It is basically a metal duct, treat it like one. I would check the interior duct liner (insulation) to see it it is has signs of water damage. Wet insulation inside can cause/allow condensation on the outside. If it is dry, reducing the R/H in the crawl or adding insulation would help keep condensation from forming. If you keep it dry, it does not have to dry out...

__________________
Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002