Just looking for a definitive opinion on the following situation(s).

My loft has only 100mm of fiberglass insulation between the joists. The
top of this insulation is, obviously, not flat. Some times the
insulation is little bit above the joists and sometimes below.

I need to use the loft for storage and am considering laying chipboard
across the top of the existing joists. Only part of the loft will be
covered. The existing insulation will, essentially, be in contact with
the chipboard. There will be no designed in air gap between the
insulation and chipboard. Is this ok?

As I understand it, the chipboard will also add a little bit of extra
insulation itself.

I could improve things by using chipboard with some sort of foam
insulation bonded to the underside. Will an air gap be needed between
this and the existing fiberglass insulation?

Would it be better to use something like celotex beneath ordinary
chipboard and would the foil covered side need to be face down in
contact with the existing insulation?

Thanks.