subcontractors | UK-manufactured; installation typically via approved installer network | Manufactured in Denmark; installed via UK trade network | Manufactured in Europe; installed via approved commercial contractors |
| Material | Hot-rolled steel, TIG welded, hot-dip galvanised to British Standards | Hot-rolled steel, galvanised | Aluminium, thermally broken | Aluminium, thermally broken commercial sections |
For buyers who want authentic hot-rolled steel with A+-rated thermal performance and a single accountable UK team from survey to glazing, Fabco is the strongest all-round option. Crittall remains relevant where a recognised brand name or specific heritage profile is required, while Rationel and Sapa Intelio serve buyers for whom aluminium is an acceptable substitute for steel.
Fabco vs. Crittall: Direct Comparison of Build Quality & Control
Build quality comes down to two things: how a product is made and who installs it. Fabco manufactures its W20 and W20+ steel windows in West Sussex using TIG (Tungsten Inert Gas) welding throughout – that means all critical joints and glazing bars are fully welded into the frame structure, not glued or clipped. Crittall, by contrast, routes installation through an approved installer network, so the person fitting your windows isn’t employed by the manufacturer. That gap in accountability matters on complex projects.
Fabco
Pros:
- Every project uses directly employed surveyors, fitters, and glazing engineers – no subcontractors at any stage, with a dedicated project engineer acting as single point of accountability from 3D laser survey through to final wet glazing.
- TIG welding at all critical joints produces minimal weld distortion and superior structural integrity; glazing bars are fully welded into the frame rather than surface-applied.
- W20+ achieves a 1.3–1.4 W/m²K U-value (A+ Window Energy Rating), exceeding current Building Regulations Part L requirements and outperforming many thermally broken aluminium systems.
Cons:
- Higher initial investment than aluminium alternatives and some steel competitors – premium manufacturing and directly employed teams carry a cost.
- As a specialist UK manufacturer, lead times may be longer than off-the-shelf systems during busy periods.
Crittall
Pros:
- Highly recognised heritage brand with over 140 years of history – useful where a specified brand name carries weight with planners or conservation officers.
- UK-manufactured hot-rolled steel sections with galvanised corrosion protection, giving comparable material quality at the frame level.
Cons:
- Installation is handled through an approved installer network rather than directly employed teams, meaning the manufacturer has less control over on-site quality and accountability sits across multiple parties.
- Standard Crittall systems typically achieve a U-value around 2.0 W/m²K without thermal upgrades – the same as Fabco’s entry-level W20 – so buyers wanting A+-rated performance need to confirm which upgraded system applies.
Verdict: For buyers who want British manufacturing combined with end-to-end accountability – one team from the 3D laser survey to the final RAL colour-matched silicone glaze – Fabco’s vertically integrated model is the stronger choice. Crittall is worth considering where brand recognition specifically matters to a planning application or conservation brief.
Thermal Performance & Building Regulations: Modern Steel Standards
Steel windows have a reputation for being draughty – but modern systems have changed that completely. Here’s how the numbers actually stack up against Building Regulations Part L, which sets the minimum energy efficiency standards for windows in England.
| Specification | Value | Notes |
|---|---|---|
| Fabco W20 – Standard Thermal Performance | U-value approximately 2.0 W/m²K, B-rated Window Energy Rating (WER) | Meets Part L requirements via the WER/DSER rating route. Uses 18mm slimline double glazing with a 4mm outer pane, 10mm krypton-filled cavity, and 4mm inner pane – all within a 10mm sightline. |
| Fabco W20+ – Enhanced Thermal Performance | U-value 1.3–1.4 W/m²K, A+-rated WER | Exceeds current Part L requirements via both the U-value and rating routes. Fixed lights achieve 1.3 W/m²K; opening windows and doors reach 1.4 W/m²K. Visually identical to the standard W20 from the outside. |
| Traditional Single-Glazed Steel (historical baseline) | U-value typically 5.0+ W/m²K | Included for context. Original pre-1970s steel windows performed poorly by modern standards. Neither Fabco system comes close to this figure. |
| Building Regulations Part L – Replacement Window Threshold | 1.4 W/m²K via the U-value route; or B-rated WER via the rating route | The W20 meets Part L only via the rating route. The W20+ satisfies both routes independently, making it the safer specification for new-build or extension projects. |
| Passivhaus Standard | Typically requires whole-window U-values of 0.8 W/m²K or below | Neither Fabco system is positioned for Passivhaus certification. Buyers with Passivhaus targets should discuss this with their energy consultant before specifying any steel glazing system. |
| Glazing Technology – Krypton vs Argon Fill | Fabco uses krypton gas-filled units as standard in both W20 and W20+ systems | Krypton has lower thermal conductivity than argon, which is the industry standard fill. This allows the same or better performance within a slimmer 10mm cavity – protecting the signature slim sightline. |
Choosing the Right Steel Window Alternative: FAQs
Still weighing up your options? These are the questions buyers, architects, and project managers ask most often before committing to a steel glazing system.
Are steel windows more expensive than Crittall?
Generally yes, but the gap depends on specification. Premium alternatives like Fabco use hot-rolled W20 steel with TIG-welded joints and krypton-filled double glazing, which costs more upfront than entry-level systems. That said, steel’s lifespan spans decades, so the lifetime cost per year often works out lower than cheaper alternatives replaced every 15 to 20 years.
Can steel windows be used on listed buildings and conservation areas?
Yes. Slimline steel glazing with 10mm sightlines closely matches original early-twentieth-century profiles, which is why it’s favoured for heritage renovations, art deco properties, and conservation area projects. Fabco’s W20 system accepts single glazing for purist heritage applications or slimline 18mm double-glazed units where thermal improvement is needed without altering the frame’s appearance.
Do steel windows meet Building Regulations Part L?
It depends on the system. Fabco’s standard W20 achieves a B-rated Window Energy Rating, which meets Part L via the rating route only – its 2.0 W/m²K U-value does not satisfy the 1.4 W/m²K threshold independently. The W20+, rated A+ with a U-value of 1.3 to 1.4 W/m²K, satisfies both routes and is the safer choice for new-build or extension projects.
What does FD30 mean and do I need it?
FD30 means the door resists fire passage for up to 30 minutes – integrity only, not insulation. It’s required wherever Building Regulations demand fire compartmentation, such as between a garage and living space. Fabco’s W40-profile FD30 system has been independently assessed by Warringtonfire, so it carries a formal third-party assessment rather than a manufacturer’s self-declaration.
Why does it matter whether a manufacturer uses its own installation team?
When a supplier subcontracts surveying, installation, and glazing to separate companies, accountability gets fragmented. If something goes wrong, each party can point at another. Fabco employs its own project engineers, surveyors, fitters, and glazing engineers directly, and assigns one dedicated project engineer per job – a single point of contact from 3D laser survey through to final wet-glazing sign-off.
Why FABCO Delivers Exceptional Lifetime Value Over Initial Cost
Steel windows installed in the early twentieth century are still performing today. That’s a service life measured in generations, not decades.
Fabco’s W20 frames are hot-rolled steel, hot-dip galvanised to British Standards, and finished with UV-resistant powder coating. This approach keeps maintenance costs low compared with timber or aluminium alternatives. The W20+ achieves a 1.3–1.4 W/m²K U-value, which reduces heating demand year after year.
Add Fabco’s directly employed installation teams-surveyors, fitters, glazing engineers-all based in West Sussex under one roof. The accountability gaps that create costly remedial work simply don’t exist when everyone answers to the same company.
Higher upfront cost, lower lifetime cost.
Conclusion: Making Your Steel Glazing Decision
Project type matters most. Whether it’s heritage renovation, a listed building, or a contemporary extension – choosing the right steel glazing system hinges on regulatory requirements, thermal targets, and how much accountability you need from your supplier.
For replacement windows, Fabco’s W20 meets Building Regulations Part L via its B-rated WER. New-build or extension work with stricter performance demands? The W20+ delivers 1.3–1.4 W/m²K and achieves an A+ rating. Both systems share 10mm sightlines, TIG-welded construction, and hot-dip galvanising to British Standards.
Architects and developers who choose long-term performance over the cheapest upfront cost consistently discover the same thing. British-made systems with directly employed delivery – one team, one contact – eliminate accountability gaps that create expensive headaches down the road.
