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.
Insulating attic dormers.
Loose fill insulation is usually less expensive to install than batt insulation and provides better coverage when installed properly.
This can be done with spray foam or rigid foam cut to fit between rafters and foamed in place with a foam gun.
Loose fill or batt insulation is typically installed in an attic.
Hold the cut insulation against the dormer walls with the paper side facing the attic or room and the fibreglass facing.
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.
To find out if you have enough attic insulation measure the thickness of the insulation.
See more on different types of insulation.