ISU is entirely correct that in retrofit, moving the insulation plane to the bottom of the roof deck is one of the singular most effective choices a person can make.
The reasons for this are logical when thought out. Our roofs, by and large, soak up sun from dawn to dusk every day during the summer. The roof and the deck underneath gain that insolation and store it. This heats the rafters and structure in the attic. We will see attic venting proposed as an answer to this.
But the minimums for this are as low as 1:350. Just like with your car, cracking the car windows open 1/2" will cool your car 3-5º--1:350 will lower attic temps about 3-5º, from around 135º to 130º. If you roll your car windows 100% down, similar to power venting your attic, you can get to 6-10º temperature change. But, you need power vents that will run 2-3000 hours per year in our climate. Which then brings our humidity into question, too. Hard to "cool" an attic with 90+º at 60%RH air.
With an attic running 120-140º, the temperature difference ("delta-T" being the term-of-art) to the inside at 78º is near 50º; the side walls of the house on a 95º day are only 17º difference.
Also, the hardest plane in any house to air-seal is the ceiling. Any leak of air will be sucked into the attic by just the Delta-T.
Most of our local houses have the HVAC ductwork in that oven-like attic. Ok, most ducts are wrapped in insulation--but that insulation is only R-5 to R-8 at best. The joint sealing on the ductwork in too many residences is nearly non-existent. So, the air in the ducts gets warm. The stat kicks the blower on, and heated air is pushed out into our conditioned spaces, all while the leaky ducts are blowing expensive air into the attic. Not ideal.
So, move the insulation away from the ceiling, and to the bottom of the roof deck. Closed-cell spray foam can be the easiest for getting into nooks and crannies. It can be installed over vent channels, if those are required. But, it is also the most expensive answer. An alternative is to use rigid insulation across the bottom of the rafters. This can be supplemented with blown-in cellulose economically. Foil-faced insulation board can add "radiant barrier" to the strategy.
Also, since you no longer have 12-24" of insulation over the ceiling joists, you get to have the attic "back" as a storage space.
This works in existing structures--if sometimes needing creative answers for low-pitch and high-pitched roofs. You almost can defer insulating the walls, having brought the ceiling plane to about the delta-T of the wall planes.
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Standing guard, even sitting behind a desk
Occupational hazard of my occupation just not being around
I think most people would agree that 88 degrees inside is pretty dang warm. We have a ton of electronic equipment that would suffer at my house, not to mention how humid the house would seem that warm. 