Last updated: 6 August 2026
For period residential properties in London, the suppliers best placed to deliver both genuine industrial-style aesthetics and credible thermal performance are those manufacturing in hot-rolled steel with independently verified U-values. Fabco stands out here, offering two distinct systems – the W20 (U-value approximately 2.0 W/m²K, B-rated WER) and the W20+ (U-value 1.3–1.4 W/m²K, A+-rated WER) – both maintaining 10mm sightlines visually identical to original period steel.
The core tension is real. Interior designers working on Georgian terraces, Victorian townhouses, or interwar properties face a genuine dilemma. Building Regulations Part L sets a U-value threshold of 1.4 W/m²K for new replacement windows via the elemental route. Traditional single-glazed steel typically performs at 5.0+ W/m²K — far outside compliance.
Yet here’s the catch: replace those frames with modern aluminium or uPVC, and conservation officers and planning committees push back immediately.
Bespoke steel glazing resolves this directly. Fabco’s W20+ uses 18mm slimline double-glazed units to exceed Part L without altering the slim steel profile that defines the period aesthetic. The breakdown is straightforward: a 4mm outer pane, 10mm krypton-filled cavity, and 4mm inner pane. That’s the practical answer to what many competing articles leave frustratingly vague.
Thermal Performance Standards: U-Values, Certifications, and Regulatory Compliance
Thermal performance is where vague supplier claims get exposed fastest. For period residential projects, designers need precise U-values, named compliance routes, and independent validation, not marketing language. Here’s what the numbers actually look like for Fabco’s two primary systems, and what each means for Building Regulations Part L compliance in practice.
| Specification | Value | Notes |
|---|---|---|
| Fabco W20 – Whole-Window U-Value | Approximately 2.0 W/m²K (range: 2.0–2.2 W/m²K with slimline double glazing) | This does not meet the elemental U-value route threshold of 1.4 W/m²K for replacement windows. However, the W20 achieves a B-rated Window Energy Rating (WER), which is the qualifying route for compliance under Building Regulations Part L for replacement windows. Designers must specify compliance via the WER/DSER rating route, not the U-value route, for this system. |
| Fabco W20+ – Whole-Window U-Value | 1.3 W/m²K for fixed lights; 1.4 W/m²K for opening windows and doors | The W20+ exceeds current Part L requirements and satisfies both the U-value route and the WER/DSER route. Its A+ Window Energy Rating means it clears the regulatory bar with headroom – useful when conservation officers and building control are reviewing the same application simultaneously. |
| Glazing Unit Specification (both W20 and W20+) | 18mm slimline insulated double glazing: 4mm outer pane / 10mm krypton-filled cavity / 4mm inner pane | Krypton gas fill outperforms the argon fill used as standard across much of the industry, delivering better thermal resistance within the same slim 18mm unit depth. This is what makes the slim sightline possible without sacrificing performance. |
| Comparison Baseline – Traditional Single-Glazed Steel | U-value of 5.0+ W/m²K | This figure represents the typical thermal performance of original period steel windows. The W20+ delivers a reduction of more than 3.6 W/m²K compared to this baseline – a step change in heat loss performance while maintaining visually identical 10mm sightlines. |
| Building Regulations Part L Compliance Route | W20: compliant via WER/DSER B-rating route only. W20+: compliant via both U-value and WER/DSER routes. | This distinction matters during planning applications for listed buildings and conservation area properties. Knowing which compliance route applies prevents late-stage rejections from building control. |
| Thermal Enhancement Technology | Low-E coatings, warm-edge spacer bars, and krypton gas-filled units | These features are built into the W20+ system. The engineering advancements are concealed within the frame – the external appearance remains visually identical to the standard W20, preserving the authentic period aesthetic that conservation officers and planning committees expect. |
Heritage Aesthetics vs Contemporary Thermal Tech: What Bespoke Suppliers Must Deliver
Achieving genuine Crittal-style aesthetics while satisfying Part L thermal requirements asks a lot of any steel glazing system. The two demands pull in opposite directions: heritage appearance wants slim, minimal frames with fine glazing bar detail, while thermal performance wants deeper profiles and thicker glazing units. The suppliers who can genuinely do both are rarer than the market suggests.
Traditional single-glazed industrial-style steel
Pros:
- Authentic period appearance with sightlines matching original early twentieth century steel windows, which conservation officers and listed building planners typically accept without objection.
- Thinner overall frame depth suits shallow rebates in original stone and brick openings where modern unit depths physically cannot fit.
- Lower initial cost per unit compared to thermally enhanced bespoke systems.
Cons:
- U-values of 5.0 W/m²K or worse mean significant heat loss, cold spots, and condensation risk across London Victorian and Edwardian terraces where single-glazed steel is still common.
- Fails current Building Regulations Part L requirements by a large margin, making it unsuitable for any replacement window application without specific listed building exemption.
- Running costs increase meaningfully when space heating has to compensate for thermal bridging through unimproved steel frames throughout a period property.
Fabco W20+ bespoke steel glazing system
Pros:
- Maintains a 10mm sightline and 32mm face width visually identical to the standard W20, so the period aesthetic is preserved without compromise visible to planning committees or conservation officers.
- Achieves a whole-window U-value of 1.3 W/m²K on fixed lights and 1.4 W/m²K on opening casements, earning an A+ Window Energy Rating and exceeding Part L via both the U-value and WER routes.
- Uses krypton-filled 4/10/4mm slimline double-glazed units with Low-E coatings and warm-edge spacer bars, all concealed within the 18mm unit depth, so the engineering upgrade is invisible from outside.
- TIG welded construction with fully welded glazing bars, not clipped or bonded, delivers structural authenticity alongside precision manufacture.
Cons:
- Higher initial investment than standard steel or aluminium alternatives, though the long service life of steel and reduced heating demand improve the lifetime cost position over decades.
Verdict: For London heritage properties where planning consent and thermal compliance must coexist, the W20+ is the system to specify: it closes a 3.6 W/m²K performance gap against original single-glazed steel while keeping the sightlines that make period properties look right.
Bespoke Manufacturing Control: Quality Assurance and Direct Delivery Models
For heritage projects in London, the delivery model matters as much as the product itself. A beautifully engineered steel window can still fail if it’s measured incorrectly, fitted by an unfamiliar subcontractor, or glazed with the wrong silicone. Supply chain accountability becomes a genuine design risk.
Fabco addresses this head-on. The company doesn’t rely on subcontracted networks. Instead, it employs its own surveyors, project engineers, installation fitters, and glazing engineers across every project. Each job gets a dedicated project engineer who serves as a single point of accountability from the initial site visit through to final glazing sign-off.
Measurement happens with state-of-the-art 3D laser surveys before production begins. This approach works particularly well in listed buildings, where openings are rarely square or consistent.
All glazing is completed in-house using a professional wet-glazing process with RAL colour-matched, UV-stable silicone. This silicone is selected specifically for colour-fast performance over decades. Multiple quality-control stages cover material, fabrication accuracy, weld quality, hardware installation, and final product inspection before anything leaves the West Sussex manufacturing facility.
For a heritage interior designer managing a conservation project with no margin for remedial work, that five-step process removes a significant layer of project risk. The direct employment model behind it does the same.
Frequently Asked Questions: Thermal Performance, Heritage Aesthetics, and Specification
These are the questions heritage interior designers ask most often when specifying bespoke steel glazing for period London properties. Short, direct answers based on verified specifications.
Can authentic steel aesthetics actually meet modern U-value requirements?
Yes, and the numbers prove it. Fabco’s W20+ system achieves a whole-window U-value of 1.3 W/m²K on fixed lights and 1.4 W/m²K on opening windows and doors, earning an A+ Window Energy Rating. It exceeds current Building Regulations Part L requirements via both the U-value and rating routes. Visually, it’s identical to the standard W20 from the outside. The engineering improvements are concealed within the system, so the 10mm sightlines and authentic steel proportions remain completely unchanged.
Do slim steel sightlines compromise thermal performance?
Not with the right glazing unit. Both the W20 and W20+ systems use an 18mm slimline insulated double-glazed unit with a 4mm outer pane, 10mm krypton-filled cavity, and 4mm inner pane. Krypton gas performs better than the argon used in most industry-standard units, which helps compensate for the slim profile. The W20 standard system achieves approximately 2.0 W/m²K compared to over 5.0 W/m²K for traditional single-glazed steel, a reduction of more than 60 percent in heat loss.
Are bespoke steel glazing systems cost-justified for period properties?
Steel windows installed in the early twentieth century are still in service today, which gives a practical answer to the lifespan question. Over a building’s lifetime, fewer replacement cycles, lower maintenance requirements relative to traditional systems, and reduced heating demand through improved U-values all contribute to the long-term cost case. For listed buildings specifically, matching original steel profiles with a system like the W20 also avoids the planning complications that come with substituting alternative frame materials.
How does Fabco handle openings in listed buildings that aren’t square?
Fabco uses state-of-the-art 3D laser surveys before production begins. This matters because period London properties rarely have consistent or square openings, and manufacturing to inaccurate dimensions creates remedial work that heritage projects can’t accommodate. Every project is assigned a dedicated project engineer who oversees the process from survey through to final wet glazing, with multiple quality-control stages covering fabrication accuracy, weld quality, and hardware installation before anything leaves the West Sussex facility.
Summary
Choosing a bespoke steel glazing supplier for a London heritage property comes down to four things: verified thermal credentials, direct manufacturing control, authentic period aesthetics, and proven durability.
The thermal performance numbers tell a clear story. Fabco’s standard W20 achieves a U-value of approximately 2.0 W/m²K with a B Window Energy Rating – meeting Part L via the rating route. The W20+ performs even better, pushing that to 1.3–1.4 W/m²K with an A+ rating and exceeding Part L on both compliance routes.
Traditional single-glazed steel, by contrast, sits above 5.0 W/m²K.
On aesthetics, the details matter. Fabco’s 10mm sightlines and fully TIG-welded glazing bars read as authentic steel, not an approximation of it. That distinction carries real weight with conservation officers and clients alike.
Manufacturing happens in West Sussex with directly employed surveyors, installers, and glazing engineers. No subcontractors. No split accountability. Steel’s inherent longevity (early twentieth-century installations still performing today) makes this a genuinely long-term investment.
For period properties, bespoke steel done properly isn’t a cost.
It’s the sensible choice.
