9.1 Duct Materials, Construction & Factory-Made Ducts

Duct Construction & Installation is 4 percent of the M2. Duct & Plenum Use is 2 percent. Together they are the non-damper half of Duct Systems (12 percent); fire and smoke dampers take the other 6 percent in the next chapter. Inspectors fail jobs here because a residential flex coil was hung as a commercial trunk, or because someone used Table 603.4 dwelling-unit gauges on a 3-inch-w.c. office main.

2024 IMC 601.1 is the scope sentence. Duct systems used for the movement of air in air-conditioning, heating, ventilating, and exhaust systems shall conform to this chapter except as otherwise specified in Chapters 5 and 7. Kitchen grease ducts, clothes-dryer exhaust, and hazardous or product-conveying exhaust stay in Chapter 5. Combustion-air ducts stay in Chapter 7. The only 601.1 exception is a duct that discharges combustible material directly into a combustion chamber, which goes to NFPA 82. Do not inspect a Type I grease riser with Chapter 6 flex and mastic rules.

This section is the materials companion to heating equipment, furnace connections, and 601.5 return locations. Fire-rated wall, floor, and shaft penetrations are not solved in 603 — they are pointed to Section 607 and taught with fire dampers.

Metallic ducts: SMACNA 006, not a photocopied house table

2024 IMC 603.3 classifies ducts by maximum operating pressure at positive or negative 0.5, 1.0, 2.0, 3.0, 4.0, 6.0, or 10.0 inches of water column. The pressure classification shall equal or exceed the design pressure of the air distribution in which the ducts are used. Shop drawings that say “standard gauge” with no pressure class are incomplete.

2024 IMC 603.4: Metallic ducts shall be constructed as specified in SMACNA 006: HVAC Duct Construction Standards—Metal and Flexible. The only exception is ducts installed within single dwelling units, which may use Table 603.4. That table is a house table. The moment the run is commercial, SMACNA 006 controls thickness, reinforcement, and joint type for the 603.3 pressure class. Table 603.4 footnote a is the same idea inside a dwelling: ductwork that exceeds 20 inches by dimension or exceeds 1-inch water gauge shall be constructed to SMACNA 006. Do not approve 30-gauge “builder duct” on a 24-by-12 VAV main because someone printed the dwelling table.

If a stem actually is a dwelling unit, Table 603.4 is the number set. Round ducts smaller than 12 inches at ½-inch water gauge are 0.013-inch galvanized or 0.018-inch aluminum. Nineteen- to 20-inch round at 1-inch water gauge is 0.024-inch galvanized or 0.034-inch aluminum. Aluminum is thicker than galvanized in every cell. Those cells are still not a commercial specification.

2024 IMC 603.4.1: Round metallic ducts shall be mechanically fastened by not less than three sheet metal screws or rivets spaced equally around the joint. Exception: a partially inaccessible connection still needs three fasteners equally spaced on the exposed portion so the joint cannot hinge. 603.4.2: Crimp joints for round and oval metal ducts shall be lapped not less than 1 inch, and the male end shall extend into the adjoining duct in the direction of airflow. A crimp pointed upstream, or two screws on the bottom of a round joint, is a 603.4 fail before you ever talk about mastic.

603.1 still requires the installed system to supply the required air distribution, to leave International Building Code fire protection intact, and to be constructed, braced, reinforced, and installed for structural strength and durability. A crushed oval, a trunk sitting on a bar joist with no reinforcement, or a fire-rated corridor wall turned into a sheet-metal chase without 607 protection fails 603.1 even if the gauge looks familiar.

Factory-made ducts: UL 181 Class 0 or Class 1

“Factory-made” on an M2 stem means a listed product, not a shop-fab rectangular main. 2024 IMC 603.5: Nonmetallic ducts shall be constructed with Class 0 or Class 1 duct material and shall comply with UL 181. Fibrous duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or the NAIMA Fibrous Glass Duct Construction Standards. The air temperature within nonmetallic ducts shall not exceed 250°F.

UL 181 Class 0 is a surface-burning classification of zero. Class 1 is the common factory-made flex and fibrous product: flame spread not more than 25 and smoke-developed not more than 50. An unlabeled roll of insulation wrap used as a duct is not Class 0 or Class 1. Fiberglass duct board is legal only as a fibrous-glass system built to SMACNA or NAIMA, listed as Class 0 or 1, and used inside the 250°F cap.

603.5.2: Nonmetallic phenolic ducts shall be constructed and installed in accordance with the SMACNA Phenolic Duct Construction Standards. Phenolic is a factory-made rigid foam-panel system. It is legal when it is built to that standard and listed for the pressure class. It is not a grease duct, a dryer exhaust, or a substitute for SMACNA metal on a high-pressure riser the listing does not cover.

Flexible air ducts versus flexible air connectors — confirm the 2024 length rules

2024 IMC 603.6 splits two UL 181 products that look identical above a ceiling.

Flexible air ducts (603.6.1), metallic or nonmetallic, shall be tested to UL 181, listed and labeled as Class 0 or Class 1 flexible air ducts, and installed in accordance with 304.1 (listing plus manufacturer’s instructions). 2024 IMC 603.6.1.1: Flexible air ducts shall not be limited in length. That is the 2024 sentence. Folklore about a 5-foot or 14-foot “flex duct” cap is connector law, not duct law.

Flexible air connectors (603.6.2) are also UL 181 Class 0 or Class 1 and also installed per 304.1. 603.6.2.1 limits connectors to 14 feet (4267 mm). 603.6.2.2: Flexible air connectors shall not pass through any wall, floor, or ceiling.

Read the printed listing. AIR DUCT and AIR CONNECTOR are different marks. A 40-foot run of listed flexible air duct can satisfy the length rule. A 16-foot run of product labeled AIR CONNECTOR is a fail. Both products stay below 250°F (603.6.3) and keep the appliance clearance in the manufacturer’s instructions (603.6.4). 607.7 adds a fire-assembly rule that applies to both products: flexible ducts and air connectors shall not pass through any fire-resistance-rated assembly.

Field contrast — residential flex used as a long commercial trunk. A listed Class 1 flexible air duct hung as a short low-pressure run to a diffuser can satisfy 603.6.1.1. The same coil used as the 3-inch-w.c. commercial trunk, crushed through a stud bay, pulled through a fire-resistance-rated corridor wall, or installed without the manufacturer’s supports, still fails. Commercial pressure class (603.3) and SMACNA metal construction (603.4) still apply to the mains the flex is attached to. A house-supply flex coil is not a designed commercial duct system.

Gypsum, combustible limits, underground, corrosion

2024 IMC 603.5.1: The use of gypsum boards to form air shafts (ducts) shall be limited to return air systems where the air temperatures do not exceed 125°F and the gypsum-board surface temperature is maintained above the airstream dew-point temperature. Supply-air ducts formed by gypsum boards shall not be incorporated in air-handling systems utilizing direct evaporative cooling. A gypsum supply shaft on a swamp-cooler, or a gypsum return next to a 140°F equipment room, is the wrong material. Distinguish gypsum ducts (return-only under 603.5.1) from gypsum plenums under 602.2 — that construction rule is the next section.

Combustible-duct limits are the Class 0 / Class 1 rule plus the underground-only rule for plastic. 603.8.3: Plastic ducts shall be constructed of PVC having a minimum pipe stiffness of 8 psi at 5 percent deflection when tested to ASTM D2412. Fittings shall be PVC or high-density polyethylene. Plastic duct and fittings shall be utilized in underground installations only. The maximum design temperature is 150°F. A PVC “duct” hung in a mechanical room is not an underground installation.

2024 IMC 603.8: Ducts shall be approved for underground installation. Metallic ducts not having an approved protective coating shall be completely encased in not less than 2 inches of concrete. 603.8.1: minimum slope of ⅛ inch per foot to a point provided with access. 603.8.2: sealed, secured, and tested prior to concrete encasement or direct burial, and leak tested as required by IECC Section C403. 603.14: Ducts shall not be installed in or within 4 inches of the earth, except where they comply with 603.8.

Corrosion is not a standalone IMC gauge table. It is the 603.8 coating-or-2-inch-concrete rule underground, the 603.16 weather-protection rule for exterior ducts (including linings, coverings, and vibration-isolation connectors), and the listing’s coating for coastal or chemical air. Uncoated spiral in wet soil, or an exterior duct with torn insulation and no weather barrier, is the field stop.

603.17 air-dispersion (fabric-sock) systems shall be installed entirely in exposed locations, used only under positive pressure, shall not pass through fire-resistance-rated construction, and shall be listed and labeled to UL 2518.

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