Eves cupboard insulation

[Andy15]

I wish to insulate the eve cupboards which run down both sides of my 1960's Chalet bungalow. The cupboards are right-angled triangle in section, the hypotenuse being the roof which has a thin layer of fibre-glass insulation directly under the roofing felt on top of the rafters. I have been told that it would be easier and cheaper to insulate under the floorboards and the stud work wall than using Kingspan (or similar) to insulate between the rafters. My concern is that the fibreglass insulation would stop air flow under the floor boards to the joists and I would need to cover the stud work wall fibreglass insulation with MDF to be able to continue using the cupboard for storage, which would mean there would be no airflow to the stud work wall timbers.
I'm also a little hesitant using Kingspan between the rafters because the existing fibre-glass insulation could restrict the airflow to the rafters.
I would be very grateful for any advice anyone can offer