Hold the cut insulation against the dormer walls with the paper side facing the attic or room and the fibreglass facing.
Insulating attic dormers.
See more on different types of insulation.
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.
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.
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.
To find out if you have enough attic insulation measure the thickness of the insulation.
Loose fill insulation is usually less expensive to install than batt insulation and provides better coverage when installed properly.