603.9 Joints and Seams of Ducts — 2025 Mechanical Code (CMC)

Joints and seams for duct systems shall comply with SMACNA HVAC Duct Construction Standards – Metal and Flexible, SMACNA Round Industrial Duct Construction Standards, or SMACNA Rectangular Industrial Duct Construction Standards, as applicable. Joints of duct systems shall be made substantially airtight by means of tapes, mastics, gasketing, or other means. All ducts shall be sealed to Seal Class A. Crimp joints for round ducts shall have a contact lap of not less than 1 1/2 inches (38 mm) and shall be mechanically fastened by means of not less than three sheet-metal screws equally spaced around the joint, or an equivalent fastening method.

DUCT SYSTEMS

603.9.1 Closure Systems. Joints and seams and reinforcements for air ducts and plenums listed and labeled to UL 181, shall be in accordance with the manufacturer’s installation instructions. Closure systems for sealing air ducts and plenums shall be listed and labeled in accordance with UL 181A or UL 181B, and marked in accordance with Table 603.9.1.

TABLE 603.9.1

CLOSURE MARKINGS

TYPE OF DUCTWORK	STANDARD	TYPE OF CLO- SURE SYSTEM	MARKING
Rigid Metallic or Rigid Fiberglass	UL 181A	Pressure Sensitive Tape	181A-P
Rigid Metallic or Rigid Fiberglass	UL 181A	Mastic Tape	181A-M
Rigid Metallic or Rigid Fiberglass	UL 181A	Heat Sensitive Tape	181A–H
Flexible Air Ducts	UL 181B	Pressure Sensitive Tape*	181B-FX*
Flexible Air Ducts	UL 181B	Mastic*	181B-M*

Mechanical fasteners shall be used in conjunction with a listed pressure sensitive tape or mastic in accordance with UL 181. Nonmetallic mechanical fasteners shall be listed and labeled in accordance with UL 181B and

labeled “181B-C.”

603.9.2 Duct Leakage Tests. [Not adopted by HCD] Ductwork shall be leak-tested in accordance with the SMACNA HVAC Air Duct Leakage Test Manual. Duct leakage tests shall be performed by a technician certified by the Associated Air Balance Council (AABC), the National Environmental Balancing Bureau (NEBB), the Testing, Adjusting and Balancing Bureau (TABB), or other equivalent approved agencies. Representative sections totaling not less than 10 percent of the total installed duct area shall be tested. Where the tested 10 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork. The permitted duct leakage shall be not more than the following:

Lmax = CLP [0.65 ] (Equation 603.9.2)

Where:

Lmax = maximum permitted leakage, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface

area.

CL = six, duct leakage class, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface area at 1 inch water column (0.2 kPa).

P = test pressure, which shall be equal to the design duct pressure class rating, inch water column (kPa).

Exception: Transfer air duct operating at less than 1 inch of water column (0.25 kPa). 603.9.2.1 Duct Leakage Tests for Buildings that Meet Air Distribution System Duct Leak- age Sealing Criteria in Title 24, Part 6. For duct leakage testing, see California Energy Code Sec- tions 150.0(m)(11) for single family buildings, Sec- tion 160.3(b) for multifamily buildings, and Section 120.4(g) for nonresidential and Hotel Motel build- ings.