Section 601.1/Duct Construction and Installation
IMC 601 and Chapter 6 cover duct system construction, materials, sizing, support, sealing, and insulation requirements for HVAC distribution systems.
IMC Chapter 6 establishes requirements for duct construction and installation. Section 601.2 requires ducts to be constructed of approved materials, with metal ducts conforming to SMACNA duct construction standards for the applicable static pressure class. Section 601.3 requires duct sizing to deliver the required airflow at the design friction rate and static pressure without exceeding the noise criteria for the space. Section 601.4 requires ducts to be supported at intervals per the SMACNA standards, with hangers and supports of adequate strength for the duct weight plus insulation. Section 603.1 requires duct joints, seams, and connections to be made substantially airtight, with sealant, gaskets, or approved tape. Section 604.1 requires supply and return ducts in unconditioned spaces to be insulated to a minimum R-value per the energy code. Flexible duct connections are limited to 14 feet in length per Section 603.5.
Why this section exists
Duct systems distribute conditioned air throughout a building and represent a major portion of the HVAC energy budget. Leaky ducts in unconditioned spaces waste 20 to 30 percent of heating and cooling energy. Improperly sized ducts create noise complaints, comfort problems, and excessive fan energy consumption. Duct materials must resist fire spread, which is why plenum requirements and fire damper provisions are integrated into duct construction standards. The flexible duct length limit prevents excess pressure drop from kinked or oversized flex runs.
What plan reviewers look for
Plan reviewers check duct sizing on the mechanical drawings against the design airflows shown on the equipment schedule. They verify duct insulation R-values on ducts routed through unconditioned spaces. They check that flexible duct lengths do not exceed 14 feet and that rigid duct is specified for all main trunk lines. They verify fire dampers are shown at all fire-rated wall and floor penetrations. They check that the duct construction pressure class is specified and matches the system static pressure. They verify duct sealing class (Seal Class A, B, or C) is noted per the energy code requirements and ASHRAE 90.1 duct insulation standards.