Why seal ducts? Research proves that typical systems lose 20-30 percent of their capacity and efficiency due to duct leaks, some much more. Duct leakage brings in significant summertime humidity in hot humid climates, contributing to mold growth, higher utility costs, and discomfort. A leaky duct system can't be balanced properly. Duct leakage also brings in dusty and potentially contaminated air from attics, garages, and crawlspaces. In some cases, duct leaks lead to CO poisoning or building rot.
Sealing and renovating ducts isn't as exciting as installing the latest high-efficiency equipment or IAQ accessory, but if pre-existing leaks are ignored, that new product simply won't deliver its potential. As Pacific Gas and Electric (PG&E) recently stated in a consumer protection booklet, it doesn't make sense to install a new, energy-efficient heating and/or air conditioning unit unless the duct system is also energy efficient.