In short
SMC composite wins on protection class, corrosion freedom, and RF transparency. Aluminium wins on thermal conductivity and recyclability. Cast iron is relevant for columns and decorative environments, not modern luminaire housings. No material is universally best — the choice should follow the installation environment and operating requirements.
Overview table
The following table summarises the five principles at material-principle level. Exact values for a specific product should always be verified against the manufacturer's datasheet.
| Property | SMC composite | Aluminium | Cast iron |
|---|---|---|---|
| Electrical insulation | Insulating (Class II) | Conductive (Class I) | Conductive (Class I) |
| Metallic corrosion | None in matrix | Corrodes; galvanic risk | Rusts |
| RF transparency | Yes (dielectric) | No (shields) | No (shields) |
| Thermal conductivity | Low (~0.2–0.5 W/mK) | High (~200 W/mK) | Medium (~50 W/mK) |
| Weight (relative) | Low–medium | Medium | High |
| Recyclability | Medium (thermoset) | Good | Good |
| Typical use | Luminaire housing | Housing, heatsink | Columns, heritage |
1. Electrical insulation — Class I vs Class II
IEC 60598 divides luminaires into protection classes. Class I requires a protective earth conductor (PE) so that a fuse or RCD can trip if current flows through the housing. Class II encloses all live parts in insulating material — the housing itself provides the protection, without requiring a protective earth.
Aluminium and cast iron are electrically conductive. A luminaire with a metal housing is by definition Class I and requires a PE conductor throughout from luminaire to earthed protection device. SMC is electrically insulating and enables Class II directly through the material — without added double insulation or special measures.
The installation consequence is concrete: with Class II luminaires, a 2-conductor cable (phase + neutral) can be run without a protective earth conductor, provided the other installation rules permit it. This reduces cable cost and simplifies replacement — particularly relevant in older networks without ground-embedded protective earth.
Note
Class II applies to the luminaire, not the entire installation. Which cable requirements apply at a given installation depends on the rest of the electrical layout and the applicable installation rules (IEC 61140, national wiring regulations). Always verify with a qualified electrician.
2. Corrosion — matrix, surface, and galvanic risk
SMC composite (Sheet Moulding Compound) is a thermoset material of cured polyester resin reinforced with glass fibres. The matrix is not metallic and therefore cannot undergo metallic corrosion. This is the difference from aluminium — not that SMC is “corrosion-proof” in all respects: SMC can age through UV exposure, heat, moisture, and chemical attack — but the way it degrades differs fundamentally from how metals corrode.
Aluminium is not particularly corrosion-sensitive in neutral environments, thanks to its oxide layer. But in coastal environments with chlorides and high humidity, corrosion accelerates. In addition, galvanic corrosion occurs when aluminium is in electrical contact with a nobler metal (e.g. stainless steel fixings and screws) in the presence of electrolyte (rainwater with salt). Aluminium is the less noble metal in the couple and corrodes preferentially.
Cast iron rusts when exposed to moisture. Correct surface treatment extends service life but does not eliminate the need for maintenance. This is why cast iron is used almost exclusively in columns and decorative heritage environments — not in modern luminaire housings.
See also moisture, condensation, and failure modes in street luminaires for a review of the most common corrosion mechanisms in the field.
3. RF transparency — what it means for wireless controls
SMC is a dielectric material — it neither conducts electricity nor significantly attenuates radio signals. An antenna inside an SMC housing can transmit and receive without the enclosure absorbing RF energy. Aluminium and cast iron are electrically conductive and act as Faraday cages: they effectively shield the frequencies used in wireless lighting controls (868 MHz, 2.4 GHz, 900 MHz ISM band).
The consequence: a luminaire with a metal housing and a built-in radio module must either have the antenna placed outside the housing — with a cable penetration and sealing — or use an external wireless node on the column. With an SMC housing, the radio node can be placed inside the enclosure without shielding, which eliminates a vulnerable external element and reduces the IPX requirement on the node mounting.
In VALDUR, the SMC housing enables the built-in AirGlow module, where RF transparency is a prerequisite for the function.
4. Thermal conductivity — SMC's limitation and the solution
SMC conducts heat poorly: typical values are around 0.2–0.5 W/mK, compared with ~200 W/mK for aluminium. This means a pure SMC housing without further measures is a poor heat conductor for high-power-density LED components.
The solution is to integrate an internal aluminium heatsink. SMC replaces the housing, not the heat sink. The LED module and driver are mounted against the heatsink, which conducts heat to the surrounding air. The SMC outer housing carries the mechanical and electrical properties (insulation, corrosion freedom, RF transparency) while the heatsink handles thermal management.
The combined construction is technically more complex than a pure aluminium housing, but enables functions that an all-metal housing cannot provide. This is a design choice, not a material deficiency.
5. Recyclability — an honest trade-off
Aluminium is one of the most recyclable materials in technical use. Recycled aluminium requires only ~5 % of the energy needed for primary production. This is a real environmental advantage at end of product life.
SMC thermoset cannot be re-melted and reformed — that is fundamental chemistry. But the recycling options are more nuanced than the simple picture of “difficult to recycle” suggests. Mechanical grinding for filler material is well established. Pyrolysis for glass fibre recovery is under commercial development. Within the EU, several industrial initiatives with substantial investment are underway to solve thermoset recycling at scale — including EuCIA’s composites recycling roadmap and Horizon-funded projects. Energy recovery (incineration) still occurs as a complement where mechanical and chemical methods are not available, but it is no longer the whole picture.
The trade-off that is rarely mentioned
Primary aluminium production is energy-intensive: aluminium manufacturing accounts for ~2 % of global electricity consumption. Since 2026, the EU Carbon Border Adjustment Mechanism (CBAM) covers aluminium imports, which is beginning to affect the price of primary aluminium from countries without carbon pricing.
Recycled aluminium, however, is straightforward environmentally. The difference between primary and secondary aluminium origin on a product declaration is therefore relevant for a correct lifecycle assessment.
SMC recycling is more complex than aluminium — but SMC does not corrode metallically and replaces no new aluminium during its service life. Which total environmental impact is lower depends on the luminaire's actual service life in the installation, not on the material alone. An SMC luminaire lasting 25 years compares differently against an aluminium luminaire replaced after 12 years due to corrosion damage.
Cast iron — columns and heritage
Cast iron is used today almost exclusively in columns and decorative luminaires for historic environments. It gives robustness and an aesthetic expression suited to preservation areas, but the weight is high, the material rusts when surface treatment is damaged, and it is not among the solutions for modern LED luminaire housings with requirements for low weight and high mechanical precision.
VALDUR and material choice
VALDUR uses SMC thermoset composite as the luminaire housing. This enables Class II, a built-in AirGlow module without an external node, and freedom from metallic corrosion in the housing. The internal heatsink is aluminium — SMC replaces the housing, not the heatsink. Full material documentation is available in the technical guide: materials.
Next level of understanding
Corrosion freedom in the housing is one thing. Moisture inside the luminaire is another.
SMC prevents metallic corrosion from outside. But condensation inside the enclosure — caused by pressure and temperature cycles — is one of the most common hidden failure modes, regardless of housing material.
Lifespan & reliability
Moisture, condensation and failure modes in street luminaires
How moisture gets in, what it does there, and what the IP rating actually guarantees.
Frequently asked questions
Does a Class II luminaire need to be earthed?
No. Class II luminaires have protection built into the double or reinforced insulation and do not require a protective earth conductor. If the luminaire is Class II, the PE conductor should in fact not be connected to the luminaire housing — it is the luminaire's construction that provides the protection. Always check the luminaire nameplate and install per the manufacturer's instructions.
Does SMC age faster than aluminium outdoors?
SMC ages differently from aluminium, not necessarily faster. UV and heat can affect surface layers and colour stability over time. A qualified SMC compound with UV stabilisers and correct glass fibre content gives long service life in outdoor environments. Aluminium ages primarily through corrosion when the surface treatment is damaged or in aggressive environments. Which holds up better long-term depends on the environment and product quality, not on the material in general.
Which material should be chosen for coastal environments with high salt content?
At high chloride exposure, the absence of metallic corrosion in the housing is the most important property. SMC composite has no metallic matrix and is not subject to the pitting and chloride-induced corrosion that can rapidly damage aluminium housings near saltwater. Metal fixings, screws and cable entries still need to be selected with galvanic compatibility in mind. For chemically aggressive environments such as wastewater plants and chemical industry, specific chemical resistance requirements should be verified per chemical agent.
Summary
The luminaire housing material affects installation, operation, and service life in five concrete ways: protection class, corrosion behaviour, RF transparency, thermal conductivity, and recyclability. No material wins on all points.
SMC wins on Class II insulation, absence of metallic corrosion, and RF transparency for built-in radio. Aluminium wins on thermal conductivity and recyclability. In modern SMC luminaires the materials are combined: SMC housing for the exposed environment and electrical protection, aluminium heatsink internally for thermal management.
Cast iron is relevant for columns and heritage environments, not for modern luminaire housings. The material choice should be based on installation environment, wireless control requirements, protection class, and expected service life relative to maintenance interval — not on a generic “better” claim about any of the materials.
Sources
- IEC 60598-1:2020. Luminaires — Part 1: General Requirements and Tests. International Electrotechnical Commission. (Class I / Class II classification.)
- Owens Corning. SMC/BMC Design Guide. (Typical material data for SMC composite.)
- European Composites Industry Association (EuCIA). Composites Recycling and End-of-Life Guidance.
- European Aluminium Association. Environmental Profile Report — Aluminium Recycling Guidance.
- Balanis, C. A. (2016). Antenna Theory: Analysis and Design (4th ed.). Wiley. (RF transparency and shielding.)
- European Commission. Regulation (EU) 2023/956. Carbon Border Adjustment Mechanism (CBAM).