I have an unusual situation and I'm wondering what I should do regarding a vapor barrier.
I have a steel building in the midwest. The building is a steel building with R-13 insulation sandwiched between the steel framing and the outside tin. The plastic coating of the insulation is visible on the inside of the building. Obviously, where the insulation is pinched at the horizontal steel frames, the R-value is considerably less than R13.
I'm building a workshop along the back wall of the building with a loft apartment above. The walls will be of 2x4 construction and I plan on insulating them with R-13. Although the building itself will be heated just enough to prevent freezing (say 40 degrees F), the shop and apartment will be heated to a warmer temperature.
The back wall of the shop/apartment will not be touching the insulation on the back wall of the building. There will be a gap of between 2" and 5 1/2" between the back of the studs and the building insulation vapor barrier. I was going to use R13 Kraft-faced in the shop/apartment walls with the Kraft face on the inside of the shop/apartment.
My buddy (whose family used to sell insulation, although I would by no means consider him an expert) said that if I did that, I would create a double vapor barrier because of the Kraft face, and the plastic face of the building insulation. He said I could get moisture/mold problems in my studded wall and suggested using no VB at all.
I've already got the R-13 Kraft-face on site, and now I'm not sure what to do. There is a 2-5.5" gap, that may allow the use of the Kraft, but I don't think that there would be a lot of airflow back there, so I wasn't sure. I worry about having a heated shop/apartment without any vapor barrier at all.