Little changes can make a big difference.
Sealing the ducts with mastic is the single most important thing that anyone should do.

Austin Energy estimates that the duct leakage
in new home construction averages 10%, and in older homes closer to 30%.

Just sealing the ducts on existing homes could reduce the need for new power plants. Sealing gets leakage rates down to about 2%

Not doing duct sealing on all new construction is very short sighted. Mastic will last the life of the system, while conventional duct tape
can fail within a year.