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The housing is the heat sink. Get the material wrong and the LED fails early regardless of the chip brand, the CRI, or the warranty length printed on the box.
Ан aluminium downlight body pulls heat away from the LED junction and dumps it into the ceiling air. A plastic body traps it. The difference in thermal conductivity between the two materials is several hundred to one.
This page covers what that gap means for lifespan, where an aluminum alloy downlight pays for itself, and when plastic is still an honest answer for dry, low-power indoor work.
If the mount family is still open, route it first:
→ LED Downlight Installation Methods
Key Takeaways
– Aluminium conducts heat roughly 700× faster than unfilled PC/ABS plastic (96–237 vs 0.2–0.25 W/(m·K) at range midpoints). That gap decides whether the LED runs below 65°C or above 85°C at the junction.
– Every 10°C reduction in junction temperature doubles LED lifespan. An aluminium housing typically keeps the junction 20°C or more cooler than a plastic equivalent.
– Anodized aluminium resists salt spray corrosion and holds structural integrity at fire-exposure temperatures. Plastic yellows, embrittles, and softens under the same conditions.
– IP-rated aluminium housings pair the material’s corrosion resistance with a seal tested to IEC 60529. The housing and the gasket work together; neither alone keeps moisture out.
– Plastic still works for low-power, dry-indoor, budget-driven jobs. It is not the wrong material everywhere — it is the wrong material where heat and moisture shorten the payback.
Before you use this page
Have these on hand:
- Target ceiling environment: dry indoor, damp (bathroom/kitchen), or wet/corrosive (outdoor soffit, pool hall, coastal)
- Shortlisted fixture datasheets showing housing material, IP rating, and rated ambient temperature (Ta)
- Project specification for fire performance if the ceiling is a rated assembly
This page does not cover downlight pricing tiers or bulk order cost comparison. That belongs on the price article when live.
1. The heat path: aluminium opens it, plastic blocks it
LEDs convert roughly 60–70% of input power to heat, not light. That heat has to leave the junction or the chip degrades.
Downlight heat dissipation follows a simple path: LED junction → metal-core PCB → housing body → ceiling air. The housing is the last and largest thermal interface. If it resists heat flow, every upstream component runs hotter.
Aluminium alloy — typically die-cast ADC12 or equivalent — has a thermal conductivity of 96–237 W/(m·K). The plastic used in budget downlight housings — typically PC/ABS — conducts at roughly 0.2–0.25 W/(m·K).
The gap is not marginal. It is two orders of magnitude.
| Материал | Thermal conductivity (W/(m·K)) | Heat behaviour |
|---|---|---|
| Die-cast aluminium (ADC12) | 96–130 | Pulls heat from PCB; radiates to air |
| Extruded aluminium | 200–237 | Even faster path; used in higher-wattage fixtures |
| PC/ABS plastic (unfilled) | 0.2–0.25 | Insulates the junction; heat builds inside |
| Stamped steel | 40–50 | Conducts better than plastic but heavier; corrosion-prone without coating |
A plastic-housed recessed aluminium fixture is a misnomer some catalogues use — the trim may be aluminium but the body behind the ceiling is plastic. Read the datasheet, not the product name.
A die-cast aluminium housing acts as a continuous heat sink from PCB to ambient. Fins, when present, increase surface area and improve convective cooling. The result is a lower steady-state junction temperature for the same wattage and ambient conditions.
🎯 What does the housing material actually decide?
Whether heat leaves the LED or sits at the junction — aluminium conducts it out several hundred times faster than unfilled plastic.
2. Junction temperature and LED life: every 10°C counts
The relationship between temperature and LED lifespan is well-established: every 10°C reduction in junction temperature roughly doubles the expected operating life.
A die-cast aluminium housing typically keeps the LED junction below 65°C under normal commercial loads. A plastic-housed equivalent of the same wattage routinely exceeds 85°C at the junction.
Under the 10°C/doubling rule, a 20°C reduction theoretically enables up to 4× the LED life under controlled lab conditions — before accounting for driver lifespan, solder fatigue, or phosphor degradation. Real-world gains are typically smaller; the table above shows the practical spread across housing types.
| Housing material | Typical junction temp | Relative LED life |
|---|---|---|
| Die-cast aluminium | <65°C | Baseline (50,000+ hrs L70) |
| Stamped steel | 70–80°C | Reduced (~35,000–40,000 hrs) |
| Plastic (PC/ABS) | >85°C | Severely reduced (~25,000–30,000 hrs) |
A aluminum alloy downlight rated at 50,000 hours L70 in a 25°C ambient will not deliver that life in a sealed insulated ceiling pocket. But the aluminium body gives the thermal headroom to handle real-world conditions that would push a plastic housing past its limit within the first year.
Lumen maintenance also suffers under heat. Plastic-housed LEDs show faster light decay — the fixture still works but delivers less light per watt after 12–18 months. The energy saving that justified the cheaper housing disappears as the output fades.
Driver life follows the same curve. Electrolytic capacitors inside LED drivers are rated for a specific maximum case temperature, often 105°C. Every 10°C below that rating roughly doubles capacitor life. A driver sitting inside a hot plastic housing ages faster than the same driver in an aluminium body, even if the LED itself survives.
🎯 How much does housing material affect LED lifespan?
Every 10°C cooler at the junction roughly doubles LED life. Aluminium typically keeps the junction 20°C or more below plastic — the body text table shows the practical spread.
3. Corrosion, IP rating, and environment matching
Heat is one failure path. Moisture and corrosion are the other.
Ан ip-rated aluminium downlight pairs a corrosion-resistant housing with a gasket seal tested to a specific IP class under IEC 60529. The housing resists the environment; the seal blocks ingress. Both are required for wet or corrosive zones.
Anodized aluminium — the most common surface treatment on commercial downlight housings — passes extended salt spray corrosion testing per manufacturer data without pitting or structural degradation. The anodized layer is aluminium oxide, chemically stable and electrically insulating. It does not peel like paint or yellow like polymer coatings.
Plastic housings do not corrode in the electrochemical sense. But they degrade under UV, embrittle with thermal cycling, and soften at temperatures that aluminium ignores. A plastic downlight in an outdoor soffit that hits 60°C in direct sun will deform over repeated cycles. The same fitting in aluminium holds its shape and seal.
| Environment | Recommended housing | Почему |
|---|---|---|
| Dry indoor (office, retail) | Aluminium or plastic | Plastic acceptable for low-power, budget work |
| Damp indoor (bathroom, kitchen) | Aluminium + IP44 min | Condensation cycles; plastic seal degrades |
| Wet indoor (shower zone 1, pool hall) | Aluminium + IP65 | Jet water + humidity; corrosion risk |
| Coastal / outdoor soffit | Anodized aluminium + IP65 | Salt spray, UV, temperature swings |
| High-temperature (plant room, canopy) | Die-cast aluminium only | Plastic softens; steel corrodes without coating |
Specify the housing material and the IP rating as a pair. An IP65 rating on a plastic body means the gasket passed a lab test — it does not mean the housing will survive five years of thermal cycling in a coastal soffit.
🎯 Does IP rating alone guarantee wet-area survival?
No — IP rating tests the seal at the time of manufacture. The housing material determines whether that seal holds its integrity after years of heat, cold, and moisture.
4. Structural and fire safety: what holds up when it counts
A downlight housing is also a fire barrier component when the ceiling is part of a rated assembly.
Die-cast aluminium does not burn, drip, or ignite. It maintains structural integrity at temperatures far above the point where plastics soften or melt, making it a predictable substrate in a fire-rated ceiling assembly. In a fire condition, the housing remains in place long enough for the intumescent material inside a fire-rated fitting to expand and seal the ceiling penetration.
Plastic housings soften and deform well below fire-exposure temperatures. A plastic-bodied downlight in a fire-rated ceiling assembly relies entirely on a separate steel can or intumescent collar to maintain compartmentation. The housing itself contributes nothing to fire resistance.
Structural integrity also matters during installation and maintenance. Die-cast aluminium bodies withstand static loads well above what a ceiling-mounted fixture encounters during handling or installation. Plastic housings can crack if dropped or over-torqued during fitting. A cracked housing compromises the IP seal and the electrical enclosure path in a single event.
Aluminium bodies also enable tighter manufacturing tolerances than plastic mouldings. That precision matters for the fit between the housing and the trim, the optic alignment, and the gasket compression. A housing that does not seat the trim squarely leaks light at the rim and moisture at the seal.
🎯 Does the housing material affect fire safety compliance?
Yes — aluminium holds structural integrity at temperatures where plastic softens or melts, and does not drip. Plastic contributes nothing to fire resistance and relies on a separate rated enclosure.
5. When plastic still works
Plastic is not the wrong answer everywhere.
For a 5W LED downlight in a dry indoor bedroom or corridor, running eight hours a day in a climate-controlled space, a plastic housing will last years without visible failure. The heat load is low. The environment is kind. The cost saving — roughly 20% less than an equivalent aluminium body — is real and defensible.
Plastic also wins on weight. A plastic-housed downlight weighs 15–60% less than the same fitting in aluminium. That weight difference matters for large-grid suspended ceilings where cumulative fixture load pushes against tee-bar limits.
The honest guidance:
- Low power (≤10W), dry indoor, budget-driven: plastic is acceptable. Check the driver and warranty, not just the housing.
- Medium power (10–25W), commercial ambient, damp zones: aluminium is the working answer. The thermal headroom and corrosion resistance justify the cost.
- High power (>25W), wet zones, coastal, fire-rated ceilings: aluminium is not optional. Plastic fails the thermal, corrosion, and fire gates simultaneously.
Match the housing to the worst condition the ceiling will see, not the average.
Часто задаваемые вопросы
Notes on evidence
Thermal conductivity values (aluminium 96–237 W/(m·K); unfilled PC/ABS plastic ~0.2–0.25 W/(m·K)) are published material properties from aluminium alloy datasheets (ADC12/A380) and polymer technical references (SABIC CYCOLOY, INEOS Styrolution). Using range midpoints (~167 vs ~0.23) yields a ratio of roughly 700×; the “several hundred to one” figure in the body is a conservative planning comparison, not a precision engineering value.
Junction temperature estimates (aluminium <65°C; plastic >85°C) and the 10°C/lifespan-doubling rule follow LED manufacturer application notes and are consistent with LM-80 test data methodology. Actual junction temperature depends on drive current, ambient, and fixture design — confirm against the product LM-80 report.
Salt spray corrosion resistance data follows manufacturer-published test data for anodized die-cast aluminium downlight housings. Fire-rated downlight assemblies are certified at the product level (housing + intumescent components together), not at the material level alone. Fire resistance claims apply to the housing material only; a fire-rated downlight assembly requires certification of the complete fitting including intumescent components.
IP ratings are verified under IEC 60529. The housing material and the gasket seal are independent factors in long-term wet-area durability.
Always confirm housing material, IP rating, fire certification, and rated ambient (Ta) against the product datasheet before ordering.
Что делать дальше?
Route the mount family first if recessed versus surface is still open:
→ LED Downlight Installation Methods
For wattage and chip quality factors that affect what you pay, see the downlight pricing guide when it is live.
For a full commercial downlight selection framework across sizes, prices, and brands, refer to the commercial downlight buying guide when published.
