AskCodeMan

I plan to run copper supply lines under the sub-floor in a bathroom that is built upon 2x10 laid upon a slab foundation. This is in Minnesota and although it gets cold the supply lines will be 8 ft from an outside wall. I don't know if the slab is not insulated. Also, this is the current plumbing scheme and I am just moving some of the supply lines into a different configuration. Does the code speak to laying supply lines in unconditioned space?

Yes, the code addresses, in various places, protection from freezing for water piping, including drain/waste/vent piping, and vent piping typically has little or no water in it but could frost up.

It the piping is not within the thermal envelope of the building, the requirements are that the piping be 12 inches minimum below grade or at least 6 inches below frost level (whichever is a greater depth), or be protected from freezing with insulation or heat tape.

The above is what the IRC calls for, I do not have your particular code with me, but you may be able to find it online. Regardless, when piping is subject to freezing temperatures the water inside the piping will be subject to freezing, and frozen water not only expands and bursts pipes, but frozen water does not flow and thus clogs the pipes up, including vent piping.

So, if the floor joist bay was packed with fiberglass insulation, would that satisfy code? Even though this is not in the thermal envelope there must be residual heat below the sub-floor as the existing supply lines never froze.

If you ran the plumbing piping alongside the floor joist, or through the floor joists at a height sufficient to allow for the minimum required insulation for floors, and had the insulation on the slab below the piping, then you could consider the piping within the thermal envelope of the structure as the insulation for the floor would be below the piping. You would also need insulate sides of the underfloor space.