We are having a steel home built, with a white roof. Anyone out there have experience to share about insulation?

The building will come with 3 inches of insulation.

Is radiant barrier more effective under the roof, or over the living space?
What about dust collecting on the barrier if it is on the floor of the attic?
If foam better? What about flammibility? (It's expensive, I already know!)

Using a radiant barrier, what is the least amount of thermal insulation needed?

What part does ventilation play with these different insulations? I am told that with the foam, there is no ventilation. Is this good?

Any advice will be carefully evaluated.

Thanks so much!!

http://www.ornl.gov/sci/roofs+walls/insulation/ins_16.html

Here is a good link for insulation values. Some considerations are window size and how many, heating plant, and location of the house.