What double-wall (double-skin) duct actually is
Double-wall duct — also called double-skin or insulated double-wall duct — is built from three layers working together. A perforated inner liner faces the airstream; an insulation core sits behind it; and a solid outer wall carries the structural and pressure load. The perforations in the liner let sound energy pass through into the insulation, where it is absorbed instead of being reflected back into the air. The same insulation core simultaneously delivers the thermal performance and condensation control of an insulated duct. In short, one assembly does the job that single-wall externally-insulated duct and internally-lined duct each do separately — with the absorber protected on both faces by metal.
That protection is the whole point. An internal liner glued to the inside of a single-wall duct works acoustically, but it can erode at high velocity, shed fibre into the airstream, and is awkward to clean. Capturing the absorber between a perforated liner and a solid outer wall removes all three problems, which is why double-wall construction is the default wherever those constraints bite.
Where double-wall and acoustic duct is specified
Double-wall and acoustic construction is not used everywhere — it is a deliberate upgrade over single-wall lined duct, justified by a specific requirement:
- Plant rooms and fan rooms — the duct sections immediately downstream of the air handler carry the most fan noise and the highest velocities, so the run nearest the AHU is the first to be specified double-wall.
- Hospitals, operating theatres and clean air — where shed fibre is unacceptable and the interior must be wipeable, the perforated liner is run in stainless and no exposed fibre is permitted.
- Recording studios, broadcast and performing-arts spaces — very low background-noise targets demand both distributed absorption and breakout control, and double-wall construction provides additional breakout reduction over single-skin lined duct.
- High-velocity supply systems — where a glued internal liner would erode, the captured absorber stays put behind a hard perforated face.
For general supply and return runs that face none of these constraints, single-wall duct with external insulation remains the economical choice — covered in the HVAC duct insulation guide.
How double-wall duct is fabricated
The construction route depends on whether the duct is round or rectangular, but the principle is identical: form the perforated liner, capture the insulation, form or set the outer wall, and close the seam.
1. Perforating the liner. The inner skin must be perforated so sound can reach the absorber. The liner is either pre-perforated coil/sheet bought in, or flat blank that is punched or plasma-cut to the specified pattern before forming. The open area of the perforation is an acoustic decision — enough to admit sound, not so much that the sheet loses rigidity — and is set by the project specification.
2. Forming the round liner and outer tube. For round duct, a spiral tubeformer rolls the perforated strip into the inner liner and lock-seams it continuously into a helical tube. The insulation is wrapped over the formed liner, and a second forming pass produces the solid outer tube around the assembly — a double-wall spiral tube. The lock seam carries the structural duty; the liner does not.
3. Assembling the rectangular double skin. For rectangular duct, an auto duct line forms the structural outer wall from coil. The insulation is laid into it, the perforated inner skin is set on standoffs or pressed hat-section spacers that hold the air gap and keep the liner off the outer wall, and the package is closed.
4. Closing the seam. The outer wall is closed with a Pittsburgh lock seam (rectangular), a spiral lock seam (round) or a transverse TDF flange at duct joints; the inner skin is lapped and sealed so the airstream sees a continuous perforated face. Seam and flange selection follows the same seal-class logic as any pressure duct — see SMACNA seal classes explained.
Liner gauges and material
The perforated inner liner is normally a lighter gauge than the structural outer wall, because it carries no pressure load — its only jobs are to retain the insulation and present a durable face to the airstream. Galvanised liner is the general-purpose choice; stainless liner is used where washdown, corrosion or hygiene requires it. Because the liner gauge is light, it is well within the range of standard duct-forming machinery: SBKJ spiral tubeformers form galvanised steel from 0.4 mm and stainless from 0.4 mm, and the dedicated stainless former runs very thin stainless from 0.05 mm. In other words the liner gauge is dictated by the specification and the acoustic/hygiene requirement, not by the machine. The exact perforation open area and gauge must be confirmed against the project — never assumed — and the gauge of the outer wall follows the usual pressure-class table in the duct sheet-metal gauge chart.
Acoustic attenuation, in brief
The mechanism is straightforward: sound energy in the airstream passes through the open area of the perforated liner into the insulation, where it is converted to heat and absorbed rather than reflected. The attenuation contributed per metre of double-wall or lined run is modest; the bulk of the insertion loss in a real system comes from discrete sound attenuators (silencers) placed near the fan. Insertion loss is verified to ASTM E477 and absorption to ASTM C423. This page is deliberately about the construction and fabrication of double-wall duct — for the acoustic design numbers (NRC by liner thickness, insertion loss versus length, regenerated noise versus face velocity, attenuator types) see the dedicated acoustic duct lining and attenuator reference.
SBKJ machinery by duct element
Double-wall and acoustic duct is fabricated on standard SBKJ duct-forming equipment — there is no exotic machine required, only the right combination of formers and a closure station. The table below maps each machine to the element it produces. All figures are taken verbatim from the SBKJ Product Catalog 2026.
| Machine | Duct element it produces | Forming range | Material gauge |
|---|
| Spiral tubeformer (SBTF series) | Round perforated inner liner & solid outer tube | Φ80–Φ2500 mm | GI 0.4–2.0 mm · SS 0.4–1.2 mm · Al 0.4–3.0 mm |
| Metal Corrugated Spiral Pipe (SBHCSP-I/II/III) | Large-diameter industrial round wall | Φ300–Φ3000 mm | Up to 3.4 mm |
| Auto Duct Line (SBAL-V, U-shape) | Rectangular structural outer wall | Max width 1250 / 1500 mm | 0.5–1.5 mm |
| Rectangular Duct Line II (SBAL-II) | Rectangular structural outer wall | Max width 1250 / 1500 mm | 0.5–1.2 mm |
| Stainless Duct Forming | Thin perforated stainless liner | Φ100–Φ500 mm | 0.05–0.1 mm stainless |
| Flexible Duct Insulation Machine | Insulated flexible duct (counterpart) | Φ100–Φ600 mm | Insulation 25–100 mm |
| Lockformer · TDF flange · Spiral flanging | Seam & flange closure | — | — |
Source: SBKJ Product Catalog 2026, manufacturer nameplate specifications. Round duct conforms to EN 1506 / SMACNA round-duct construction; rectangular duct to SMACNA / AS‑NZS 4254. The perforated liner gauge and open area are set by the project acoustic and hygiene specification. Insulation material is sourced separately by the customer.
Standards that govern double-wall duct
Double-wall and lined-duct construction is referenced across the major construction codes. SMACNA HVAC Duct Construction Standards — Metal and Flexible details liner attachment, double-wall make-up and the gauge tables by pressure class. AS/NZS 4254 governs ductwork construction for Australian projects and is cross-referenced in our AS/NZS 4254 reference. EN 1506 covers round metal duct dimensions, and the acoustic test methods are ASTM E477 (attenuator insertion loss) and ASTM C423 (sound absorption / NRC). Specify the seal class, the liner gauge and open area, and the absorber type against these standards rather than against a generic description.
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FAQ
What is double-wall (double-skin) HVAC duct?
A three-layer duct: a perforated inner liner facing the airstream, an insulation core, and a solid outer wall that carries the pressure and structural load. The perforations let sound pass into the insulation to be absorbed, while the same core gives the thermal and condensation control of an insulated duct. The absorber is captured between two metal skins instead of glued to the inside.
Where is double-wall or acoustic duct specified?
Plant and fan rooms near the air handler, hospitals and operating theatres where shed fibre is unacceptable, recording studios and performing-arts spaces with low noise targets, clean-air and washdown systems, and high-velocity runs where a glued liner would erode. General supply and return runs typically stay single-wall with external insulation.
How is double-wall duct fabricated?
Round: a spiral tubeformer lock-seams a perforated strip into the inner liner, insulation is wrapped over it, and a second pass forms the solid outer tube. Rectangular: an auto duct line forms the outer wall, insulation is laid in, the perforated inner skin is set on standoffs to hold the air gap, and the assembly is closed with a Pittsburgh lock seam or TDF flange. The liner is pre-perforated coil or is punched/plasma-cut before forming.
What gauge is the perforated inner liner?
Lighter than the structural outer wall, because it carries no pressure load. Galvanised is general-purpose; stainless is used for washdown and hygiene. SBKJ spiral formers run galvanised from 0.4 mm and the stainless former runs thin stainless from 0.05 mm, so the gauge is set by the specification, not the machine. Confirm the exact perforation open area and gauge against the project.
How does double-wall duct attenuate noise?
Sound energy passes through the open area of the perforated liner into the insulation, where it is absorbed rather than reflected. Attenuation per metre of run is modest; discrete attenuators near the fan provide most of the insertion loss. Insertion loss is verified to ASTM E477 and absorption to ASTM C423 — the acoustic lining reference covers the design numbers.
Which SBKJ machines fabricate insulated and double-wall duct?
Round liners and outer walls on the spiral tubeformer (SBTF, Φ80–Φ2500 mm) and the corrugated spiral machine for large industrial duct; rectangular double-skin on the SBAL-V (0.5–1.5 mm) and Rectangular Duct Line II (0.5–1.2 mm); thin perforated stainless liners on the stainless former; closure by lockformer, TDF flange or spiral flanging; insulated flexible duct on the flexible duct insulation machine (Φ100–Φ600 mm, 25–100 mm insulation). Insulation material is sourced separately.
