I like polyethylene foam, comes in 10 foot long lengths, sized for 1/2 inch up to 2 inch diameter pipe, inexpensive, available at any big box store. I have never seen rubber insulation for hot water pipe. I don't think I have ever seen polypropylene insulation for pipes either. I don't care for fiberglass, it itches, and does not slip on like preslit polyethylene.
Home depot sells rubber pipe insulation in 6 ft sections. It feels like it will do a far better job than the polyethylene (sorry I said polypropylene by mistake) it does cost about $1 per foot but feels like it will hold all the heat in.