What's the best way to seal a 30-year-old AC duct system in an attic?
It's a rigid system with some tape joints leaking. We want to minimize any leaks in the ductwork.
Mastic is always the best way to seal ductwork. You can get it at any HVAC supply house. Be generous with the application of the mastic around all joints, seams and connections.
If you can't reach parts of the ductwork and it's leaky at these inaccessible places, you can use Aeroseal -- which you'd have to hire someone to install for $1,500 to $2,000.