A detailed engineering, economic, and regulatory analysis of steel versus aluminium in façade, curtain wall, and window systems.
Aluminium has dominated curtain-wall and window-framing since the 1960s — not because it is the strongest material, but because it offers an unmatched combination of lightness, corrosion resistance, thermal treatability, and extrusion flexibility. As global aluminium prices surge due to energy costs and supply chain disruptions, the construction industry has revisited steel — a material that is structurally superior in raw tensile strength. Yet steel has not displaced aluminium in facades. Why?
This document examines the fundamental technical, economic, and regulatory barriers that prevent steel from being a viable drop-in substitute in façade and window applications.
Tensile strength of 250–690 MPa (structural grades). Extremely stiff with an elastic modulus of ~200 GPa. Dense at 7,850 kg/m³. Excellent in compression and bending. Widely available and cost-effective in bulk tonnage. However, heavy, prone to corrosion, and a severe thermal bridge without complex thermal break engineering.
Tensile strength 150–570 MPa depending on alloy/temper. Elastic modulus ~69 GPa. Density 2,700 kg/m³ — one-third that of steel. Naturally oxidises to form a protective layer. Infinitely extrudable into complex thermal-break profiles. Dominant façade material globally, but energy-intensive to produce and price-volatile.
Despite steel's raw mechanical superiority, seven fundamental barriers restrict its use in modern building envelopes:
Steel's thermal conductivity is ~50 W/m·K vs aluminium's ~160 W/m·K — but both are devastatingly high compared to UPVC (~0.17) or timber (~0.15). Standard steel sections create severe cold bridges, causing condensation, mould, energy loss, and failing modern energy codes (ASHRAE 90.1, UK Part L, EU EPBD). Thermally broken steel profiles do exist but are prohibitively complex and costly to manufacture.
Steel is 2.9× heavier than aluminium by volume. A curtain wall system using steel framing adds enormous dead load to the building structure — requiring larger structural columns, beams, and foundations. For high-rises and slender towers, this is often structurally and economically untenable, eliminating any cost savings from cheaper section material.
Carbon steel rusts aggressively in humid, coastal, and industrial environments. Maintenance cycles of 5–10 years involving abrasive blasting and recoating add enormous lifecycle cost. Stainless steel (304/316) avoids corrosion but costs 4–8× more than carbon steel, and roughly 2–3× more than aluminium, negating any procurement advantage.
Aluminium is uniquely suited to extrusion — it can be pushed into almost any complex cross-sectional profile, including multi-chambered thermal break cavities, integrated drainage channels, and snap-fit gasket slots, all in one die pass. Steel cannot be extruded; it is rolled, formed, or welded. Achieving equivalent profile complexity in steel requires expensive secondary fabrication, multiple press operations, or welded assemblies — driving labour costs dramatically.
Steel's coefficient of thermal expansion (~12 × 10⁻⁶/°C) is roughly half that of aluminium (~23 × 10⁻⁶/°C). When combined with glass (glass CTE ≈ 9 × 10⁻⁶/°C), differential movement in mixed systems risks seal failure, glass stress fracture, and joint cracking. Designing façade systems with mixed materials or replacing aluminium profiles with steel requires complete re-engineering of all movement joints and glazing bite dimensions.
Aluminium is lightweight, easily cut with standard saws, drilled without special tooling, and handled by two workers. Steel systems require cutting disc equipment, larger cranes for lifting, more robust temporary fixings, and specialist welders for any site modifications. On-site modifications — routine in facade installation — are straightforward in aluminium and highly challenging in steel, especially for stainless grades.
Contrary to intuition, steel performs poorly in fire without protection — it rapidly loses load-bearing capacity above 550°C. Aluminium melts at 660°C but loses structural integrity at ~150–200°C. Both require fire protection coating or cladding in many façade specifications. However, the certification ecosystem — tested system approvals, fire-rated glazing assemblies, CWCT test data — is mature and deep for aluminium, and largely absent for steel in façade applications, making specification and insurance sign-off very difficult.
Decades of aluminium-centric R&D have produced a vast ecosystem: tested gaskets, thermal break polyamide struts, pressed fittings, neoprene glazing packers, sealant compatibility data, and software tools all calibrated to aluminium section properties. There is no equivalent ecosystem for steel façade systems. Specifying steel requires custom engineering from scratch for virtually every component, raising design cost and programme risk.
The bars below score each material out of 100 across key façade performance criteria. Raw structural strength alone does not determine suitability for building envelopes.
| Properties evaluated for curtain wall, window, and cladding applications | |||
|---|---|---|---|
| Property / Criterion | Steel (Carbon/Structural) | Aluminium (6063/6082 Alloy) | Façade Winner |
| Tensile Strength | 250–690 MPa | 150–310 MPa | Steel |
| Yield Strength | 250–500 MPa | 110–270 MPa | Steel |
| Elastic Modulus | ~200 GPa | ~69 GPa | Steel (stiffer) |
| Density | 7,850 kg/m³ | 2,700 kg/m³ | Aluminium (3× lighter) |
| Strength-to-Weight Ratio | Moderate | High | Aluminium |
| Thermal Conductivity | ~50 W/m·K | ~160 W/m·K | Both poor — need breaks |
| Thermal Break Feasibility | Very difficult / costly | Standard polyamide strut — simple | Aluminium |
| Thermal Bridging Risk | Severe — large section footprint | Moderate — manageable with breaks | Aluminium |
| Coefficient of Thermal Expansion | ~12 × 10⁻⁶/°C | ~23 × 10⁻⁶/°C | Steel less movement, but mismatch risk |
| Corrosion Resistance (bare) | Very poor — rusts | Excellent — self-passivating oxide | Aluminium |
| Required Surface Protection | Paint, galvanising, or stainless | Anodise or powder coat (optional) | Aluminium |
| Maintenance Interval (facade) | 5–10 years (carbon); minimal (SS) | 20–30 years (powder coat) | Aluminium |
| Extrusion / Profile Complexity | Not extrudable; rolled/formed only | Fully extrudable — unlimited profiles | Aluminium |
| On-site Modification | Difficult; requires disc cutters, welders | Easy; standard saw, drill, files | Aluminium |
| Structural Dead Load Added | High — significant sub-structure needed | Low — minimal sub-structure impact | Aluminium |
| Fire Resistance (structural) | Fails ~550°C (rapid strength loss) | Fails ~200°C (lower but rarely structural) | Both require fire protection |
| Raw Material Cost (current) | Lower (carbon); higher (SS) | Moderate — but volatile (energy-linked) | Carbon steel cheaper; SS comparable |
| Total Installed System Cost | Higher (fabrication, weight, protection) | Moderate — mature supply chain | Aluminium |
| Lifecycle Cost (30-year) | High (maintenance, painting cycles) | Low to moderate | Aluminium |
| System Test Data / Certifications | Scarce — bespoke engineering required | Extensive — CWCT, BS EN, AAMA, AS2047 | Aluminium |
| Glazing Compatibility (tested) | Very limited standardised data | Vast library of tested assemblies | Aluminium |
| Recyclability | ~90%+ recycled globally | ~75%+ recycled; high energy to re-smelt | Steel (slightly) |
| Embodied Carbon | ~2.0–2.5 kgCO₂e/kg (EAF) | ~8–12 kgCO₂e/kg (primary smelting) | Steel (lower embodied carbon) |
| Aesthetic / Slim Sightlines | Possible (steel is stiff — slim profiles) | Good — but slimmer with steel's stiffness | Steel niche advantage |
| Colour / Finish Options | Painted; limited powder coat options | Full RAL range; anodising; PVD | Aluminium |
| Speed of Installation | Slow — heavy lifts, site welding | Fast — light components, click-fit systems | Aluminium |
Thermal bridging is the single biggest barrier to steel in energy-regulated façades. The numbers below illustrate why this is not a minor issue:
Steel is not without merit in the building envelope. In specific, non-thermally-regulated or structurally demanding contexts it is highly appropriate:
Steel's high stiffness allows structural steel fins and couplers in point-fixed glass systems and minimal-frame glazed facades to achieve sightlines as narrow as 10–15mm — impossible in aluminium which would deflect excessively at the same section depth. Architects specify steel for premium glass box aesthetics.
Warehouses, factories, power stations, and heritage steel-framed buildings routinely use steel window systems (Crittal-style) where thermal performance is not the primary driver and the visual language demands metal frames with heritage character.
For blast-rated, bullet-resistant, and forced-entry-resistant openings, steel frames and structural steel glazing systems are frequently specified because the section stiffness and weld-ability to surrounding structure provides superior blast impulse resistance beyond what aluminium can achieve.
While aluminium wins for the visible frame, steel dominates as the hidden structural substrate — the brackets, rails, and sub-structure that transfer cladding loads back to the primary structure, where weight matters less and stiffness matters more.
| Cost Element | Steel System | Aluminium System | Difference |
|---|---|---|---|
| Raw Section Material | Lower (carbon steel) | Higher (volatile) | Steel advantage |
| Anti-corrosion coating | Mandatory — significant cost | Optional — low cost | Steel penalty |
| Fabrication complexity | Very high (welding, forming) | Low (extrusion, clip assembly) | Steel penalty |
| Thermal break engineering | Custom design; costly | Standard profile; negligible | Steel penalty |
| Structural sub-frame uplift | Significant (heavier cladding) | Minimal | Steel penalty |
| Installation labour | High (heavy lifting, welding) | Low (light, snap-fit) | Steel penalty |
| Certification / test cost | High (bespoke test required) | Low (off-shelf tested systems) | Steel penalty |
| Maintenance (30-year) | High (repainting cycles) | Very low | Steel penalty |
| Overall System Cost | 15–40% more than aluminium equivalent | Baseline | Steel loses overall |
Steel's superior tensile strength and stiffness are real — but they are largely irrelevant to the principal challenges of façade engineering. Curtain walls and windows fail not from tensile overload but from thermal underperformance, corrosion, movement accommodation, and weight-driven structural cost. Aluminium excels precisely in those dimensions where steel is most deficient. Rising aluminium costs create economic pressure, but they do not eliminate the technical barriers. The industry response is more likely to be optimised aluminium designs, increased use of UPVC-aluminium hybrids, thermally broken steel for niche premium applications, and greater recycled content in aluminium sections — rather than a wholesale migration to steel facades.