Question-and-Answer Resource for the Building Energy Modeling Community
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I'd say first that this isn't really the appropriate spot for this question, unless, for example, it's about how to enter thermal properties of plastic into EnergyPlus for use in thermal calculations. :-)
But I think to answer your question, it has to do with safety/pressure guidelines, the lifetime of the material, and as with most all decisions, material cost.
Editorial note: I imagine that the plastic might actually help in at least one facet of the thermal behavior of the system. If the pipe was extremely conductive, it is possible that the fluid would radially conduct a significant amount of heat in a very short length of the pipe, reducing the uniformity of the system across the entire floor/wall/ceiling. Having plastic may help dampen the heat transfer in the system to spread out the heat transfer over the entire surface.
