What would it take???

I just bought a house that has a finished attic. It is three rooms. The middle room has two full sized doors that lead into the unfinished attic space on either of the finished portion. The unfinished spaces run the length of the house and are about 8-10 feet deep. These areas are currently insulated with typical fiberglass insulation. The insulation is against the roof and covered with cardboard. I would like to pull out that insulation and put in new insulation. I've been told by several people that the correct way to insulate this is to leave the roof area without insulation and put the insualtion on the floor. My only problem with that is that those two doors to the unfinished space are regular inside doors and uninsulated. I would also like to use some portion of the unfinished for storage. So, based on my limited knowledge of these things, I thought I would remove the existing insulation, lay insulation on the floor of most of the unfinished space and have closets that are about ten feet wide built into the portion of the unfinished space that I would like to use. In that space, I would put insulation on the backside of those new walls and against the roof. Any I crazy to think this is the correct paln? I would appreciate any advice.

I am inclosing a porch at my house that has a tin roof atop 2x6 rafters with an appropriate slope. I do not want to put in a drop ceiling. Rather I wish to insulate against the tin roof and attach sheet rock directly to the rafters underside.. The 2x6 rafters should allow adequate space for the insulation. Please advise me on how to do this insulation. What type moisture barrier do I need against the tin to keep the insulation from touching? Do I need air space between the tin and the insulation? If so how much space and how do I get that space? All help will be appreciated.