If the attic kneewall spaces are used for storage then you ll have to insulate along the roof slope down to the eaves and create an air barrier at the rafters.
Insulating attic dormers.
In case the space on the existing rafters is not enough you can fix a batten to the rafters underside in order to enhance the depth.
Insulating the floor of the void or the ceiling of the storey below depending which way you look at it is straightforward with a number of options available and is in effect the same as.
The rigid insulation method requires the utilisation of 150mm thick insulation in between the rafters along with a 50mm air gap in order to avert cold bridging.
This can be done with spray foam or rigid foam cut to fit between rafters and foamed in place with a foam gun.
Hold the cut insulation against the dormer walls with the paper side facing the attic or room and the fibreglass facing.
Loose fill insulation is usually less expensive to install than batt insulation and provides better coverage when installed properly.
Loose fill or batt insulation is typically installed in an attic.