Where to place vapour barrior

My house is located in a cold climate and the usual rule is to place the vapor barrier on the inside of the insulated walls. I am adding radiant heating on a thin slab with insulated joist below. The floor sheathing and flooring acts as the vapor barrier to the warm heated side, but what do you do at the outside walls where the joists are located?
Do you put vapor barrier on the rim joists/blocking, put nothing, just air barrier, only caulking? Or do you put insulation next to the rim joist, then vapor barrier and then insulation in the joist space?
Thanks, R
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