MRC 2015
N1103.3.2 (R403.3.2) Sealing (Mandatory).
Ducts, air handlers and filter boxes shall be sealed. Joints and seams shall comply with either the International
Mechanical Code or Section M1601.4.1 of this code, as applicable.
Exceptions:
1. Air-impermeable spray foam products shall be permitted to be applied without additional joint seals.
2. For ducts having a static pressure classification of less than 2 inches of water column (500 Pa), additional closure
systems shall not be required for continuously welded joints and seams, and locking-type joints and seams of other
than the snap-lock and button-lock types.
M1601.4.1 Joints, seams and connections.
Longitudinal and transverse joints, seams and connections in metallic and nonmetallic ducts shall be constructed as
specified in SMACNA HVAC Duct Construction Standards—Metal and Flexible and NAIMA Fibrous Glass Duct
Construction Standards. Joints, longitudinal and transverse seams, and connections in ductwork shall be securely
fastened and sealed with welds, gaskets, mastics (adhesives), mastic-plus-embedded-fabric systems, liquid sealants or
tapes. Tapes and mastics used to seal fibrous glass ductwork shall be listed and labeled in accordance with UL 181A
and shall be marked “181A-P” for pressure-sensitive tape, “181 A-M” for mastic or “181 A-H” for heat-sensitive tape.
Tapes and mastics used to seal metallic and flexible air ducts and flexible air connectors shall comply with UL 181B
and shall be marked “181 B-FX” for pressure-sensitive tape or “181 BM” for mastic. Duct connections to flanges of air
distribution system equipment shall be sealed and mechanically fastened. Mechanical fasteners for use with flexible
nonmetallic air ducts shall comply with UL 181B and shall be marked 181B-C. Crimp joints for round metallic ducts
shall have a contact lap of not less than 1 inch (25 mm) and shall be mechanically fastened by means of not less than
three sheet-metal screws or rivets equally spaced around the joint.
Closure systems used to seal all ductwork shall be installed in accordance with the manufacturers’ instructions.
Exceptions:
1. Spray polyurethane foam shall be permitted to be applied without additional joint seals.
2. Where a duct connection is made that is partially inaccessible, three screws or rivets shall be equally spaced on the
exposed portion of the joint so as to prevent a hinge effect.
3. For ducts having a static pressure classification of less than 2 inches of water column (500 Pa), additional closure
systems shall not be required for continuously welded joints and seams and locking-type joints and seams of other than
the snap-lock and button-lock types.