This can be done with spray foam or rigid foam cut to fit between rafters and foamed in place with a foam gun.
Insulating attic dormers.
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.
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.
See more on different types of insulation.
Put on a long sleeved shirt safety glasses and a dust mask.
To find out if you have enough attic insulation measure the thickness of the insulation.
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.
Loose fill or batt insulation is typically installed in an attic.