I have central air with ducts running through the hot, uninsulated attic (there is blown-in insulation between the joists to insulate the rooms below). Fortunately it's only about 25 feet of ductwork, but it's a super key length!
The ducts are the standard insulated ducts--about R-8--which seems far too little insulation for the temperature differential between the cold air inside and the hot air outside. So I want to add more insulation to the ducts.
They run across the top of the ceiling joists (perpendicular) which makes it slightly more tricky.
What I'm struggling with is how to balance the importance of an air-tight seal vs. a thick, circular seal--let me explain with two contrasting approaches.
Approach 1 Buy fiberglass insulation batts and wrap them like a spiral around the ducts. There will be a lot of gaps.
Approach 2 Use rigid foamboard to create a box around the ducts, and tape the corners airtight. This will be more airtight but will leave an air-gap between the duct and the insulation.
I suppose another approach would be to buy some pre-insulated ductwork that is large enough to completely envelop the smaller ducts, so I'd get a perfect air seal plus a circular insulation, but this would lead to a lot of potential confusion for future contractors, and would probably be needlessly expensive.