I currently have uninsulated HVAC ductwork running the width of my garage. There's no gap between the ducts and the plaster ceiling above it. There's also very little clearance between the ducts and vehicles.
Based on some recent plumbing work I've done, I doubt there is any insulation between the garage plaster ceiling and the living space above the garage.
Most guides I've reviewed start with the assumption there is a gap where insulation can be wrapped entirely around the duct. Since that really isn't an option here, how can I successfully insulate them ?