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?

3 Answers 3


You are correct. The field of building science is gradually moving away from friendliness toward vapor barriers in exterior walls, and you certainly should not put one in an interior wall or floor. Not a great idea.

In addition, it would make more sense to use batt or spray foam insulation between the floor joists as opposed to cutting rigid foam to fit in those irregular spaces, which would be a nightmare. Finally, most foam insulations are themselves vapor barriers, so if you use foam, you will be creating much of a vapor barrier anyway. For this reason, ideally you would use mineral wool batts (sold under the name "Roxul" in the USA) in the floor joist bays, as they are highly-insulating and vapor-permeable. As an added bonus, they block sound very well too--useful for insulation between floors of a house. I am re-insulating my house with this stuff and just love it. Unlike fiberglass, mineral wool batts are not moisture sensitive, easy to work with, and highly effective.


You are correct: this is a situation where you want no vapor barrier at all, not even the insulation.

However, your better bet may working with the plumbing, so you don't need a heater, don't pay for that electricity, and don't need to worry about power outages. I assume when you said "rubber pipes" you meant plastic like PEX, but even then you face the issue of defrosting them prior to use.

The place could be re-plumbed with all pipes sloped to a central drain. Drain the system before you leave, or even install a freeze sensitive automatic system.


I had a client's weekend home completely freeze with pex lines we installed. Nothing broke.

Unfaced batt insulation is the fastest, cheapest stuff. Use Ecobatt if you can find it, you can sleep in that stuff, no itching or irritation.

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