Context: I'm planning to take my self-inflating sleeping pad on a bike trip this year. The bike doesn't have any mounting points for a rack, so space is somewhat limited. Currently, my plan is to simply strap the pad to the handlebars in the manufacturer-provided stuff sack.
Question: Do self-inflating pads need to be protected from rain? Will water from the outside do any damage? I'm wondering whether I should get a dry bag or whether the (not fully waterproof) stuff sack is enough.