Doctor Drivel wrote:

As he is removing the old insulation, it is best seal up the holes where
cable and pipes penetrate with silicon, then lat a poly vapour barrier
on the plasterboard/joist. Pin it with a staple gun to side of the
joists and tape the joins with big overlaps. Then lay the insulation and
lay the new joists. Then make sure the hatch is sealed and insulated too.

Is that just to stop drafts, or for any other reason? The ceiling
appears to be chipboard rather than plasterboard here.

Dave