Insulating ceiling in garage?

[smike]

I have to break down and insulate the ceiling of my pole barn with metal roof. Gets to cold to work out there. I am going with r-30 batts above the ceiling joists. I was going to put up a 6 mill vapor barrier on the bottom of the joists. Also it will help hold the insulation. Is the vapor barrier nesessary in a garage? Will it hurt if the insulation is 6" above the vapor barrier when I go over the joists? Do you think r-30 is an over kill?

 

[Jbullfrog]

You have to have the vaper barrier if you are going to heat it and don't want it to rain. The insulation needs to be against the plastic with no air gap or you will defeat the point of having insulation. In my shop I put 2x4s between the trusses on 4' centers. I tab nailed the vapor barrier and then screwed used steel to the 2x4's. The exhisting holes in the steel are benificial in disipating noise, so used is actually better then new for a ceiling. Once the ceiling is there, blow in 13" of fluff and your golden.