Insulation, Radiant and Vapor Barriers - Insulating Garage Attic

I am planning to insulate the attic above my garage and I have a couple of questions. I live in MN and will be heating the garage with radiant heat to a constant 45-50 degrees, with the occasional weekend where I heat it to 65-70 degrees.

First, the walls are 2x6 construction and they were insulated and drywalled at the time of construction. The rafters in the ceiling are 2x4, and since there is a floor going into the attic, I am limited to 3 1/2 of depth for the insulation. Is this enough? I am also unsure if a vapor barrier is necessary or if I should just place bats of insulation between the rafters. I have soffits that extend 2-3 ft beyond the dry walls ceiling (from above) which are vented. Do I simply run the insulation to the end of the drywall? Do I need any kind of a barrier or other item to tie it out?

Finally, I as I mentioned, there is a floor going into the attic for storage. Does this affect how this area should be insulated?

Thanks.

http://www.certainteed.com/cinsulate/cict00301p.html

This site shows a product that is known as "high-performance insulation" for walls, R-15 at 3 1/2 inches thich for walls.

Understanding that you intend to floor the attic and you are only left with 2 x 4 cavities, you could use the product mentioned above to obtain optimum performance for this space. It will cost you more to purchase but you will save in the long run.

When installing insulation with a vapor barrier, just make sure there aren't any gaps, rips or openings in the vapor barrier. Vapor barrier tape usually solves this. What you are trying to accomplish is to create a continuous vapor barrier over the entire area where you insulate.

Is a vapor barrier required for my application? What typically dictates whether or not a vapor barrier is used?