Our contractor ran 3/8 PEX up through the floor to several radiators. Exposed to daylight from the windows, the PEX needed protection from the ultra violet. I found a solution by sleeving the PEX with pre-slit, flexible plastic conduit used for organizing wiring to electronics and automobile engines. It’s available in several diameters, slips neatly over the PEX, but all I find locally is black at Lowes and Advance Auto.