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

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

A further advantage of Option 1 is that you can install a reflective layer
between the tiles and the insulation. This vastly helps in the "hot in
summer" stakes. It will also result in a thinner result for the same
U-value. It is also possible to ensure that correct ventilation has been
installed. Someone may have stuffed the entire cavity, leading to rotting
problems in the rafters.

Christian.