Well, that IMHO is the wrong way to do it. You will be effectively putting ZERO insulation between you and the unconditioned space of the attic. The heat that the attic will still gain - because of the static air - will permeate your ceiling right into your conditioned space.

You need to insulate between your conditioned living space and the NON conditioned NON living space of the attic. That is between the lower floor rooms and the attic. There is SOME benefit to insulating the roof to keep the heat out of the attic, but without something between the attic and the living area it's not going to do much good. Roof insulation will keep SOME of the heat from radiating though the roof, but it's still gonna get plenty hot and cold up there.

I've seen some people that think their heating and cooling bills will be cut in half by isolating an unused upper floor of a house and not heat or cool it. The savings are more in the order of 10 to 30% if that much. Why? Because they have an unconditioned space that gets hot in the summer and cold in the winter right up against the heated and cooled living space with no insulation to keep the transfer of energy from occurring.