Is wood fiber insulation on rafters useful for uninsulated attics?

no, the attic is too small to be used as living space. However, the idea has now come up to place the [Kontrollierte-Wohnraumlüftung] in the attic and additionally insulate the roof with blow-in insulation alongside the top floor ceiling. That seems quite too complex to me; I would rather put the [Kontrollierte-Wohnraumlüftung] in the basement. But that would be a new topic again