I was told that I shouldn’t use the insulation that I picked up at the plumbing supply house. Is that right?
There are a number of different types of pipe insulation on the market. The most common type is Polyethylene insulation. This is a semi-rigid foam insulation that is similar to the swimming noodles that kids use in the pool. Frequently, water heaters will come supplied with 2-24” long pieces in the box. Polyethylene insulation is found at most hardware and plumbing supply stores. Polyethylene insulation should not be used on solar heating systems!!! Polyethylene insulation melts at 160°F – 180°F.
A more appropriate choice is Rubetex insulation. Rubetex (or other brands) rubberized insulation can be found at HVAC supply stores. Rubetex can be identified since it is much more flexible than Polyethylene with a higher melting temperature. The high temperature limit of standard rubetex insulation is 230 degrees which is suitable for most solar installations. If your installation has higher temperature needs then Rubetex has a higher temperature version that has a high limit of 300 degrees. Some installers prefer fiberglass insulation, which also works fine at the high temperatures.
Keep your installation looking good by choosing the right insulation for the job.