Our home is approximately 1600 square feet an is located in Missouri. It is approximately 50 years old. We are currently replacing the HVAC system as well as running new insulated ductwork in the attic of our home. It originally ran in the slab, and is extremely corroded and rusted beyond repair.

My question is, what is the best way to fill/insulate the old ductwork in the slab?

Thanks,

AWeck