Spiral duct is round metal ductwork manufactured by helically winding a continuous strip of sheet metal into a rigid tube. The interlocking spiral seam gives it structural strength that longitudinal-seam round duct cannot match — allowing lighter gauge metal, longer unsupported spans, and tighter air leakage rates. It is the dominant duct type in commercial HVAC systems worldwide and is increasingly replacing rectangular duct in new construction due to lower material cost, faster installation, and superior aerodynamic performance.
This guide covers spiral ductwork sizing, material selection, fitting types, installation methods, and real-world cost data. Our parent company, Airwise Parts, manufactures spiral duct at our facilities in Dongyang, China and Houston, TX — so the specifications and pricing in this guide reflect actual production data.
What Is Spiral Duct?
Spiral duct starts as a flat coil of sheet metal — typically galvanized steel, aluminum, or stainless steel — that is fed into a spiral forming machine. The machine bends the strip into a helix and locks each revolution to the previous one with a continuous raised seam. The result is a round tube with a single helical seam running its entire length at a consistent pitch (typically 1.5-3 inches between seam ridges).
This manufacturing process gives spiral duct three advantages over other round duct types:
Structural strength. The spiral seam acts as a continuous rib, resisting collapse under negative pressure and deflection under its own weight. A 12-inch spiral duct in 26 gauge can span 12 feet between hangers; the equivalent snap-lock duct needs support every 8 feet.
Air tightness. The lockseam is mechanically formed and continuous — there are no butt joints or open seams along the duct body. SMACNA Seal Class A (all seams and joints sealed) is easily achievable. Standard spiral duct leaks at 1-3% per 100 feet, vs 5-10% for rectangular duct with the same sealing effort.
Material efficiency. A round cross-section encloses 27% more area than a square of the same perimeter. A 12-inch round duct (113 sq in area, 37.7 in perimeter) handles the same airflow as a 10x12 rectangular duct (120 sq in area, 44 in perimeter). That is 14% less sheet metal for the same capacity.
Spiral Duct Sizes
Standard spiral ductwork is manufactured in the following diameters. The table shows recommended CFM ranges based on ASHRAE velocity limits of 600-1200 FPM for low-velocity commercial systems and up to 2000 FPM for industrial applications.
| Diameter (in) | Area (sq in) | CFM @ 600 FPM | CFM @ 1200 FPM | Gauge | Typical Use |
|---|---|---|---|---|---|
| 4 | 12.6 | 52 | 105 | 26 | Bathroom exhaust, dryer vent |
| 6 | 28.3 | 118 | 236 | 26 | Branch runs, small rooms |
| 8 | 50.3 | 209 | 419 | 26 | Branch runs, bedrooms |
| 10 | 78.5 | 327 | 654 | 26 | Sub-main, open offices |
| 12 | 113.1 | 471 | 942 | 26 | Main trunk, small commercial |
| 14 | 153.9 | 641 | 1,282 | 26 | Main trunk |
| 16 | 201.1 | 838 | 1,675 | 24 | Main trunk, commercial |
| 18 | 254.5 | 1,060 | 2,120 | 24 | Risers, large commercial |
| 20 | 314.2 | 1,309 | 2,618 | 24 | Main supply/return |
| 24 | 452.4 | 1,885 | 3,770 | 24 | Riser shafts, AHU connections |
Standard spiral duct lengths are 5 feet and 10 feet. Custom lengths up to 20 feet reduce the number of joints and improve air tightness, but are harder to transport and handle on-site. For sizing based on CFM requirements, see our duct CFM chart which covers both spiral and flex duct.
Spiral Duct vs Rectangular Duct
The choice between spiral (round) and rectangular ductwork is one of the most impactful decisions in HVAC system design. Here is how they compare across every performance dimension:
| Factor | Spiral Duct | Rectangular Duct |
|---|---|---|
| Pressure drop | Lower (smooth flow, no corners) | Higher (turbulence at corners) |
| Air leakage | 1-3% (one seam) | 5-10% (four seams + joints) |
| Material use | 20-40% less metal | More metal per CFM |
| Installation speed | Faster (slip joints, clamps) | Slower (TDC/S-slip flanges) |
| Noise | Quieter (laminar flow) | Noisier (rumble, breakout) |
| Clearance needed | More height (round profile) | Less height (flat profile) |
| Cost | 15-30% lower installed | Higher material + labor |
| Aesthetics (exposed) | Clean industrial look | Utilitarian |
When to choose rectangular: Low ceiling plenums where vertical clearance is limited (a 12x8 rectangular duct is only 8 inches tall vs 12 inches for equivalent round), applications that require internal duct liner for sound attenuation, and renovations where existing rectangular openings cannot be modified.
When to choose spiral: Everything else. New construction, exposed ductwork (restaurants, retail, lofts), data centers, industrial ventilation, and any project where energy efficiency and air tightness matter.
Material Selection
Galvanized steel (G60 or G90 coating) is the standard material for HVAC spiral duct. G90 provides better corrosion resistance and is recommended for coastal or high-humidity environments. Most commercial spiral duct is G90 galvanized, 26 gauge for diameters up to 14 inches, stepping up to 24 gauge for 16-26 inches.
Aluminum spiral duct is used in food processing, pharmaceutical, and cleanroom applications where galvanized coatings may contaminate the process. It is also lighter (about 1/3 the weight of steel) which reduces structural support requirements. Cost is approximately 2x galvanized steel.
Stainless steel (304 or 316 grade) is specified for kitchen exhaust, laboratory fume extraction, and corrosive environments. 316 stainless is required for saltwater exposure and strong chemical fumes. Cost is 3-4x galvanized steel.
Spiral Duct Fittings
A spiral duct system requires fittings to change direction, branch, reduce diameter, and connect to equipment. Standard fittings include:
Elbows: 90° and 45° elbows in smooth-radius (1.5D centerline radius) or adjustable (gore) configurations. Smooth-radius elbows produce lower pressure drop but require more clearance. Adjustable elbows are more compact and can be set to any angle from 0-90°.
Tees and laterals: Branch takeoffs for splitting airflow. Conical tees provide better airflow distribution than straight tees. Saddle taps are used for smaller branch connections drilled into the main trunk.
Reducers: Concentric (centered) and eccentric (offset) reducers for transitioning between diameters. Eccentric reducers keep the bottom of the duct flat — important for horizontal runs where condensation drainage matters.
End caps: Flat or domed caps for terminating duct runs. Domed caps handle positive pressure better; flat caps are cheaper.
Installation Best Practices
Spiral duct installation is straightforward but requires attention to joint sealing and support spacing.
Joints. Spiral duct sections connect with slip joints — one end is crimped to slide inside the next section, overlapping by 1.5-2 inches. The joint is secured with three self-drilling sheet metal screws at 120° spacing, then sealed with mastic or foil tape. For SMACNA Seal Class A (required in many commercial codes), all transverse joints must be sealed with mastic.
Hangers. Support spiral duct with adjustable clevis hangers, trapeze hangers, or band hangers. Maximum hanger spacing for horizontal runs: 10 feet for ducts up to 12 inches, 8 feet for 14-24 inches, and 6 feet for larger sizes. Vertical risers need support at every floor penetration plus intermediate supports every 12-16 feet.
Seismic bracing. In seismic zones 2-4 (most of the western US), spiral duct requires lateral and longitudinal bracing per SMACNA seismic guidelines. Transverse braces every 12 feet, longitudinal braces at every 24 feet, and bracing at all turns and branch connections.
Flexible connections. Install flexible duct connectors between air handling units and the duct system to isolate vibration. A 4-6 inch length of fabric or rubber connector prevents fan vibration from transmitting through the ductwork and creating noise.
Spiral Duct Cost per Foot
| Diameter | Galv. Steel (26ga) | Aluminum | 304 Stainless |
|---|---|---|---|
| 6" | $2.00-3.00 | $4.00-6.00 | $8.00-12.00 |
| 8" | $2.50-3.50 | $5.00-7.00 | $10.00-14.00 |
| 10" | $3.00-4.50 | $6.00-9.00 | $12.00-18.00 |
| 12" | $3.50-5.00 | $7.00-10.00 | $14.00-20.00 |
| 16" (24ga) | $6.00-8.00 | $12.00-16.00 | $24.00-32.00 |
| 20" (24ga) | $8.00-12.00 | $16.00-24.00 | $32.00-48.00 |
| 24" (24ga) | $10.00-15.00 | $20.00-30.00 | $40.00-60.00 |
Pricing note: These are material-only prices at typical US distributor pricing (2026). Bulk orders (500+ linear feet) typically receive 10-20% discounts. Custom lengths, non-standard gauges, and specialty coatings (painted, lined) add 15-40% to base pricing. For factory-direct pricing on spiral duct and fittings from our production facilities, contact our sales team.
Exposed Spiral Duct: The Industrial Aesthetic
Exposed spiral ductwork has become a defining design element in restaurants, breweries, co-working spaces, loft apartments, and retail stores. The clean round profile and consistent spiral seam pattern create an industrial aesthetic that complements exposed concrete, brick, and steel.
For exposed installations, specify painted spiral duct in matte black, white, or custom colors. The paint is applied at the factory for a uniform finish that field-painting cannot match. Powder-coated spiral duct is also available for high-traffic areas where scratch resistance matters.
Design considerations for exposed spiral: plan hanger locations to align with ceiling grid or structural members, use concealed fasteners (rivets instead of screws) at joints, and route the duct system as a deliberate architectural element rather than trying to hide it. The best exposed spiral installations look intentional, not accidental.
Looking for spiral duct, fittings, or flexible duct connectors for your project? Our parent company Airwise Parts manufactures spiral ductwork at our facilities in China and Houston. We also carry the full range of flexible ducts and duct accessories for complete HVAC duct system solutions.