Section 5.1.1/Residential Duct Requirements
NFPA 90B Section 5.1 covers duct construction, materials, and installation for residential warm air heating and cooling systems.
Ducts for residential systems must be constructed of metal, duct board, or flexible duct listed for the application. Metal ducts must be a minimum 26 gauge for round ducts up to 14 inches and thicker for larger sizes. Flexible duct must be listed per UL 181 and limited in length (typically no more than 25 feet with minimal bends). All duct connections and joints must be sealed with mastic or listed tape. Ducts in unconditioned spaces must be insulated to the applicable energy code R-value.
Why this section exists
Residential ductwork operates at lower pressures than commercial systems but still must maintain integrity over the life of the building. Leaky duct connections waste energy by losing conditioned air into unconditioned spaces. Excessive flexible duct creates high friction losses that reduce system airflow and comfort. Uninsulated ducts in attics and crawl spaces gain or lose significant heat, reducing system efficiency.
What plan reviewers look for
Plan reviewers check duct material specifications, flexible duct lengths, and insulation R-values on the mechanical plans. They verify that duct sealing is specified (mastic or listed tape, not standard cloth duct tape). For energy code compliance, they check that all ducts in unconditioned spaces are insulated to the required R-value.
Common violations
Related NFPA 90B requirements
Section 4.1 covers equipment installation. NFPA 90A covers commercial duct requirements. UMC Section 504 covers duct construction in the Uniform Mechanical Code. The IECC covers duct insulation requirements by climate zone.