It is unfortunately a complex topic and there are a myriad of options. A lot depends on what your intended use is for the building, the climate zone you are designing for, and your budget. I am not an expert and am just sharing the things I *think* that I've learned from going through the process myself over the last few months. The way I'm going is probably on the "high end" of cost and complexity but I'm planning on building out my building to have some living quarters and kitchen and such. If I were building a straight workshop/outbuilding then I might go a different route.
I'm sure lots of people have put up the plastic/vinyl wrapped batts directly under the metal as I believe that is one of the more common insulation methods. You can call the suppliers and maybe check the specs to see if they recommend (or require for warranty) that you ventilate behind it. Seems like that is the system that the metal suppliers are geared for and it would be kind of the bare bones insulation package. I'd keep asking and maybe start a new thread and/or search for people who have used the stuff.
My hunch is that you DO need to provide a bit of ventilation so that there is a way for the cavity to dry out. The vinyl/plastic facing acts as a moisture barrier so you don't want to trap any moisture between that and your metal. Also keep in mind that it could be difficult to eliminate any condensation in there with the normal installation method, which I believe is to run it over purlins and then compress it when you fasten the metal. The area that is compressed essentially has ZERO R-Value.
In all the research I've done over the last few months, I can't recall coming across any insulating system with fiberglass that does not provide a way for the fiberglass to dry out. It seems that it either needs to dry to the outside with ventilation behind, or it must NOT have facing or enclosing layers that act as a significant moisture/vapor barrier (such as vinyl/plastic facing) on the interior.
Even for spray foam insulation, I believe you need to allow everything on the inside from the spray foam to dry to the interior of the building. (Even drywall and some types of paint will allow drying to the interior.)
Perhaps they make some of the vinyl faced insulation with perforations throughout the surface to allow moisture through?
My advice in a nutshell would be to make sure any fiberglass insulation can breathe, either to the interior or exterior. That seems to be the recommended way to use the stuff and I don't see much risk in following that plan. There is a pretty big risk of bad things happening if you set yourself up to trap moisture between your metal and an interior moisture barrier though!
Maybe this link will be helpful...
http://www.insulation.org/metalbuilding/pages/design/condensationfacts.html
