[johnsjmc]

The type of duct in a crawl space depends a lot on your location. In Canada we always see metal duct (which never wears out here)but in Florida insulated flex duct is much more common.
I just had the heat pump replaced in a Tampa winter home and was told the metal duct rusts away very quickly because of the high local humidity so it,s rarely used there.
I was also told by more than one hvac contracter that ductwork in a crawlspace wears out in about 5-10 yrs. avg life for a heat pump there is 7 yrs . The crawlspace is open to the outdoors with the house raised on piers, and fruit rats and other vermin will eat the ductwork,plus exposure to wind driven rain
I had quotes to replace ductwork for 7 rooms $800-1500. I ended up having the existing repaired for $300, with $150 came from the local electric co.(Progress energy)
I found detailed info on flex duct recommendedon on the net,but didn,t bookmark it.