Insights · Materials
A practical, number-first material selection guide for HVAC ductwork. Coating weights, gauge equivalents, corrosion resistance, cost per kilogram — and when each metal is the right choice rather than the default choice.
Galvanized steel is cold-rolled carbon steel with a zinc coating bonded to both surfaces. The grade number refers to the total zinc weight on both sides combined, in ounces per square foot:
Do not accept "galvanized" without a grade number in the mill certificate. The difference between G60 and G90 is about 5% in raw material cost and roughly 3x in service life under humid conditions.
304 stainless is the general workhorse: 18% chromium, 8% nickel, no molybdenum. It handles kitchen grease, most lab chemicals and normal condensation well. 316 stainless adds 2–3% molybdenum, which dramatically improves resistance to chlorides — sea air, pool chlorine vapor, de-icing salt spray. 316 is roughly 25–30% more expensive than 304. Do not default to 316 — specify it only where chloride exposure genuinely warrants it, because the price delta adds up fast on a 10,000 m² project.
| Gauge | Thickness (mm) | Thickness (inch) | Typical use |
|---|---|---|---|
| 26 ga | 0.55 mm | 0.0217" | Residential, small commercial supply |
| 24 ga | 0.70 mm | 0.0276" | Commercial supply, low pressure |
| 22 ga | 0.85 mm | 0.0336" | Medium-pressure supply, 2" w.g. |
| 20 ga | 1.00 mm | 0.0396" | High-pressure, large rectangular duct |
| 18 ga | 1.31 mm | 0.0516" | Industrial, kitchen exhaust |
| 16 ga | 1.61 mm | 0.0635" | Heavy industrial, dust collection |
SMACNA Table 1-3 maps gauge to duct size and pressure class. Always confirm against the project specification.
Market prices move constantly, so treat these as ratios, not absolutes:
Specifying stainless where G90 would do adds 200% to the raw material bill. The opposite mistake — specifying G90 for kitchen exhaust — costs 10x more in replacement duct five years later.
On an SBAL-V auto duct line, material choice affects throughput:
Any galvanized or aluminised coating takes damage at cut edges, punched holes and TDF flange bends. For most interior supply, the sacrificial zinc at the edge self-protects and does not need touch-up. For exterior and high-humidity applications, brush zinc-rich paint on all cut edges before installation. Stainless does not need edge touch-up but welded joints must be passivated to restore corrosion resistance.
Aluminium is routinely overlooked. It weighs roughly one-third of steel, which matters on high-rise branch duct where dead load is limited, and on long horizontal runs where hanger spacing is a cost driver. It does not rust. The downside: it is more expensive per kilogram, softer at the TDF profile, and sensitive to galvanic corrosion if bolted directly to carbon steel hangers without isolation.
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G60 has 0.60 oz/sq ft of zinc total (both sides), G90 has 0.90 oz/sq ft. G90 is the SMACNA default for HVAC duct because it resists corrosion better in humid conditions. Metric equivalents are Z180 and Z275 grams per square meter.
Kitchen exhaust, laboratory fume exhaust, swimming pool supply and exhaust, coastal installations, and any application where condensation on the duct interior is expected long-term. 304 is the default; 316 for severe chloride exposure.
26 ga = 0.55 mm, 24 ga = 0.70 mm, 22 ga = 0.85 mm, 20 ga = 1.00 mm, 18 ga = 1.31 mm, 16 ga = 1.61 mm. SMACNA Table 1-3 specifies gauge by duct size and pressure class.
Yes. SBAL-V auto duct lines handle galvanized, stainless (304 and 316), aluminium and aluminised steel up to 1.5 mm thickness. Stainless requires slightly slower line speed and harder tooling.
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