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The average homes ductwork leaks out 10-30 percent of the air through joints, seams and holes instead of making it out the designed areas or registers.
If your ductwork is located inside the home envelope this is not really an issue.
When this air leaks out, your system is going to run longer to satisfy the desired temperature. Heated or cooled air is escaping into unconditioned spaces such as the attic or crawlspace resulting in wasted energy.
If your ductwork is located in the crawlspace or attic, you can definitely benefit from a sealed duct system.
The seams where the ductwork is pieced together is not airtight but often times, there are even larger gaps resulting in even more lost air and lower system efficiency.