The way you insulate your attic depends on whether you want a space in the ceiling, "cold" or "hot". A cold roof requires insulation at the level of the joist to stop the heat from escaping through the space in the ceiling without the use of. A hot roof is insulated between and under the rafters of the roof itself. The recommended depth for insulation has been increased recently, so you may have to increase the depths of the joists or rafters if you want to create platforms usable storage space.