Ductwork Materials

Material selection for air distribution systems directly impacts system performance, longevity, energy efficiency, and maintenance requirements. Each ductwork material exhibits distinct thermal, mechanical, and chemical properties that determine suitability for specific applications, pressure classes, and environmental conditions.

Galvanized Steel Duct

Galvanized steel represents the predominant material for commercial and industrial air distribution systems due to favorable strength-to-weight ratio, formability, and cost-effectiveness.

Material Composition and Properties

Galvanized steel duct consists of mild carbon steel (typically ASTM A653 or A924) with hot-dip zinc coating applied for corrosion protection. The zinc coating designation G60 (0.60 oz/ft² per surface) or G90 (0.90 oz/ft²) specifies minimum coating weight, with G90 providing superior corrosion resistance for demanding environments.

Physical Properties:

Gauge Specifications

SMACNA standards specify minimum gauge thickness based on duct dimensions, pressure class, and construction class. The relationship between gauge number and thickness follows the Manufacturers’ Standard Gauge for sheet steel:

Gauge selection depends on maximum dimension (width or diameter) and static pressure class. For rectangular ducts, the governing dimension determines minimum gauge per SMACNA Table 1-2. Reinforcement requirements reduce as gauge increases.

Pressure Classifications

SMACNA defines pressure classes that establish maximum allowable static pressure for duct systems:

Positive Pressure Applications:

Negative Pressure Applications:

Negative pressure ductwork (exhaust and return systems) requires different construction standards due to buckling potential. SMACNA specifies heavier gauge requirements or closer reinforcement spacing for equivalent negative pressure compared to positive pressure applications.

Corrosion Resistance and Environmental Considerations

Zinc coating provides galvanic protection, sacrificing zinc to protect underlying steel. Corrosion rate depends on environmental factors:

• Indoor conditioned spaces: 0.1 - 0.3 mils/year zinc loss (50+ year service life)
• High humidity environments: 0.5 - 1.0 mils/year (10 - 20 year coating life)
• Coastal/marine exposure: 1.5 - 3.0 mils/year (requires G90 or alternative material)
• Chemical environments: Variable (may require stainless steel)

White rust (zinc hydroxide formation) occurs when galvanized surfaces remain wet without adequate ventilation. This accelerates zinc depletion and compromises corrosion protection.

Aluminum Duct

Aluminum ductwork provides superior corrosion resistance with lower weight compared to galvanized steel, making it advantageous for corrosive environments, roof-mounted equipment, and weight-sensitive installations.

Material Properties

Aluminum duct typically utilizes 3003-H14 alloy, providing optimal formability and strength characteristics.

Gauge and Thickness Requirements

Aluminum’s lower strength requires thicker material for equivalent structural performance. SMACNA standards account for this through adjusted gauge specifications:

The reduced modulus of elasticity necessitates closer reinforcement spacing or heavier gauge for equivalent deflection limits.

Galvanic Corrosion Considerations

Aluminum exhibits electrochemical potential differences with other metals, creating galvanic corrosion risk when dissimilar metals contact in presence of electrolyte:

• Acceptable contact: Aluminum to aluminum, aluminum to stainless steel (with caution)
• Problematic contact: Aluminum to carbon steel, aluminum to copper
• Prevention: Insulating gaskets, dielectric coatings, isolation materials

Salt water or coastal environments accelerate galvanic corrosion. Proper isolation between aluminum ductwork and steel supports prevents galvanic couples.

Stainless Steel Duct

Stainless steel ductwork serves highly corrosive environments, high-temperature applications, and installations requiring long-term durability without maintenance.

Alloy Selection

Physical Properties (Type 304)

Stainless steel maintains strength at elevated temperatures where galvanized steel would fail (above 400°F, zinc coating deteriorates and steel weakens).

Cost-Benefit Analysis

Stainless steel material costs 4 - 8× galvanized steel but eliminates:

• Painting and coating maintenance
• Corrosion-related failures and replacements
• System downtime for repairs

Life-cycle cost analysis often favors stainless steel for:

• Chemical exhaust systems
• Pharmaceutical cleanrooms
• Food processing facilities
• Marine/coastal installations
• High-temperature exhaust (>400°F)

Fiberglass Duct Board

Fiberglass duct board integrates air barrier and thermal insulation into single material, reducing installation labor and providing inherent acoustic attenuation.

Material Construction

Duct board consists of rigid fiberglass insulation (typically 3 - 6 lb/ft³ density) with reinforced foil-scrim-kraft (FSK) facing on interior surface. The FSK facing provides:

• Air barrier (vapor retarder when sealed)
• Erosion resistance
• Smooth interior surface (reduces friction losses)
• Reflective thermal barrier

Thermal Properties

A 1.5-inch thick duct board provides approximately R-6.3, significantly reducing heat gain/loss compared to uninsulated metal ductwork.

Acoustic Performance

Fiberglass duct board provides superior sound attenuation compared to sheet metal:

This internal absorption reduces need for separate duct silencers in many applications.

Pressure Limitations

UL 181 Class 1 duct board withstands:

• Positive pressure: Up to 2 inches wg (standard), 3 inches wg (reinforced)
• Negative pressure: Up to 2 inches wg (standard), 3 inches wg (reinforced)
• Maximum velocity: 2500 fpm (standard), 4000 fpm (erosion-resistant facing)

Reinforcement options include:

• Fiberglass strapping
• Metal angles at corners
• Full metal wrap at high-pressure connections

Moisture and Indoor Air Quality Considerations

Fiberglass duct board requires proper installation and environmental conditions:

Acceptable conditions:

• Indoor conditioned spaces with controlled humidity
• Relative humidity consistently below 80%
• No direct water contact or condensation
• Proper vapor barrier installation on exterior surfaces

Problematic conditions:

• Outdoor installations or unconditioned spaces
• High humidity environments (>80% RH sustained)
• Below-grade installations with moisture migration
• Return air plenums in crawl spaces

ASHRAE Standard 62.1 requires fiberglass duct materials maintain clean, dry conditions. Moisture accumulation supports microbial growth and deteriorates material integrity.

Flexible Duct

Flexible duct consists of helical wire coil supporting interior polymer film and exterior insulation, providing adaptable connections between trunk ducts and terminals.

Construction Components

1. Inner core: Polymer film (polyester, metalized polyester, or aluminum laminate)
2. Wire helix: Spring steel supporting structure
3. Insulation: Fiberglass blanket (R-4.2 to R-8.0)
4. Vapor barrier: Exterior polymer film or FSK jacket

Physical and Thermal Properties

Pressure Drop Considerations

Flexible duct exhibits significantly higher friction loss compared to smooth metal duct. ASHRAE friction charts provide adjustment factors:

Critical installation requirements:

• Extend to full length (eliminate compression)
• Support every 4 feet maximum spacing
• Minimize bends (maximum two 90° bends per run)
• Secure connections with draw bands or clamps
• Maintain straight runs without sagging

A 5-foot flex duct run compressed to 3 feet can increase pressure drop by 200 - 300%, significantly impacting system performance and energy consumption.

Code and Application Limitations

International Mechanical Code (IMC) Section 603.4 restricts flexible duct application:

• Maximum length: 5 feet (some jurisdictions allow 8 feet)
• Not permitted to penetrate fire-rated assemblies
• Not permitted in plenums unless listed for plenum use
• Prohibited for use as vertical risers without support
• Connections must be accessible for inspection

Despite code allowances, engineering best practice limits flex duct to 3 - 5 feet for optimal performance.

Spiral Pipe and Fittings

Spiral lockseam pipe provides circular duct with continuous helical seam, offering superior strength and air tightness compared to longitudinal seam pipe.

Manufacturing and Construction

Spiral pipe forms from continuous coil stock, creating helical seam with 4 - 5 locks per foot. The spiral construction provides:

• Enhanced rigidity (resists buckling and collapse)
• Improved air tightness (continuous seam vs intermittent joints)
• Lower fabrication labor (automated production)
• Smooth interior surface (minimal pressure drop)

Gauge Specifications

Spiral pipe gauge selection depends on diameter and pressure class:

Structural Performance

The spiral construction provides 30 - 50% greater resistance to buckling compared to equivalent gauge longitudinal seam pipe, allowing:

• Longer unsupported spans
• Reduced reinforcement requirements
• Lighter gauge for equivalent performance
• Superior negative pressure resistance

SMACNA Standards and Construction Classes

SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association) provides comprehensive standards for duct construction, establishing minimum requirements for materials, reinforcement, joints, and sealing.

Pressure Class System

SMACNA construction standards organize by pressure class, specifying:

1. Minimum gauge thickness based on duct size
2. Reinforcement spacing (tie rod, angle, or channel)
3. Joint type and spacing (drives, standing seams, proprietary)
4. Seal requirements (mastic, tape, gaskets)

Duct Seal Classes (ASHRAE 90.1)

Achieving specified seal class requires:

• Proper joint construction per SMACNA standards
• Continuous mastic or tape application at all seams and joints
• Gasketed or welded connections at equipment
• Testing and verification per SMACNA HVAC Air Duct Leakage Test Manual

Reinforcement Requirements

Reinforcement prevents duct deflection and structural failure:

Rectangular duct reinforcement:

• External reinforcement: Angles, channels, or Z-shapes
• Internal reinforcement: Tie rods or solid bar stock
• Spacing: Determined by duct dimensions, gauge, and pressure class

Round duct reinforcement:

• Bell stiffeners or rolled beads
• Intermediate reinforcing angles for large diameters
• Support spacing: 10 - 12 feet for suspended ductwork

Material Selection Decision Framework

Selection of appropriate ductwork material requires evaluation of multiple performance criteria:

Performance Comparison Matrix

Application-Specific Selection

High-pressure systems (>3 in wg):

• Galvanized or stainless steel
• Avoid fiberglass and flexible duct
• Consider spiral construction for round ducts

Corrosive environments:

• Stainless steel (chemical exhaust, coastal)
• Aluminum (moderate corrosion, weight sensitive)
• Avoid galvanized steel without protective coatings

Low-velocity, acoustic-sensitive:

• Fiberglass duct board (internal attenuation)
• Double-wall metal duct with insulation
• Minimize flexible duct length

Energy efficiency priority:

• Fiberglass duct board (integral insulation)
• Externally insulated metal duct (controlled installation)
• Minimize flex duct (high friction losses)

Cost-constrained residential:

• Galvanized steel trunk and branches
• Fiberglass duct board alternative
• Flexible duct for final connections (minimize length)

Installation and Quality Assurance

Proper material installation ensures design performance:

Galvanized/metal duct:

• Support per SMACNA spacing requirements
• Seal all joints and seams per seal class
• Install vibration isolation at equipment connections
• Insulate exterior surfaces per energy code

Fiberglass duct board:

• Score and snap cuts (avoid crushing fibers)
• Apply continuous bead of FSK tape or mastic at joints
• Install vapor barrier on exterior in cooling applications
• Protect during construction from moisture and damage

Flexible duct:

• Extend to full length without compression
• Support every 4 feet maximum
• Secure connections with draw bands (minimum 2)
• Maintain straight runs without kinks or sagging

Quality assurance includes visual inspection, duct leakage testing per SMACNA protocols, and verification of gauge, reinforcement, and sealing per specifications and applicable codes.

Components

• Galvanized Steel Duct
• Aluminum Duct
• Stainless Steel Duct
• Fiberglass Duct Board
• Flexible Duct Insulated
• Spiral Pipe Duct
• Smacna Standards Duct Construction