I have a small ski cabin in upstate New York (cold climate) that we only visit on weekends. It is heated by a wood stove. During winter we had many problems with bursting water pipes so they are now all replaced with rubber except in the bathroom upstairs where I also have my water tank. The bathroom walls and the ceiling are well insulated but not the floor underneath which is only double 3/4 inch sheathing.
During the week the temperature in the bathroom is kept above the freezing point by a small electric heater controlled by a thermostat. I was planning to staple or nail solid foam insulation between the floor joists underneath the bathroom only to save on electricity when I stumbled on this site and the subject of vapor barriers.
Do I need one and if so, on what side of the 1" styrofoam insulation (pink stuff)? There is no ceiling drywall underneath, only suspended ceiling panels. So you now see that my cold side of the insulation will be toward the living room below during the week and to a lesser degree toward the floor above during the winter weekends.
My intuition tells me that I should not be using any barrier at all. Am I correct?