Duct mastic (RCD) is applied to a joint with a brush. Mastic remains flexible and holds a seal as the duct expands and contracts.
All joints and connections of ducts in non-conditioned space should be sealed with mastic. (Never use standard duct tape; it does not last.)
Fiberglass mesh tape is embedded in the mastic around the joint for added reinforcement.
Another coating of mastic should be applied over the mesh tape for a long-lasting seal.
The LSU AgCenter and the LSU College of Agriculture