I have a largish loft room (approx 8m * 5m * 3m at roof apex) as part of
the house extension, which is partially above the garage. The previous
owners have installed white polystyrene insulation of unknown thickness
between the rafters and then plasterboarded and skimmed over this.

In the summer the room gets very hot, and in winter very cold. I've been
told I have two options:

1. Tear down existing boards and polystyrene and install Celotex/Kingspan
type insulation, then re-dryline and skim.

2. Install insulated plasterboard on top of what's already there, and
skim.

I would rather go for option 2 as it would be quicker and cheaper, but is
it going to be sufficient to insulate the room properly, and what
thickness of insulated plasterboard would be best?

TIA