Leaky Locations Joints between duct sections, at their connection to the plenum, and the plenum itself are common leak locations—especially when runs are installed in tight spaces and required to weave around structural elements. Researchers at Lawrence Berkeley National Laboratory (LBNL) found that duct tape almost always peeled away as its glue degraded. Panduit straps (large zip ties) connecting flexible duct sections can become loose or tear a section to create and expose leaks.
Insulate the Spaces - Consider insulating the areas where ducts run, such as “cathedralizing” the underside of an unfinished attic roof deck with an open-cell foam or lining the foundation with rigid foam panels. Doing so will reduce the temperature (and pressure) differences between conditioned and unconditioned spaces to lessen the potential for leaks, as well as moderate ambient heat (in the attic, especially) that might degrade duct tape at connection joints.
Block the Leaks - Among several methods, consider a pressurized aerosol sealant. Block all return and supply-air registers, connect the machine to the plenum, and force vinyl acetate adhesive particles through the ducts. The particles naturally seek out leaks to escape, but instead adhere, build up, and cure at those locations until the gaps are sealed from the inside. The process costs an estimated $300 to $1,800 and purports to reduce duct leakage by up to 90 percent, assuming a good initial duct installation job.