HVAC Ducting Materials: Galvanized Steel vs Aluminum vs Flexible Duct
The material you choose for HVAC ducting determines system lifespan, air leakage, fire safety, noise, and installed cost. This guide compares the four main ducting materials used in commercial and residential HVAC — galvanized steel, aluminum, stainless steel, and flexible duct — with pressure ratings, cost data, and application-specific recommendations. Whether you are specifying ductwork for a new build or sourcing ducting fittings for a retrofit, this comparison will help you make the right material choice.
In This Guide
1. HVAC Ducting Materials Overview
HVAC ductwork carries conditioned air from the air handling unit (AHU) to every room in a building and returns spent air back for reconditioning. The ducting material must withstand decades of thermal cycling, resist corrosion from condensation, maintain air-tight seals under positive and negative pressure, and meet fire safety codes. Four materials dominate the market, each with distinct advantages:
- Galvanized steel — the industry standard for 90%+ of commercial and residential ductwork. Best balance of strength, cost, and fire resistance.
- Aluminum — lightweight alternative for marine, food processing, and weight-sensitive applications. Higher cost but better corrosion resistance.
- Stainless steel — premium choice for extreme environments (chemical exposure, high temperature, coastal). Highest cost but longest lifespan.
- Flexible duct — spiral wire core with insulated jacket. Used only for short final connections between rigid duct and plenum boxes or diffusers.
The choice is rarely one material for the entire system. Most commercial projects use galvanized steel for trunk ducts and branch runs, with flexible duct for the final 1-2 metre connection to ceiling diffusers. Understanding when to use each material — and when to mix them — is the key to specifying a cost-effective, long-lasting duct system.
2. Galvanized Steel Duct
Galvanized steel is carbon steel coated with a zinc layer through hot-dip galvanizing. The zinc coating (G60 = 0.84 oz/sq ft or G90 = 1.25 oz/sq ft) provides sacrificial corrosion protection — the zinc corrodes preferentially, protecting the underlying steel. This material accounts for over 90% of commercial HVAC ductwork installed worldwide.
Key Properties
| Property | Value |
|---|---|
| Density | 7,850 kg/m3 |
| Typical gauge | 26-20 gauge (0.55-1.00 mm) |
| Zinc coating | G60 (standard), G90 (humid environments) |
| Melting point | 1,500°C (steel) / 420°C (zinc coating) |
| Pressure rating | Up to 10" w.g. (spiral), 6" w.g. (rectangular) |
| Fire rating | Non-combustible (ASTM E136) |
| Lifespan | 30-50 years (indoor), 10-20 years (corrosive) |
| Cost index | 1.0x (baseline) |
Advantages
- Structural rigidity: Resists collapse under negative pressure and supports long spans without excessive hangers.
- Smooth interior: Low friction loss (0.08-0.12 in. w.g. per 100 ft for spiral duct), maximizing fan efficiency.
- Fire safety: Non-combustible per ASTM E136. Meets NFPA 90A requirements without additional treatment.
- Sealability: Spiral lock seams and flanged connections achieve SMACNA Class A air-tightness (leakage < 4 CFM per 100 sq ft of duct surface at 1" w.g.).
- Cost: Lowest material cost per linear metre of any rigid duct material. Widely available from multiple suppliers.
- Recyclability: 100% recyclable. Scrap value offsets demolition costs.
Limitations
- Heavy — 3.5x the weight of aluminum at equal gauge, increasing structural support requirements.
- Zinc coating degrades in acidic or high-chloride environments (pool rooms, chemical plants, coastal buildings).
- Condensation on uninsulated exterior surfaces in cooling applications — requires insulation.
3. Aluminum Duct
Aluminum duct is manufactured from 3003-H14 or 5052-H32 aluminum alloy. It weighs approximately 35% of equivalent galvanized steel duct and offers superior corrosion resistance in marine and humid environments. Aluminum is the standard choice for food processing plants, pharmaceutical facilities, and offshore applications where weight and corrosion are primary concerns.
Key Properties
| Property | Value |
|---|---|
| Density | 2,700 kg/m3 (35% of steel) |
| Typical gauge | 24-20 gauge (0.60-1.00 mm) |
| Alloy | 3003-H14 (general), 5052-H32 (marine) |
| Melting point | 660°C |
| Pressure rating | Up to 6" w.g. (spiral) |
| Fire rating | Non-combustible (ASTM E136) |
| Lifespan | 40-60 years (forms protective oxide layer) |
| Cost index | 1.4-1.6x galvanized steel |
Advantages
- Lightweight: 65% lighter than galvanized steel, reducing structural load and installation labor. One worker can handle sections that require two for steel.
- Corrosion resistance: Forms a natural aluminum oxide layer that resists corrosion without coatings. Ideal for coastal, marine, and high-humidity applications.
- No zinc off-gassing: Safe for food processing, pharmaceutical, and cleanroom environments where zinc contamination is unacceptable.
- Thermal conductivity: Higher than steel (205 vs. 50 W/mK), which can be an advantage or disadvantage depending on the application.
Limitations
- 40-60% higher material cost than galvanized steel.
- Lower structural rigidity at equal gauge — may require heavier gauge or closer hanger spacing.
- Lower melting point (660°C vs. 1,500°C) — though still classified as non-combustible, it softens at temperatures that steel resists.
- Cannot be welded with standard steel welding equipment — requires TIG or MIG aluminum welding.
- Galvanic corrosion risk when in contact with steel or copper without isolation.
4. Stainless Steel Duct
Stainless steel duct is the premium choice for environments that destroy galvanized steel and aluminum. The two grades used in HVAC are 304 (18-8 chromium-nickel, general corrosion resistance) and 316 (added molybdenum for chloride resistance). Stainless steel is specified for laboratory fume exhaust, chemical plant ventilation, pool natatorium dehumidification, and coastal high-rise buildings.
When to Specify Stainless Steel
- Laboratory fume exhaust: Acid fumes (HCl, H2SO4, HNO3) destroy galvanized steel within 1-3 years. 316 stainless resists most acid concentrations encountered in lab exhaust.
- Pool/natatorium: Chloramine-laden air corrodes galvanized steel aggressively. 316L stainless is the standard for pool HVAC ductwork.
- Coastal buildings: Salt spray corrodes galvanized steel within 5-10 years. 316 stainless provides 50+ year lifespan in coastal environments.
- Kitchen exhaust: Grease-laden exhaust at high temperatures requires stainless steel per NFPA 96 (Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations). Typically 18 gauge (1.2 mm) minimum.
Cost reality: Stainless steel duct costs 3-5x galvanized steel. For a typical 500-metre commercial duct system, the material cost difference can be $15,000-40,000. This premium is justified only when the environment would destroy less expensive materials within their expected service life — replacing corroded galvanized duct in a pool natatorium costs far more than the stainless premium.
5. Flexible Duct
Flexible duct consists of a spiral wire core wrapped with a vapor barrier, insulation (typically R-4.2 or R-6 fiberglass), and an outer jacket. It is designed for one purpose: short final connections between rigid ductwork and ceiling diffusers or plenum boxes. Flexible duct should never be used as primary ductwork.
Proper Use of Flexible Duct
- Maximum length: 2 metres (6 feet). SMACNA and most building codes limit flex duct runs to prevent excessive friction loss and air leakage.
- Fully extended: Flex duct must be pulled taut during installation. A 2-metre section compressed to 1.5 metres has 50% higher friction loss due to the accordion effect on the inner liner.
- Supported: Hanger straps every 1-1.5 metres to prevent sagging. Sags create low points that trap condensation and restrict airflow.
- Sealed: Connect with zip ties and mastic at both ends. Flex-to-rigid connections are the most common air leak point in residential systems.
Why Flexible Duct Is Not a Substitute for Rigid
| Metric | Flexible Duct | Spiral Steel Duct |
|---|---|---|
| Friction loss (per 100 ft) | 0.25-0.35 in. w.g. | 0.08 in. w.g. |
| Noise (NC rating at 800 fpm) | NC 35-45 | NC 15-25 |
| Air leakage class | SMACNA Class C (24 CFM/100 sq ft) | SMACNA Class A (4 CFM/100 sq ft) |
| Lifespan | 10-15 years | 30-50 years |
| Fire rating | Class 1 (UL 181) | Non-combustible |
The friction loss difference is critical for energy consumption. A system with excessive flex duct runs requires a larger fan (or higher fan speed) to overcome the added resistance, consuming 15-30% more energy annually. For a commercial system running 3,000+ hours per year, this translates to thousands of dollars in wasted electricity over the duct's lifetime.
6. Material Comparison Table
| Property | Galvanized Steel | Aluminum | Stainless Steel | Flexible Duct |
|---|---|---|---|---|
| Relative cost | 1.0x | 1.4-1.6x | 3-5x | 0.6-0.8x |
| Weight (relative) | 1.0x | 0.35x | 1.0x | 0.2x |
| Max pressure | 10" w.g. | 6" w.g. | 10" w.g. | 2" w.g. |
| Friction loss | Low | Low | Low | High (3-5x) |
| Fire rating | Non-combustible | Non-combustible | Non-combustible | UL 181 Class 1 |
| Corrosion resistance | Good (indoor) | Excellent | Superior | Good |
| Lifespan | 30-50 yrs | 40-60 yrs | 50+ yrs | 10-15 yrs |
| Best application | General commercial/residential | Marine, food, lightweight | Chemical, pool, coastal | Short final connections |
7. Ducting Fittings by Material
Every duct system requires ducting fittings — elbows, reducers, tees, take-offs, transitions, and connectors — to route air around obstacles, change duct sizes, and branch to multiple zones. The fittings must match the duct material to prevent galvanic corrosion and ensure compatible joining methods. For a complete guide to fitting types and selection, see our HVAC Duct Fittings Types: Contractor's Guide.
Galvanized Steel Fittings
The most extensive range of standard fittings. Our duct fittings catalog includes 206 SKUs of galvanized steel fittings covering elbows (45° and 90°, pressed and segmented), reducers (concentric and eccentric), tees, wyes, take-offs (conical and saddle), transitions (round-to-rectangular), and spiral duct connectors. Standard gauge matches the connected duct (26-22 gauge). All fittings are compatible with standard joining methods: slip joint with screws and mastic, flanged (TDF/TDC), or welded.
Aluminum and Stainless Fittings
Available as custom fabrication. Because aluminum and stainless steel duct installations are less common, standard inventories are smaller. We fabricate aluminum and stainless fittings to order with 30-45 day lead time. Minimum order quantities are typically 50-100 pieces per fitting type. For large projects, provide a complete fitting schedule and we will quote the full package.
8. Which Material for Which Application?
| Application | Recommended Material | Reason |
|---|---|---|
| Commercial office (standard) | Galvanized steel (G60) | Lowest cost, excellent performance, 30+ year life |
| Residential | Galvanized steel (G60) + flex final | Steel for trunk/branch, flex for last 2 m to diffuser |
| Hospital / cleanroom | Galvanized steel (G90) | Sealed system, higher coating for humidity resistance |
| Data center | Galvanized steel (G60/G90) | Standard for data center cooling systems |
| Restaurant / commercial kitchen | Stainless steel (304 or 316) | NFPA 96 requirement for grease-laden exhaust |
| Indoor swimming pool | Stainless steel (316L) | Chloramine resistance, 50+ year lifespan |
| Coastal building (within 1 km of ocean) | Stainless steel (316) or aluminum (5052) | Salt spray destroys galvanized in 5-10 years |
| Food processing | Aluminum (3003) or stainless (304) | No zinc contamination risk, easy cleaning |
| Marine / offshore | Aluminum (5052-H32) | Lightweight, excellent marine corrosion resistance |
| Laboratory fume exhaust | Stainless steel (316) or FRP | Acid resistance for chemical fumes |
9. Sourcing Ducting Materials from Airwise
Our Ducts & Metal Fittings catalog includes 206 SKUs of galvanized steel ducting fittings and spiral duct sections. All products are manufactured from G60 galvanized steel at our Dongyang factory, with G90 coating available on request. Custom aluminum and stainless steel fabrication is available for specialty projects.
For complete systems, source your ducting fittings, plenum boxes, diffusers and grilles, and dampers from one manufacturer. Single-source procurement eliminates dimensional mismatches between components from different suppliers and simplifies your import logistics.