What is spiral duct? Spiral duct is round sheet metal duct wound from a steel strip into a tube with one continuous helical lock seam. Fully sealed, round duct reaches leakage class 3 against 6 for rectangular duct in DOE’s figures, which is why commercial and exposed systems favor it.
Key Takeaways
- Unsealed rectangular duct averages leakage class 48; sealing every joint, seam and penetration brings rectangular to 6 and round to 3, per the DOE and AMCA fan sourcebook.
- ASHRAE’s official interpretation of Standard 90.1 says all duct pressure classes require Seal Class A (IC 90.1-2013-14).
- Washington’s IECC-based code requires ducts at 3 in. w.g. or more to test at leakage class 4.0 or less on at least 25 percent of duct area (C403.10.2.3).
- That code exempts locking-type longitudinal seams below 2 in. w.g. from sealing, never the joints between pieces.
- Eastern Sheet Metal lists round spiral from 3 to 84 inches, in lengths up to 12 feet (maker data).
What is spiral duct?
Spiral duct (also called spiral pipe or spiral ductwork) is factory made round or flat oval duct whose seam winds around the tube in a helix. A forming machine locks the strip edges together as it rolls, so each straight piece arrives rigid with no field seam to close.
Eastern Sheet Metal, a US maker, lists round spiral from 3 to 30 inches in 1 inch steps and 32 to 84 inches in 2 inch steps, in galvanized, paint grip, stainless and aluminum. Its single wall catalog notes galvanized spiral 6 inches and larger ships corrugated unless ordered smooth.
Designers use it for trunks feeding a variable air volume terminal, for stainless fume exhaust, and for exposed ceilings in gyms, restaurants and lofts.
Why does round spiral duct lose less pressure than rectangular duct?
Round duct wraps the most cross section in the least metal, so less wall surface drags on the air. A 2002 AIVC literature survey found pressure loss through a rectangular duct is significantly higher than through a round duct of equal area, and it grows with aspect ratio.
Worked geometry (our arithmetic): a 20 by 8 inch rectangle has 160 square inches of area and a 56 inch perimeter. A round duct with the same area is 14.3 inches across with a 44.8 inch perimeter, about 20 percent less metal per foot. That is less surface to leak and to gain or lose heat.
Diameter matters even more than shape. The DOE sourcebook states that doubling a round duct’s diameter cuts frictional head loss by a factor of 32, and that stepping from 10 to 12 inches can cut it by 60 percent. Lower friction means lower total external static pressure at the blower, the root of many duct airflow problems.
Does spiral ductwork leak less? Seal classes and leakage classes
Yes, at every sealing level. DOE puts fully sealed round duct at leakage class 3 against 6 for rectangular, because round duct has less surface per unit of cross section and no corners to seal. Leakage class (CL) is leakage in cfm per 100 square feet of duct surface, normalized to test pressure.
| Sealing level | What gets sealed (SMACNA class) | Rectangular CL | Round CL |
|---|---|---|---|
| Unsealed | Nothing | 48 (average) | Not stated |
| Transverse joints only | Joints between pieces (Seal Class C) | 24 | 12 |
| Everything | Joints, longitudinal seams, wall penetrations (Seal Class A) | 6 | 3 |
Leakage classes are DOE’s; seal class definitions follow the University of Michigan duct design guideline, which caps tested leakage on its projects at SMACNA Leakage Class 3.
Worked example (our arithmetic): at 1 in. w.g., 100 feet of fully sealed 20 by 8 rectangular duct has about 467 square feet of surface and leaks roughly 28 cfm. The equal area round run, about 373 square feet, leaks roughly 11 cfm, 40 percent as much.
Correction
Supplier pages often say spiral duct “needs no sealing.” Only the factory spiral seam can skip it. Every slip joint, coupling and fitting is a transverse joint, and even Seal Class C seals those.
What do energy codes require for spiral duct sealing and testing?
Commercial energy codes tie spiral duct sealing to pressure class, and at 3 in. w.g. and up they require a leakage test. The clearest text we could read in full is Washington’s IECC-based code, WAC 51-11C-40391, in the version in force until October 2023. Section numbers shift between IECC editions, so check your adopted code.
| Section | Pressure class | Requirement |
|---|---|---|
| C403.10.2.1 | 2 in. w.g. or less | Seal all joints and seams; continuously welded and locking-type longitudinal seams exempt |
| C403.10.2.2 | Over 2, under 3 in. w.g. | Insulate and seal per C403.10.1 |
| C403.10.2.3 | 3 in. w.g. or more | Leak test per SMACNA test manual; CL 4.0 or less on at least 25 percent of duct area |
The test math is CL = F / P0.65, with F the measured leakage in cfm per 100 square feet of duct surface and P the test pressure. ASHRAE 90.1 Section 6.4.4.2.2 requires leak tests for ductwork designed above 3 in. wc and for ductwork outdoors, and ASHRAE’s 2017 interpretation says all pressure classes require Seal Class A.
A failed test means resealing joints, as in our guide to duct leak repair. The Michigan guideline requires testing before insulation goes on.
What spiral duct sizes, gauges and fittings do makers list?
Spiral duct sizes run from 3 to 84 inches at one major maker, and the wall gets heavier as diameter grows. The table is Eastern Sheet Metal’s galvanized single wall column for 10 in. WG positive pressure, labeled 2005 SMACNA. It is maker data; your specified pressure class governs.
| Diameter | Spiral duct gauge | Fitting gauge |
|---|---|---|
| 3 to 24 in. | 26 gauge | 26 (24 at 19 to 24 in.) |
| 25 to 42 in. | 24 gauge | 22 |
| 43 to 66 in. | 22 gauge | 20 (18 at 61 to 66 in.) |
| 67 to 84 in. | 20 gauge | 18 |
Spiral duct fittings come as full bodied elbows, tees, reducers, couplings and taps, spot welded or gorelocked through 40 inches per the catalog. Branch taps often carry a balancing volume damper. The maker’s slip joint sequence:
- Angle the fitting collar into the duct and work it in until about one inch of collar remains exposed.
- Apply duct sealer to that exposed band (skipped only with factory gasketed fittings).
- Push the fitting home until the stop bead meets the duct end.
- Fasten with screws evenly spaced around the joint, about 1/2 inch back from the edge.
Single wall vs double wall spiral duct: what does the liner buy?
Double wall spiral duct adds a perforated inner tube and insulation inside the outer pipe, buying sound absorption and thermal insulation with no outside wrap. Single wall is one lighter metal shell, the usual choice where noise and condensation are not concerns.
Eastern Sheet Metal’s dual wall catalog specifies an inner shell with 3/32 inch holes on 3/16 inch staggered centers, a 23% free area, around standard 1 inch insulation rated R4 or better. The maker lists a noise reduction coefficient of 0.75 for that insulation. The Michigan guideline adds a film wrap between liner and perforation to keep fiber out of the airstream.
Choose double wall near quiet rooms or for exposed runs that would sweat. Choose single wall plus exterior wrap where the duct is hidden; the duct insulation R-value rules by location still apply. For a fibrous glass alternative, see our explainer on fiberglass duct board.
Can you use exposed spiral duct in a house?
Yes. Spiral duct is sheet metal duct, and the residential rules that matter turn on where it runs, not on its seam. Exposed spiral inside the living space has a code advantage: it sits inside the thermal envelope. Under the 2015 IECC as adopted in Washington, DC, the residential duct leakage test (R403.3.3) is waived where all ducts and the air handler are inside the envelope.
Run the same spiral through a vented attic and DC’s R403.3.4 caps post construction leakage at 4 cfm per 100 square feet of conditioned floor area, tested at 0.1 in. w.g. (25 Pa), and the duct needs insulation for its location. Exposed indoor duct may also skip insulation; Washington’s commercial code exempts ductwork exposed to view within the zone it serves.
In houses, rigid spiral is stiffer than flexible duct, so plan layouts without tight offsets. Joints still get mastic or sealer, as in any ductwork installation built to standard. For painted exposed runs, Eastern Sheet Metal sells a paint grip steel alongside standard galvanized.
How we researched this
Research date: October 14, 2026. We read the top ranking pages for “spiral duct” (supplier and fabricator pages, none citing a standard), then primary documents: the DOE and AMCA fan sourcebook, an ASHRAE 90.1 interpretation, Washington and DC code text, a University of Michigan guideline and Eastern Sheet Metal catalogs. We consulted 16 sources and cite 9. SMACNA’s own manuals are paywalled, so SMACNA classes appear only as restated by those documents. Excluded: vendor price ranges (no federal series), and unsourced supplier percentages for material, labor and fan energy savings. Reddit was not used.
Frequently asked questions
Is spiral duct more expensive than rectangular duct?
No federal or national lab source publishes installed spiral duct prices, so we quote none. Eastern Sheet Metal claims its long lengths eliminate 30% or more of field connections, a maker claim. Our ductwork cost explainer covers what cost data does exist.
Does spiral duct need to be sealed?
The spiral seam does not below 2 in. w.g., under Washington’s C403.10.2.1 exception for locking-type longitudinal seams. Joints, fittings and taps do. ASHRAE’s 2017 interpretation of Standard 90.1 says all pressure classes require Seal Class A; Eastern Sheet Metal states 90.1 still excuses the spiral seam, which we could not confirm in the paywalled text.
What gauge is spiral duct?
In Eastern Sheet Metal’s catalog column for 10 in. WG positive pressure, galvanized spiral duct is 26 gauge from 3 to 24 inches, 24 gauge from 25 to 42 inches, 22 gauge from 43 to 66 inches and 20 gauge from 67 to 84 inches. Negative pressure and other materials use different tables.
Is spiral duct quieter than rectangular duct?
Makers market round duct as resistant to breakout noise because it has no flat panels; we could not verify a test. For measured absorption, use double wall: Eastern Sheet Metal lists a noise reduction coefficient of 0.75 for its standard 1 inch liner insulation.
What does seal class mean for spiral ductwork?
Seal class says which parts of the duct get sealant. Per the University of Michigan guideline, Seal Class C seals transverse joints only, while Seal Class A seals all transverse joints, longitudinal seams and duct wall penetrations. DOE’s figures put fully sealed round duct at leakage class 3.
What is the biggest drawback of spiral duct?
It needs depth. The round duct equal in area to a 20 by 8 inch rectangle is 14.3 inches across, so spiral can fail to fit a shallow joist or ceiling space. Rigid pieces up to 12 feet long, per Eastern Sheet Metal, are also awkward in finished houses.
Sources
- U.S. Department of Energy and AMCA International. “Improving Fan System Performance: A Sourcebook for Industry” (2003). https://www.energy.gov/sites/prod/files/2014/05/f16/fan_sourcebook.pdf. Accessed October 2026.
- ASHRAE. “Interpretation IC 90.1-2013-14 of ANSI/ASHRAE/IES Standard 90.1-2013.” https://www.ashrae.org/file%20library/technical%20resources/standards%20and%20guidelines/standards%20intepretations/ic-90.1-2013-14.pdf. Accessed October 2026.
- Washington State Legislature. “WAC 51-11C-40391, Section C403.10, Construction of HVAC system elements” (effective until October 29, 2023). https://lawfilesext.leg.wa.gov/law/WACArchive/2023/htm/WAC%20%2051%20%20TITLE/WAC%20%2051%20-%2011C%20CHAPTER/WAC%20%2051%20-%2011C-40391.htm. Accessed October 2026.
- DC Department of Buildings. “Building and Duct Leakage Testing.” https://dob.dc.gov/node/1620966. Accessed October 2026.
- University of Michigan. “Design Guideline 230020, Duct System Design.” https://umaec.umich.edu/desguide/tech/23/DG230020.pdf. Accessed October 2026.
- Hassan, O.A.B. and Yue, Z. “Pressure Drop in and Noise Radiation from Rectangular and Round Ducts: Literature Survey.” AIVC, 2002. https://www.aivc.org/resource/pressure-drop-and-noise-radiation-rectangular-and-round-ducts-literature-survey. Accessed October 2026.
- Eastern Sheet Metal. “Topics: Why Spiral Duct” (maker data). https://www.easternsheetmetal.com/Topics. Accessed October 2026.
- Eastern Sheet Metal. “Single-Wall Round Spiral Duct and Fittings” (maker catalog). https://www.easternsheetmetal.com/Portals/6/Documents/SWR040709.pdf. Accessed October 2026.
- Eastern Sheet Metal. “Dual-Wall Round Spiral Duct and Fittings” (maker catalog). https://www.easternsheetmetal.com/Portals/6/Documents/DWR040709.pdf. Accessed October 2026.
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