5m of exposed ducting? That seems an incredible amount. Just out of interest what make and model of caravan do you have?
I doubt the ducting is actually unprotected. It will most likely have a length of DR ducting with the standard UR ducting inside. The DR ducting is plasticised and so is water proof, and simply by having a second skin, that will reduce heat loss quite considerably especially as teh outers skin is less thermally conductive. So there is a little to be gained by extra insulation, but not as much as you might imagine.