See more on different types of insulation.
Insulating attic dormers.
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.
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.
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 insulation is usually less expensive to install than batt insulation and provides better coverage when installed properly.
To find out if you have enough attic insulation measure the thickness of the insulation.