wrote:
Looking to board my attic for storage; it has some scrappy Rockwool
and joists are 4"x2". I was wondering whether to plant 2"x2" timbers
on joists to allow for greater depth of Rockwool, or maybe the
chipboard would provide enough extra insulation? Another thought -
better grade insulation material in place of Rockwool but in the same
4" space? Any ideas welcomed.

If you add 2x2 timbers on top of the existing joists you are still 4 inches
short of the recommended 10 inches of loft insulation. Have you considered
also putting insulation between the rafters, say 4 inches?