I was poking around under the house yesterday, and noticed that the main duct coming out of the furnace(which is in the garage, just adjacent to the house), is not insulated at all. The 6"-10" ducts that split off from it are insulated quite nicely, but there's this ducting, about 18" in diameter and 4 or 5 feet long, that's just bare metal. Home Depot didn't have duct-wrapping material; the closest they have is a roll of R-13 fiberglass, with a paper facing. Could I wrap that around that big duct and tape it in place, with the facing out? Or is that duct uninsulated for a reason?