From the 2021 IRC

Duct Material

Per Section M1502.4

  • The exhaust duct shall be made of rigid metal that has a smooth interior finish and not be less than 0.0157 inches thick (No. 28 gage)
  • The exhaust duct shall be at least 4 inches minimum in diameter.
  • Transition ducts shall not be more than 8 feet in length and shall be in accordance with UL 2158A.
  • Booster fans are prohibited.
  • Exhaust ducts shall be supported at intervals not exceeding 12 feet in length.

Transition Duct VS Exhaust Duct

A transition duct is used to connect the dryer to the exhaust duct. Transition ducts are typically flexible ducts that are constructed of metal foil fabric or plastic on a spiral wire frame. This duct must be listed and labeled UL 2158A.

An exhaust duct is the actual exhausting system that connects from the transition duct to the termination point (outside). It must be a metal smooth-wall duct. Sections must not be fastened with screws.

Exhaust Duct Length

Per Section IRC M1502.4.6

  • The maximum length of the exhaust duct shall not exceed 35 feet from the transition duct connection to the outlet termination point, but... the maximum length of the duct shall be reduced where fitting are used.

The code provides a reduction table: