When pipes go from a cottage down through the crawl space and into the ground, they risk freezing. Simply wrapping them in insulation will not protect them, especially if the wind manages to blow though the fiberglass.
Dig down about a foot where the pipes go into the ground. Remove the insulation above the pipes on the cottage floor. Build a box about 2 or 3 feet square around the pipes with plywood (to block animals and wind) that goes from the cottage floor to the bottom the hole in the ground. Heavily insulate the inside of this box, but leave a little space around the pipes. Close it all in. Now, the heat from the cottage will come down into this box from the heated room above, and be trapped inside this insulated space, together with that plumbing pipe.