Galvanized Ductwork 101: Gauges, G-Ratings, and What We Use at K & E

When you’re choosing duct, two specs matter most: steel gauge (thickness) and galvanized coating weight (zinc layer, e.g., G60 or G90). Thicker steel resists dents and spans farther; more zinc slows rust at seams and scratches. (Intro to coating weights and ASTM overview: GalvInfoNote, U. S. Steel quick conversions.)

Gauge (Thickness): What it Means

  • 30 ga (thin, light): common on small-diameter round pipe for low-pressure runs.
    Pros: light, inexpensive. Cons: dents easily; not ideal for long spans or rough handling.

  • 28 ga: step up in stiffness for round and small fittings.
    Pros: better durability. Cons: slightly higher cost/weight.

  • 26 ga: standard for many residential trunks and larger fittings.
    Pros: sturdier panels, cleaner seams. Cons: heavier, pricier than 28–30.

  • 24 ga: for larger trunks, long spans, light commercial, or tougher handling.
    Pros: strong, holds shape, reduces oil-canning. Cons: more cost/weight; may need heavier hangers.

Rule of thumb: As duct gets larger, longer, or exposed to abuse, go thicker (lower gauge number). (SMACNA context: see the HVAC Duct Construction Standards.)

G-Ratings (Zinc Coating): Why G60 vs. G90 Matters

The G-number is total zinc weight on both sides (ASTM A653). More zinc = better corrosion resistance.

  • G60: balanced protection for interior duct in conditioned spaces.

  • G90: extra margin for damp areas, basements, crawlspaces, garages, rooftop curbs, or where condensation risk is higher.

Important: G60 vs. G90 does not change steel thickness—only the protective zinc layer. (Definitions and conversions: GalvInfoNote, G60 vs G90 explainer.)

K & E Shop Standards

  • We stock and fabricate 26 ga and 24 ga as our core thicknesses.

  • For both 26 ga and 24 ga we use G60 and G90 zinc coatings depending on environment.

  • Heavier gauges (22 ga, 20 ga) are used as the job and specifications require (size, pressure class, span, and reinforcement).

  • For sealing, we follow UL 181 listed mastics/tapes for HVAC duct systems. (UL 181 context.)

Quick Picks (Save & Share)

  • Small round runs (dry interior): 28–30 ga, G60

  • Typical residential trunks/returns: 26 ga, G60

  • Basement/crawl/garage or any condensation risk: 26 ga, G90 (or 24 ga if large spans)

  • Large trunks/long spans/light commercial: 24 ga, G60/G90 per environment

  • Outdoor/rooftop/curb: G90 minimum; consider 24 ga or heavier

Advantages & Tradeoffs at a Glance

  • Thicker gauge (24 vs. 26): more rigidity, fewer oil-cans, longer spans → higher cost/weight.

  • Higher zinc (G90 vs. G60): longer rust resistance → slightly higher material cost.

  • Smart combo: match gauge to size/span and G-rating to moisture risk.

Helpful Definitions & Deep Dives

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