Leaky ductwork means your conditioned air is going to another space in your building or home - instead of where it's supposed to go. This can be expensive if your heated air is leaking into your ceiling or wall space. Yes, some of the heat will find it's way back into the space it was intended for, but why heat (or cool) these interstitial spaces? Another problem with leaky ductwork is on the return side. Think about the lovely spaces through which the return ducts run. Now imagine the air, particles, spores and fibers from these dark dingy spaces being drawn into the return duct through the leaks. Not a pretty picture as all this stuff starts recirculating through your home, school or work area. Sealing leaks can be challenging since typically not all the ductwork is accessible. I like the product offered by Carrier Corporation - called AEROSEAL It seals leaks from the inside - amazing process. Check it out: (www.aeroseal.com).