Why would you not insulate between the rafters in an attic? I see a lot of houses with no insulation on the inside of the roof.
A typical composite shingle would get too hot if you insulated the roof directly underneath it, decreasing life of shingle. You want good air flow in attic, insulate above the ceilings only.
as long as the attic floor is insulated that is all you need. The insulation will keep the area below it warm. If you insulate the rafters then the attic stays even warmer while the area below not so much.
Why would you want to waste money heating the attic? There are several reasons to put the insulation directly above the living space. Increases the life of your roof, prevents ice damming, saves money, helps avoid moisture issues in the attic and roof sheathing. Is that enough?
Usually the floor of the attic is insulated unless you have cathedral ceilings. Then they have to insulate above the sloped ceiling and leave a little air space directly above the insulation that vents to perforated soffits and a ridge vent. You can also install foil faced plywood on the roof to improve energy performance.
