The best way to insulate the attic is to insulate the floor or the attic only.
There are several reasons for this:
1) By insulating the floor of the attic you keep the living space below warmer
2)It cost you less to heat and cool your home because by insulating the rafters, you must then also heat and cool the attic. This will only increase your heating and cooling bills.
3)Insulating the floor along with proper ventilation will keep the roof system cooler thereby preventing ice dams (if you live in a cold climate) and premature shingle damage from excess heat buildup in summer.
4)Insulating between the rafters, though doable, must be accompanied by proper thickness of insulation as well as maintaining and providing additional insulation in the form of rafter vents, but also may necessitate add to the thickness of the rafters themselves to accomodate R-30 insulation. (R-30 can usually only be placed in 2×12 rafters).
If all you wish to do is have added insulation AND still be able to use the attic floor for storage, consider simply raising the attic floor with perpendicular joists to add increased floor insulation and then install a walking, storage area upon them.
Otherwise, if you need to use the attic for living space, you will need to add kneww walls and add rafter insulation and vents.
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