This can be achieved through the use of truss joists scissor truss framing or sufficiently large rafters.
Proper insulating roof ceiling.
To insulate the roof itself you either use spray foam or retrofit with it with rigid foam board.
Set up a stepladder tall enough to work comfortably from.
For example cathedral ceilings built with 2x12 rafters have space for standard 10 inch batts r 30 and ventilation.
But its open cell structure still allows moisture to escape.
Cathedral ceilings must provide space between the roof deck and home s ceiling for adequate insulation and ventilation.
Sprayed against the underside of the roof deck it seals against air movement eliminating the need for a vapor barrier or ventilation.
If you re insulating the roof directly.
Purchase rolls or packages of insulation that will fit in the spaces between the ceiling joists.
When tom insulates a cathedral ceiling or a finished attic he also turns to foam.
The standard width for joists is 24 inches.
Always cover the tops of the ceiling joists to make sure the insulation is deep enough to reach your target r value and to prevent thermal bridging the heat loss that occurs through the wood framing.
Illustration by ian worpole.