A friend's 1960's era home's crawlspace has rigid steel air ducts. They are insulated with what looks like fiberglass with a layer of cardboard on top. The insulation is coming off in a few places. In some other places the cardboard (if that is what it is) is deteriorated somewhat.
There are no air leaks according to the local duct contractor. He says it would be too difficult to replace the damaged or deteriorated insulation. His suggestion is to fix the damaged areas on the main ridgid trunk line and replace the supplies with flex duct.
It looks to me that the worst of the damaged supply areas could be fixed. Also, the fact that there are no leaks in the existing system would lead me to keep what is there.
What would you suggest in this situation?