General Duct Sealing

Select only UL 181 approved materials that:

are compatible with their intended surfaces

allow for differential expansion and contraction between dissimilar materials

meet the requirements of the applicable fire safety code (e.g., thermal or ignition barriers)

Select low volatile organic compound (VOC) sealants for use inside the pressure boundary that meet independent testing and verification protocols

Clear surrounding insulation to expose joints being sealed, salvage for reuse if possible

If duct must be cut open to gain access, position the hole to make repair with appropriate materials feasible

Remove loose debris using a vacuum

Remove any substance that will prevent sealant adhesion (tape, oil, etc.) using appropriate solvent.

Securely fasten all duct connections using appropriate mechanical fasteners according to the SWS detail "Ducts: Mechanical Fastening"

Seal all accessible seams, cracks, joints, holes, and penetrations of duct system

Select method according to physical leak size

Seal leaks less than 1/4" using fiberglass mesh and mastic

Mastic alone is acceptable for holes less than 1/8" in size that are more than 10' from air handler if static operating pressure is less than 1" of Water Column

Seal leaks between 1/4" and 3/4" using a two stage process:

Install temporary tape as a backing material

Seal with fiberglass mesh and mastic that extends at least 1" past the temporary tape on all sides

Repair leaks larger than 3/4" using a rigid duct patch

Mechanically fasten patch before applying mastic

Install fiberglass mesh and mastic over the seam, overlapping repair joint by at least 1" on all sides

Seal gaps between boot and surface connections using fiberglass mesh tape and mastic or appropriate flexible caulking

Ensure sealant is dry before reinstalling the register

Ensure the register can be removed and reinstalled by the dwelling occupant

Seal any joints, cracks, and holes that are not gasketed or weatherstripped and are not needed for proper function or service of the unit using removable sealant (e.g., foil tape, gaskets, etc.)

Seal the filter slot with a durable, client removable filter slot cover (e.g., magnetic strip)