Mr PEX Pre-Insulated PEX Tubing for outdoors Wood Boilers and other outdoor insatllation where you want hot water to move from one location to another. Mr PEX line of Superior Quality Insulated Underground Supply. Mr PEX offers to lines of Insulated PEX Tubing in various thickness of insulation and either 1 or 2 pipes. PEX-A is the type of PEX Tubing used.