HVAC Duct Materials Comparison: Galvanized Steel, Aluminum, and Fiberglass Board
Duct material selection directly affects system efficiency, installation labor, acoustic performance, fire resistance rating, and long-term maintenance cost in both residential and commercial HVAC applications. This page examines the three dominant materials used in North American duct fabrication — galvanized steel, aluminum, and fiberglass duct board — across the criteria that determine suitability for a given installation. The comparison draws on classifications established by SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) and requirements codified in ASHRAE Standard 90.1 and the International Mechanical Code (IMC). Understanding material boundaries is prerequisite knowledge before consulting resources like Air Duct Types Overview or Duct System Codes and Standards.
Definition and scope
The three materials addressed here represent the primary rigid and semi-rigid duct construction categories recognized in SMACNA's HVAC Duct Construction Standards — Metal and Flexible and the IMC Section 603. Each material carries distinct physical properties, pressure class ratings, fire and smoke performance characteristics, and application constraints.
Galvanized steel is cold-rolled carbon steel coated with zinc by hot-dip process, typically 26-gauge to 16-gauge for HVAC duct work. The zinc coating provides corrosion resistance without surface treatments.
Aluminum duct uses alloy sheet (most commonly 3003-H14 or 5052-H32) in gauges ranging from 0.016 inches to 0.040 inches. Aluminum is lighter per unit area than galvanized steel and inherently corrosion-resistant without a supplemental coating.
Fiberglass duct board (also called duct board or fiber glass rigid duct) consists of inorganic glass fibers bonded with a thermosetting resin, faced on the airstream side with a foil-scrim-kraft (FSK) or black glass vapor barrier, and on the exterior with FSK laminate. NAIMA (North American Insulation Manufacturers Association) and UL publish the fabrication and performance standards governing duct board products. Duct board panels are typically 1 inch or 1.5 inches thick with an R-value of approximately R-4.2 per inch (NAIMA Duct Insulation Guide).
These three materials do not exhaust all duct options — flexible duct, stainless steel, and PVC duct exist for specialty applications — but they account for the dominant share of installed duct systems in US construction. Permitting implications and inspection criteria vary by material; a broader treatment appears in HVAC Duct Permits and Inspections.
How it works
Galvanized steel duct — mechanism
Galvanized steel duct is fabricated by cutting flat sheet stock and forming it into rectangular, round, or oval sections using brake presses, Pittsburgh seam locks, and snap-lock connectors. Seams and joints are sealed with mastic or UL 181-listed tapes per IMC Section 603.9. The rigidity of steel allows galvanized systems to handle all four SMACNA pressure classes (from ½ inch w.g. to 10 inches w.g. static pressure), making galvanized steel the default choice for high-pressure commercial systems. Sheet metal duct construction details are covered in depth in the Sheet Metal Ductwork Guide.
Thermal performance requires external insulation — galvanized steel alone provides no meaningful R-value. ASHRAE 90.1-2022 Table 6.8.2 specifies minimum duct insulation levels by system type and location, ranging from R-6 for supply ducts in unconditioned spaces in most climate zones to R-8 in the most demanding zones (ASHRAE 90.1-2022).
Aluminum duct — mechanism
Aluminum duct is formed using the same sheet metal processes as galvanized steel but requires different tooling settings due to aluminum's lower yield strength and greater flexibility at thin gauges. Aluminum is the predominant material for flexible duct inner cores and for kitchen exhaust ductwork where steel's zinc coating could degrade from grease exposure. Like galvanized steel, aluminum duct requires field-applied or factory-jacketed insulation to meet code thermal requirements.
Aluminum duct is approximately one-third the weight of equivalent galvanized steel duct, which reduces hanging and support load calculations. SMACNA specifies support spacing requirements by gauge and duct width in its Duct Construction Standards — the maximum hanger spacing for rectangular aluminum duct at low-pressure class is 8 feet, matching galvanized steel equivalents at similar gauge.
Fiberglass duct board — mechanism
Duct board is fabricated by scoring and folding flat panels into rectangular duct sections on-site or in shop. The panel score-fold technique eliminates the need for seam crimping equipment. The airstream-side FSK facing serves as the vapor barrier and provides a smooth interior surface with an absolute roughness factor of approximately 0.0003 feet, comparable to flexible duct but rougher than smooth sheet metal (absolute roughness ≈ 0.0001 feet for galvanized steel), which affects static pressure loss calculations addressed in Duct Static Pressure Explained.
Duct board systems are limited to SMACNA pressure classes of 1 inch w.g. positive and ½ inch w.g. negative, making them unsuitable for high-velocity or variable air volume applications. However, the integral insulation — typically R-6.0 at 1.5-inch thickness — satisfies ASHRAE 90.1 requirements in most climate zones without additional field wrapping.
Common scenarios
Material selection aligns with specific installation contexts:
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High-pressure commercial systems — Galvanized steel is the standard material for systems operating above 2 inches w.g. static pressure, including variable air volume main trunks, rooftop unit supply plenums, and kitchen exhaust systems requiring UL 2221-listed construction. The HVAC Duct Fire Safety Requirements page details the Class 1 air duct rating requirements governing these installations.
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Residential new construction supply and return mains — Galvanized steel rectangular or round duct is typical for trunk lines; duct board is widely used for branch runouts and plenum boxes in low-pressure residential systems where its thermal performance eliminates a separate insulation labor step.
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Coastal and high-humidity environments — Aluminum is preferred over galvanized steel in installations subject to salt air or sustained condensation because zinc coatings degrade in chloride-rich environments, leading to premature corrosion and potential duct system IAQ impact from particulate release.
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Renovation and retrofit work — Duct board's tool-light fabrication process makes it practical for attic or crawl space retrofits where sheet metal brake equipment cannot be easily deployed. Galvanized round duct with snap-lock seams is also common for retrofit branch duct runs.
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Kitchen exhaust and grease duct — IMC Section 506 requires grease duct to be constructed of minimum 16-gauge black carbon steel or 18-gauge stainless steel. Neither aluminum nor duct board is permitted for grease duct applications. This is a hard code boundary, not a best-practice preference.
Decision boundaries
The following structured comparison identifies the primary selection criteria and how each material ranks or qualifies:
| Criterion | Galvanized Steel | Aluminum | Fiberglass Duct Board |
|---|---|---|---|
| Maximum pressure class (SMACNA) | 10 in. w.g. | 2 in. w.g. (typical) | 1 in. w.g. positive |
| Integral thermal insulation | None — requires field wrap | None — requires field wrap | R-4.2/inch (R-6.0 at 1.5 in.) |
| Interior surface roughness | ~0.0001 ft | ~0.0002 ft | ~0.0003 ft |
| Weight (relative) | Heaviest | ~65% lighter than steel | Lightest rigid panel |
| Corrosion resistance | Moderate (zinc coating) | High (inherent) | High (glass fiber/FSK) |
| Grease duct permitted | Yes (16-gauge or heavier) | No | No |
| Typical fire/smoke rating | UL 181 Class 1 | UL 181 Class 1 | UL 181 Class 1 (board) |
| Mold risk | Low with proper insulation | Low | Moderate if FSK damaged |
| Field fabrication complexity | Moderate–High (brake/seam tools) | Moderate | Low (score-fold) |
Three hard decision rules govern material selection independent of installer preference:
- Grease and kitchen exhaust duct must use steel at gauges specified in IMC Section 506 — no substitution is code-compliant.
- Systems exceeding 2 inches w.g. positive static pressure must use sheet metal (galvanized steel or equivalent); duct board is excluded by its published pressure class ceiling.
- Locations requiring vapor barrier continuity (attics, unconditioned crawlspaces in humid climates) require either pre-insulated duct board with intact FSK facing or metal duct with continuous external vapor-barrier insulation
References
- ASHRAE Standard 90.1 – Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings (ASHRAE, Standard 90.1)
- International Mechanical Code (IMC) – Section 603: Duct Construction and Installation (ICC, International Mechanical Code 2021)
- SMACNA HVAC Duct Construction Standards — Metal and Flexible (SMACNA, HVAC Duct Construction Standards)
- UL 181 – Factory-Made Air Ducts and Air Connectors (UL, Standard 181)
- NAIMA – Fibrous Glass Duct Construction Standards (NAIMA, Fibrous Glass Duct Construction Standards)
- OSHA 29 CFR 1910 – Occupational Safety and Health Standards: Hazardous Materials and Ventilation
- U.S. Department of Energy – Building Technologies Office: HVAC Systems and Duct Efficiency
- EPA Indoor Air Quality – Ventilation and Duct Systems
- NFPA 90A – Standard for the Installation of Air-Conditioning and Ventilating Systems (NFPA, Standard 90A)
- 10 CFR Part 433 – Energy Efficiency Standards for New Federal Commercial and Multi-Family High-Rise Residential Buildings