First let me start by saying I am not in the HVAC business. I am the owner of a home built in the 50's. We have the original galvanized sheet metal ducts in our basement for the HVAC. It was suggested (by a moisture control co.) to have the ducts looked at--some needed to be repaired. So far, we have gotten 4 estimates--2 companies say we need to tear out all the old ducts because they are metal and the seams will always leak. The other 2 co. say that the old metal ducts are the best there is for air flow and not to take them out. They suggested sealing the seams with mastic. Is there a benefit of 1 type of duct over the other? We have not had any problems w/ our system (the house heats and cools efficiently). The co. only found one duct that needs repair. Any suggestions would be greatly appreciated!